feat(scheduler,runtime): non-panicking try_spawn for at-capacity load shedding

allocate_slot() panics on a full slab; for a load-shedding caller (an
accept loop spawning one actor per connection) that panic lands in the
spawning actor, which then crash-loops under Restart::Transient into the
still-full slab until its restart budget is spent — and the service stops
accepting entirely. Observed live (urus slowloris scaling, 2026-08-10).
A full slab is a routine overload condition for such callers, not an
invariant violation.

- RuntimeInner::try_allocate_slot() -> Option<u32>: the non-panicking
  core; a single pop under the free-list lock, so the claim is atomic
  (claim-or-report — no check-then-spawn TOCTOU, no headroom margin).
  allocate_slot() is now a thin panicking wrapper over it.
- scheduler::try_spawn / try_spawn_under_with -> Result<JoinHandle,
  SpawnError>: parity with spawn/spawn_under_with except a full slab
  returns Err(SpawnError::AtCapacity) instead of panicking. Minimal
  surface per the agreed strategy; the remaining _with/_addr mirrors are
  trivial wrappers if ever needed.
- Slot-first ordering on the try path (reverse of spawn's stack-first):
  under overload Err is the hot path, and a rejection costs one mutex
  pop — no mmap/pool-pop + init + recycle per shed unit of work. A
  drop-guard returns the claimed slot if stack allocation panics in the
  claim-to-install window (would otherwise leak and trip run()'s
  teardown slot-leak debug_assert).
- SpawnError: non_exhaustive, Display + std::error::Error.
- spawn and every existing call site untouched: the panic remains the
  correct loud invariant check at internal/bounded spawn sites.

tests/try_spawn.rs: parity when slots free; exact slab accounting at
capacity (Err, no panic, repeatable); custom-shape try refuses before
stack allocation; self-heal after slots free; plain spawn still panics
(surfaced via JoinError payload); 4-thread race for the last slots
claims exactly the free count; SpawnError impl checks.

Design doc: smarm-suggestion-try-spawn.md. Downstream consumer change
(canned 503 on AtCapacity in urus's accept loop) is urus scope, not
smarm.

(cherry picked from commit 36de4b36aeaa72b2a5f9f3797b9854652656dcf6)

.githooks/pre-commit

@@ -2,7 +2,23 @@
 # smarm pre-commit gate: clippy the library (src/) with warnings as errors.
 # unwrap_used / expect_used are denied (Cargo.toml [lints.clippy]): library
 # code must not hide a panic behind unwrap/expect. Tests/examples are not gated.
+#
+# Toolchain resolution: prefer an installed cargo-clippy; on machines whose
+# rust comes without the clippy component (e.g. NixOS home-manager), fall
+# back to an ephemeral nix-shell toolchain. The fallback uses its own target
+# dir (target/clippy) because the shell's rustc version may differ from the
+# default toolchain's — mixed-compiler artifacts in one target dir are an
+# E0514 hard error. MSRV (Cargo.toml rust-version) keeps the older shell
+# toolchain a legitimate gate.
 set -eu
 [ -f "$HOME/.cargo/env" ] && . "$HOME/.cargo/env"
 cd "$(git rev-parse --show-toplevel)"
-cargo clippy --lib -- -D warnings
+if cargo clippy --version >/dev/null 2>&1; then
+    cargo clippy --lib -- -D warnings
+elif command -v nix-shell >/dev/null 2>&1; then
+    nix-shell -p clippy -p cargo -p rustc \
+        --run 'CARGO_TARGET_DIR=target/clippy cargo clippy --lib -- -D warnings'
+else
+    echo "pre-commit: cargo clippy unavailable and no nix-shell fallback" >&2
+    exit 1
+fi

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "smarm"
-version = "0.4.0"
+version = "0.6.1"
 edition = "2021"
 rust-version = "1.95"
 
@@ -39,6 +39,9 @@ rq-mutex = []
 rq-mpmc = []
 rq-striped = []
 
+[build-dependencies]
+cc = "1"
+
 [dependencies]
 libc = "0.2"
 

benches/general.rs

@@ -26,7 +26,9 @@ use std::time::Instant;
 const ITERS: u32 = 15;
 
 fn available_threads() -> usize {
-    std::thread::available_parallelism().map(|n| n.get()).unwrap_or(1)
+    std::thread::available_parallelism()
+        .map(|n| n.get())
+        .unwrap_or(1)
 }
 
 fn env_sets() -> u32 {
@@ -108,17 +110,15 @@ fn bench_chained_smarm(threads: usize) -> (u64, u128) {
 fn bench_chained_tokio_current() -> (u64, u128) {
     let counter = Arc::new(AtomicU64::new(0));
     let c2 = counter.clone();
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
         // Use a oneshot done channel like tokio's own chained_spawn bench.
         let (done_tx, done_rx) = tokio::sync::oneshot::channel();
-        fn iter(
-            c: Arc<AtomicU64>,
-            done: tokio::sync::oneshot::Sender<()>,
-            n: u64,
-        ) {
+        fn iter(c: Arc<AtomicU64>, done: tokio::sync::oneshot::Sender<()>, n: u64) {
             if n == 0 {
                 let _ = done.send(());
             } else {
@@ -186,7 +186,9 @@ fn bench_yield_smarm(threads: usize) -> (u64, u128) {
 }
 
 fn bench_yield_tokio_current() -> (u64, u128) {
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -235,11 +237,22 @@ const PRIME_N: u64 = 400_000;
 const PRIME_WORKERS: u64 = 64;
 
 fn is_prime(n: u64) -> bool {
-    if n < 2 { return false; }
-    if n < 4 { return true; }
-    if n % 2 == 0 { return false; }
+    if n < 2 {
+        return false;
+    }
+    if n < 4 {
+        return true;
+    }
+    if n % 2 == 0 {
+        return false;
+    }
     let mut i = 3u64;
-    while i * i <= n { if n % i == 0 { return false; } i += 2; }
+    while i * i <= n {
+        if n % i == 0 {
+            return false;
+        }
+        i += 2;
+    }
     true
 }
 
@@ -250,7 +263,11 @@ fn count_primes(lo: u64, hi: u64) -> u64 {
 fn primes_slice(w: u64) -> (u64, u64) {
     let per = PRIME_N / PRIME_WORKERS;
     let lo = w * per;
-    let hi = if w + 1 == PRIME_WORKERS { PRIME_N } else { lo + per };
+    let hi = if w + 1 == PRIME_WORKERS {
+        PRIME_N
+    } else {
+        lo + per
+    };
     (lo, hi)
 }
 
@@ -267,7 +284,9 @@ fn bench_primes_smarm(threads: usize) -> (u64, u128) {
                 tc.fetch_add(count_primes(lo, hi), Ordering::Relaxed);
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
     });
     (total.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -275,7 +294,9 @@ fn bench_primes_smarm(threads: usize) -> (u64, u128) {
 fn bench_primes_tokio_current() -> (u64, u128) {
     let total = Arc::new(AtomicU64::new(0));
     let t2 = total.clone();
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -287,7 +308,9 @@ fn bench_primes_tokio_current() -> (u64, u128) {
                 tc.fetch_add(count_primes(lo, hi), Ordering::Relaxed);
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
     });
     (total.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -309,7 +332,9 @@ fn bench_primes_tokio_multi() -> (u64, u128) {
                 tc.fetch_add(count_primes(lo, hi), Ordering::Relaxed);
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
     });
     (total.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -344,7 +369,9 @@ fn bench_pp_smarm(threads: usize) -> (u64, u128) {
 }
 
 fn bench_pp_tokio_current() -> (u64, u128) {
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -395,7 +422,6 @@ fn bench_pp_tokio_multi() -> (u64, u128) {
 // main
 // ---------------------------------------------------------------------------
 
-
 // ---------------------------------------------------------------------------
 // Knob helper — reads SMARM_ALLOC_INTERVAL / SMARM_TIMESLICE_CYCLES env vars
 // so the sweep script can override the preemption knobs without recompiling.
@@ -404,10 +430,14 @@ fn bench_pp_tokio_multi() -> (u64, u128) {
 fn bench_cfg(threads: usize) -> smarm::runtime::Config {
     let mut cfg = smarm::runtime::Config::exact(threads);
     if let Ok(v) = std::env::var("SMARM_ALLOC_INTERVAL") {
-        if let Ok(n) = v.parse::<u32>() { cfg = cfg.alloc_interval(n); }
+        if let Ok(n) = v.parse::<u32>() {
+            cfg = cfg.alloc_interval(n);
+        }
     }
     if let Ok(v) = std::env::var("SMARM_TIMESLICE_CYCLES") {
-        if let Ok(n) = v.parse::<u64>() { cfg = cfg.timeslice_cycles(n); }
+        if let Ok(n) = v.parse::<u64>() {
+            cfg = cfg.timeslice_cycles(n);
+        }
     }
     cfg
 }
@@ -417,7 +447,10 @@ fn main() {
     println!("smarm general benchmarks");
     println!("available parallelism: {n} threads");
     let sets = env_sets();
-    println!("ITERS={ITERS}×{sets} sets = {} samples (+1 warmup, discarded)", ITERS * sets);
+    println!(
+        "ITERS={ITERS}×{sets} sets = {} samples (+1 warmup, discarded)",
+        ITERS * sets
+    );
     println!(
         "CHAIN_DEPTH={CHAIN_DEPTH}, YIELD_TASKS={YIELD_TASKS}×{YIELD_ROUNDS}, \
          PRIME_N={PRIME_N}/{PRIME_WORKERS} workers, PP_ROUNDS={PP_ROUNDS}"
@@ -426,21 +459,29 @@ fn main() {
     // ---- 1. chained_spawn ----
     print_header(&format!("chained_spawn: depth {CHAIN_DEPTH}"));
     run_n("smarm 1-thread", ITERS, || bench_chained_smarm(1));
-    run_n(&format!("smarm {n}-thread"), ITERS, || bench_chained_smarm(n));
+    run_n(&format!("smarm {n}-thread"), ITERS, || {
+        bench_chained_smarm(n)
+    });
     run_n("tokio current_thread", ITERS, bench_chained_tokio_current);
     run_n("tokio multi-thread", ITERS, bench_chained_tokio_multi);
 
     // ---- 2. yield_many ----
-    print_header(&format!("yield_many: {YIELD_TASKS} tasks × {YIELD_ROUNDS} yields"));
+    print_header(&format!(
+        "yield_many: {YIELD_TASKS} tasks × {YIELD_ROUNDS} yields"
+    ));
     run_n("smarm 1-thread", ITERS, || bench_yield_smarm(1));
     run_n(&format!("smarm {n}-thread"), ITERS, || bench_yield_smarm(n));
     run_n("tokio current_thread", ITERS, bench_yield_tokio_current);
     run_n("tokio multi-thread", ITERS, bench_yield_tokio_multi);
 
     // ---- 3. fan_out_compute ----
-    print_header(&format!("fan_out_compute: primes in [2, {PRIME_N}) across {PRIME_WORKERS}"));
+    print_header(&format!(
+        "fan_out_compute: primes in [2, {PRIME_N}) across {PRIME_WORKERS}"
+    ));
     run_n("smarm 1-thread", ITERS, || bench_primes_smarm(1));
-    run_n(&format!("smarm {n}-thread"), ITERS, || bench_primes_smarm(n));
+    run_n(&format!("smarm {n}-thread"), ITERS, || {
+        bench_primes_smarm(n)
+    });
     run_n("tokio current_thread", ITERS, bench_primes_tokio_current);
     run_n("tokio multi-thread", ITERS, bench_primes_tokio_multi);
 

benches/multi_scheduler.rs

@@ -64,11 +64,22 @@ const PRIME_N: u64 = 400_000;
 const WORKERS: u64 = 64;
 
 fn is_prime(n: u64) -> bool {
-    if n < 2 { return false; }
-    if n < 4 { return true; }
-    if n % 2 == 0 { return false; }
+    if n < 2 {
+        return false;
+    }
+    if n < 4 {
+        return true;
+    }
+    if n % 2 == 0 {
+        return false;
+    }
     let mut i = 3u64;
-    while i * i <= n { if n % i == 0 { return false; } i += 2; }
+    while i * i <= n {
+        if n % i == 0 {
+            return false;
+        }
+        i += 2;
+    }
     true
 }
 
@@ -96,7 +107,9 @@ fn bench_primes_smarm(threads: usize) -> (u64, u128) {
                 tc.fetch_add(count_primes(lo, hi), Ordering::Relaxed);
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
     });
     (total.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -104,7 +117,9 @@ fn bench_primes_smarm(threads: usize) -> (u64, u128) {
 fn bench_primes_tokio_current() -> (u64, u128) {
     let total = Arc::new(AtomicU64::new(0));
     let t2 = total.clone();
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -116,7 +131,9 @@ fn bench_primes_tokio_current() -> (u64, u128) {
                 tc.fetch_add(count_primes(lo, hi), Ordering::Relaxed);
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
     });
     (total.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -138,17 +155,21 @@ fn bench_primes_tokio_multi() -> (u64, u128) {
                 tc.fetch_add(count_primes(lo, hi), Ordering::Relaxed);
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
     });
     (total.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
 
 fn bench_primes_baseline() -> (u64, u128) {
     let start = Instant::now();
-    let total: u64 = (0..WORKERS).map(|w| {
+    let total: u64 = (0..WORKERS)
+        .map(|w| {
             let (lo, hi) = primes_slice(w);
             count_primes(lo, hi)
-    }).sum();
+        })
+        .sum();
     (total, start.elapsed().as_micros())
 }
 
@@ -167,15 +188,17 @@ fn bench_pingpong_smarm(threads: usize) -> (u64, u128) {
             tx_a.send(0).unwrap();
             loop {
                 let v = rx_b.recv().unwrap();
-                if v >= PING_ROUNDS { break; }
+                if v >= PING_ROUNDS {
+                    break;
+                }
                 tx_a.send(v + 1).unwrap();
             }
         });
-        let hb = smarm::spawn(move || {
-            loop {
+        let hb = smarm::spawn(move || loop {
             let v = rx_a.recv().unwrap();
             tx_b.send(v + 1).unwrap();
-                if v + 1 >= PING_ROUNDS { break; }
+            if v + 1 >= PING_ROUNDS {
+                break;
             }
         });
         ha.join().unwrap();
@@ -198,7 +221,9 @@ fn bench_pingpong_tokio_current() -> (u64, u128) {
             tx_a.send(0).unwrap();
             loop {
                 let v = rx_b.recv().await.unwrap();
-                if v >= PING_ROUNDS { break; }
+                if v >= PING_ROUNDS {
+                    break;
+                }
                 tx_a.send(v + 1).unwrap();
             }
         });
@@ -206,7 +231,9 @@ fn bench_pingpong_tokio_current() -> (u64, u128) {
             loop {
                 let v = rx_a.recv().await.unwrap();
                 tx_b.send(v + 1).unwrap();
-                if v + 1 >= PING_ROUNDS { break; }
+                if v + 1 >= PING_ROUNDS {
+                    break;
+                }
             }
         });
         let _ = ha.await;
@@ -229,7 +256,9 @@ fn bench_pingpong_tokio_multi() -> (u64, u128) {
             tx_a.send(0).unwrap();
             loop {
                 let v = rx_b.recv().await.unwrap();
-                if v >= PING_ROUNDS { break; }
+                if v >= PING_ROUNDS {
+                    break;
+                }
                 tx_a.send(v + 1).unwrap();
             }
         });
@@ -237,7 +266,9 @@ fn bench_pingpong_tokio_multi() -> (u64, u128) {
             loop {
                 let v = rx_a.recv().await.unwrap();
                 tx_b.send(v + 1).unwrap();
-                if v + 1 >= PING_ROUNDS { break; }
+                if v + 1 >= PING_ROUNDS {
+                    break;
+                }
             }
         });
         let _ = ha.await;
@@ -264,7 +295,9 @@ fn bench_spawn_smarm(threads: usize) -> (u64, u128) {
                 cc.fetch_add(1, Ordering::Relaxed);
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
     });
     (counter.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -272,7 +305,9 @@ fn bench_spawn_smarm(threads: usize) -> (u64, u128) {
 fn bench_spawn_tokio_current() -> (u64, u128) {
     let counter = Arc::new(AtomicU64::new(0));
     let c = counter.clone();
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -283,7 +318,9 @@ fn bench_spawn_tokio_current() -> (u64, u128) {
                 cc.fetch_add(1, Ordering::Relaxed);
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
     });
     (counter.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -304,7 +341,9 @@ fn bench_spawn_tokio_multi() -> (u64, u128) {
                 cc.fetch_add(1, Ordering::Relaxed);
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
     });
     (counter.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -320,22 +359,32 @@ fn main() {
     println!("PRIME_N={PRIME_N}, WORKERS={WORKERS}, PING_ROUNDS={PING_ROUNDS}, SPAWN_COUNT={SPAWN_COUNT}");
 
     // ---- Primes ----
-    print_header(&format!("Fan-out/fan-in: count primes in [2, {PRIME_N}) across {WORKERS} workers"));
+    print_header(&format!(
+        "Fan-out/fan-in: count primes in [2, {PRIME_N}) across {WORKERS} workers"
+    ));
     run_n("baseline (serial)", ITERS, bench_primes_baseline);
     run_n("smarm single-thread", ITERS, || bench_primes_smarm(1));
-    run_n(&format!("smarm {n}-thread"), ITERS, || bench_primes_smarm(n));
+    run_n(&format!("smarm {n}-thread"), ITERS, || {
+        bench_primes_smarm(n)
+    });
     run_n("tokio current_thread", ITERS, bench_primes_tokio_current);
     run_n("tokio multi-thread", ITERS, bench_primes_tokio_multi);
 
     // ---- Ping-pong ----
-    print_header(&format!("Ping-pong: {PING_ROUNDS} round-trips between two actors"));
+    print_header(&format!(
+        "Ping-pong: {PING_ROUNDS} round-trips between two actors"
+    ));
     run_n("smarm single-thread", ITERS, || bench_pingpong_smarm(1));
-    run_n(&format!("smarm {n}-thread"), ITERS, || bench_pingpong_smarm(n));
+    run_n(&format!("smarm {n}-thread"), ITERS, || {
+        bench_pingpong_smarm(n)
+    });
     run_n("tokio current_thread", ITERS, bench_pingpong_tokio_current);
     run_n("tokio multi-thread", ITERS, bench_pingpong_tokio_multi);
 
     // ---- Spawn throughput ----
-    print_header(&format!("Spawn throughput: {SPAWN_COUNT} actors spawned and joined"));
+    print_header(&format!(
+        "Spawn throughput: {SPAWN_COUNT} actors spawned and joined"
+    ));
     run_n("smarm single-thread", ITERS, || bench_spawn_smarm(1));
     run_n(&format!("smarm {n}-thread"), ITERS, || bench_spawn_smarm(n));
     run_n("tokio current_thread", ITERS, bench_spawn_tokio_current);

benches/primes.rs

@@ -16,12 +16,20 @@ const WORKERS: u64 = 16;
 const ITERATIONS: u32 = 5;
 
 fn is_prime(n: u64) -> bool {
-    if n < 2 { return false; }
-    if n < 4 { return true; }
-    if n % 2 == 0 { return false; }
+    if n < 2 {
+        return false;
+    }
+    if n < 4 {
+        return true;
+    }
+    if n % 2 == 0 {
+        return false;
+    }
     let mut i = 3u64;
     while i * i <= n {
-        if n % i == 0 { return false; }
+        if n % i == 0 {
+            return false;
+        }
         i += 2;
     }
     true
@@ -30,7 +38,9 @@ fn is_prime(n: u64) -> bool {
 fn count_primes_in(lo: u64, hi: u64) -> u64 {
     let mut count = 0u64;
     for n in lo..hi {
-        if is_prime(n) { count += 1; }
+        if is_prime(n) {
+            count += 1;
+        }
     }
     count
 }
@@ -38,7 +48,11 @@ fn count_primes_in(lo: u64, hi: u64) -> u64 {
 fn slice(worker: u64) -> (u64, u64) {
     let per = N / WORKERS;
     let lo = worker * per;
-    let hi = if worker + 1 == WORKERS { N } else { (worker + 1) * per };
+    let hi = if worker + 1 == WORKERS {
+        N
+    } else {
+        (worker + 1) * per
+    };
     (lo, hi)
 }
 
@@ -125,7 +139,10 @@ fn main() {
         "Counting primes in [2, {}) across {} workers, {} iterations each\n",
         N, WORKERS, ITERATIONS
     );
-    println!("{:>12} | {:>15} | {:>16} | {:>15} | {:>15}", "runtime", "primes found", "median", "min", "max");
+    println!(
+        "{:>12} | {:>15} | {:>16} | {:>15} | {:>15}",
+        "runtime", "primes found", "median", "min", "max"
+    );
     println!("{}", "-".repeat(80));
 
     run_n("baseline", ITERATIONS, bench_baseline);

benches/rq_micro.rs

@@ -27,12 +27,19 @@ use std::sync::Arc;
 use std::time::Instant;
 
 fn env_usize(key: &str, default: usize) -> usize {
-    std::env::var(key).ok().and_then(|v| v.parse().ok()).unwrap_or(default)
+    std::env::var(key)
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(default)
 }
 
 fn env_threads() -> Vec<usize> {
     std::env::var("SMARM_BENCH_THREADS")
-        .map(|v| v.split_whitespace().filter_map(|t| t.parse().ok()).collect())
+        .map(|v| {
+            v.split_whitespace()
+                .filter_map(|t| t.parse().ok())
+                .collect()
+        })
         .unwrap_or_else(|_| vec![1, 2, 4])
 }
 
@@ -53,7 +60,11 @@ fn drive<Q: Send + Sync + 'static>(
     for p in 0..producers {
         let q = q.clone();
         // Give the last producer the remainder.
-        let n = if p == producers - 1 { items - per * (producers - 1) } else { per };
+        let n = if p == producers - 1 {
+            items - per * (producers - 1)
+        } else {
+            per
+        };
         hs.push(std::thread::spawn(move || {
             let pid = Pid::new(p as u32, 0);
             for _ in 0..n {
@@ -132,7 +143,12 @@ fn main() {
     for &t in &threads_sweep {
         for (p, c) in ratios_for(t) {
             for s in ["mutex", "mpmc", "striped"] {
-                cases.push(Case { structure: s, threads: t, producers: p, consumers: c });
+                cases.push(Case {
+                    structure: s,
+                    threads: t,
+                    producers: p,
+                    consumers: c,
+                });
             }
         }
     }
@@ -147,7 +163,14 @@ fn main() {
                         if case.threads < 2 {
                             drive_single(&*q, MutexQueue::push, MutexQueue::pop, items)
                         } else {
-                            drive(q, MutexQueue::push, MutexQueue::pop, case.producers, case.consumers, items)
+                            drive(
+                                q,
+                                MutexQueue::push,
+                                MutexQueue::pop,
+                                case.producers,
+                                case.consumers,
+                                items,
+                            )
                         }
                     }
                     "mpmc" => {
@@ -155,7 +178,14 @@ fn main() {
                         if case.threads < 2 {
                             drive_single(&*q, MpmcRing::push, MpmcRing::pop, items)
                         } else {
-                            drive(q, MpmcRing::push, MpmcRing::pop, case.producers, case.consumers, items)
+                            drive(
+                                q,
+                                MpmcRing::push,
+                                MpmcRing::pop,
+                                case.producers,
+                                case.consumers,
+                                items,
+                            )
                         }
                     }
                     "striped" => {
@@ -163,7 +193,14 @@ fn main() {
                         if case.threads < 2 {
                             drive_single(&*q, StripedRing::push, StripedRing::pop, items)
                         } else {
-                            drive(q, StripedRing::push, StripedRing::pop, case.producers, case.consumers, items)
+                            drive(
+                                q,
+                                StripedRing::push,
+                                StripedRing::pop,
+                                case.producers,
+                                case.consumers,
+                                items,
+                            )
                         }
                     }
                     _ => unreachable!(),

benches/rq_runtime.rs

@@ -54,12 +54,19 @@ fn variant() -> &'static str {
 }
 
 fn env_usize(key: &str, default: usize) -> usize {
-    std::env::var(key).ok().and_then(|v| v.parse().ok()).unwrap_or(default)
+    std::env::var(key)
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(default)
 }
 
 fn env_threads() -> Vec<usize> {
     std::env::var("SMARM_BENCH_THREADS")
-        .map(|v| v.split_whitespace().filter_map(|t| t.parse().ok()).collect())
+        .map(|v| {
+            v.split_whitespace()
+                .filter_map(|t| t.parse().ok())
+                .collect()
+        })
         .unwrap_or_else(|_| vec![1, 2, 4])
 }
 
@@ -238,12 +245,22 @@ fn main() {
                 );
                 println!(
                     "RQCSV,runtime,{},{},{},{},{},{},{}",
-                    variant(), slot_str, name, t, work, mid.us, per_s
+                    variant(),
+                    slot_str,
+                    name,
+                    t,
+                    work,
+                    mid.us,
+                    per_s
                 );
                 if slot {
                     println!(
                         "RQSLOT,{},{},{},{},{}",
-                        variant(), name, t, mid.hits, mid.displacements
+                        variant(),
+                        name,
+                        t,
+                        mid.hits,
+                        mid.displacements
                     );
                 }
             }

benches/smarm_favored.rs

@@ -37,7 +37,9 @@ use std::time::Instant;
 const ITERS: u32 = 15;
 
 fn available_threads() -> usize {
-    std::thread::available_parallelism().map(|n| n.get()).unwrap_or(1)
+    std::thread::available_parallelism()
+        .map(|n| n.get())
+        .unwrap_or(1)
 }
 
 fn env_sets() -> u32 {
@@ -116,7 +118,9 @@ fn bench_recurse_smarm(threads: usize) -> (u64, u128) {
 fn bench_recurse_tokio_current() -> (u64, u128) {
     let counter = Arc::new(AtomicU64::new(0));
     let c2 = counter.clone();
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -199,7 +203,9 @@ fn bench_hot_smarm() -> (u64, u128) {
 }
 
 fn bench_hot_tokio_current() -> (u64, u128) {
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -249,7 +255,9 @@ fn bench_unc_smarm() -> (u64, u128) {
 }
 
 fn bench_unc_tokio_current() -> (u64, u128) {
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -297,8 +305,12 @@ fn bench_panic_smarm(threads: usize) -> (u64, u128) {
         }
         for h in handles {
             match h.join() {
-                Ok(()) => { ok2.fetch_add(1, Ordering::Relaxed); }
-                Err(_) => { err2.fetch_add(1, Ordering::Relaxed); }
+                Ok(()) => {
+                    ok2.fetch_add(1, Ordering::Relaxed);
+                }
+                Err(_) => {
+                    err2.fetch_add(1, Ordering::Relaxed);
+                }
             }
         }
     });
@@ -312,7 +324,9 @@ fn bench_panic_tokio_current() -> (u64, u128) {
     let err = Arc::new(AtomicU64::new(0));
     let ok2 = ok.clone();
     let err2 = err.clone();
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let prev_hook = std::panic::take_hook();
     std::panic::set_hook(Box::new(|_| {}));
     let start = Instant::now();
@@ -328,8 +342,12 @@ fn bench_panic_tokio_current() -> (u64, u128) {
         }
         for h in handles {
             match h.await {
-                Ok(()) => { ok2.fetch_add(1, Ordering::Relaxed); }
-                Err(_) => { err2.fetch_add(1, Ordering::Relaxed); }
+                Ok(()) => {
+                    ok2.fetch_add(1, Ordering::Relaxed);
+                }
+                Err(_) => {
+                    err2.fetch_add(1, Ordering::Relaxed);
+                }
             }
         }
     });
@@ -361,8 +379,12 @@ fn bench_panic_tokio_multi() -> (u64, u128) {
         }
         for h in handles {
             match h.await {
-                Ok(()) => { ok2.fetch_add(1, Ordering::Relaxed); }
-                Err(_) => { err2.fetch_add(1, Ordering::Relaxed); }
+                Ok(()) => {
+                    ok2.fetch_add(1, Ordering::Relaxed);
+                }
+                Err(_) => {
+                    err2.fetch_add(1, Ordering::Relaxed);
+                }
             }
         }
     });
@@ -375,7 +397,6 @@ fn bench_panic_tokio_multi() -> (u64, u128) {
 // main
 // ---------------------------------------------------------------------------
 
-
 // ---------------------------------------------------------------------------
 // Knob helper — reads SMARM_ALLOC_INTERVAL / SMARM_TIMESLICE_CYCLES env vars
 // so the sweep script can override the preemption knobs without recompiling.
@@ -384,10 +405,14 @@ fn bench_panic_tokio_multi() -> (u64, u128) {
 fn bench_cfg(threads: usize) -> smarm::runtime::Config {
     let mut cfg = smarm::runtime::Config::exact(threads);
     if let Ok(v) = std::env::var("SMARM_ALLOC_INTERVAL") {
-        if let Ok(n) = v.parse::<u32>() { cfg = cfg.alloc_interval(n); }
+        if let Ok(n) = v.parse::<u32>() {
+            cfg = cfg.alloc_interval(n);
+        }
     }
     if let Ok(v) = std::env::var("SMARM_TIMESLICE_CYCLES") {
-        if let Ok(n) = v.parse::<u64>() { cfg = cfg.timeslice_cycles(n); }
+        if let Ok(n) = v.parse::<u64>() {
+            cfg = cfg.timeslice_cycles(n);
+        }
     }
     cfg
 }
@@ -397,7 +422,10 @@ fn main() {
     println!("smarm smarm-favored benchmarks");
     println!("available parallelism: {n} threads");
     let sets = env_sets();
-    println!("ITERS={ITERS}×{sets} sets = {} samples (+1 warmup, discarded)", ITERS * sets);
+    println!(
+        "ITERS={ITERS}×{sets} sets = {} samples (+1 warmup, discarded)",
+        ITERS * sets
+    );
     println!(
         "RECURSE_DEPTH={RECURSE_DEPTH}, HOT_YIELDS={HOT_YIELDS}×2, \
          UNCONT_MSGS={UNCONT_MSGS}, PANIC_TASKS={PANIC_TASKS}"
@@ -406,22 +434,30 @@ fn main() {
     // ---- 9. deep_recursion ----
     print_header(&format!("deep_recursion: depth {RECURSE_DEPTH}"));
     run_n("smarm 1-thread", ITERS, || bench_recurse_smarm(1));
-    run_n(&format!("smarm {n}-thread"), ITERS, || bench_recurse_smarm(n));
+    run_n(&format!("smarm {n}-thread"), ITERS, || {
+        bench_recurse_smarm(n)
+    });
     run_n("tokio current_thread", ITERS, bench_recurse_tokio_current);
     run_n("tokio multi-thread", ITERS, bench_recurse_tokio_multi);
 
     // ---- 10. yield_in_hot_loop ----
-    print_header(&format!("yield_in_hot_loop: 2 actors × {HOT_YIELDS} yields (single thread)"));
+    print_header(&format!(
+        "yield_in_hot_loop: 2 actors × {HOT_YIELDS} yields (single thread)"
+    ));
     run_n("smarm 1-thread", ITERS, bench_hot_smarm);
     run_n("tokio current_thread", ITERS, bench_hot_tokio_current);
 
     // ---- 11. uncontended_channel ----
-    print_header(&format!("uncontended_channel: 1→1, {UNCONT_MSGS} msgs (single thread)"));
+    print_header(&format!(
+        "uncontended_channel: 1→1, {UNCONT_MSGS} msgs (single thread)"
+    ));
     run_n("smarm 1-thread", ITERS, bench_unc_smarm);
     run_n("tokio current_thread", ITERS, bench_unc_tokio_current);
 
     // ---- 12. catch_unwind_panics ----
-    print_header(&format!("catch_unwind_panics: {PANIC_TASKS} tasks, 50% panic"));
+    print_header(&format!(
+        "catch_unwind_panics: {PANIC_TASKS} tasks, 50% panic"
+    ));
     run_n("smarm 1-thread", ITERS, || bench_panic_smarm(1));
     run_n(&format!("smarm {n}-thread"), ITERS, || bench_panic_smarm(n));
     run_n("tokio current_thread", ITERS, bench_panic_tokio_current);

benches/switch_cost.rs

@@ -73,7 +73,10 @@ fn variant() -> &'static str {
 }
 
 fn env_usize(key: &str, default: usize) -> usize {
-    std::env::var(key).ok().and_then(|v| v.parse().ok()).unwrap_or(default)
+    std::env::var(key)
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(default)
 }
 
 // --------------------------------------------------------------------------
@@ -226,7 +229,11 @@ fn main() {
     let mean_cyc = pooled_cyc.iter().map(|&v| v as f64).sum::<f64>() / n.max(1) as f64;
     // Derived effective frequency: cycles per ns = GHz. Cross-checks the two
     // lenses against the box's known base clock.
-    let derived_ghz = if mean_ns > 0.0 { mean_cyc / mean_ns } else { 0.0 };
+    let derived_ghz = if mean_ns > 0.0 {
+        mean_cyc / mean_ns
+    } else {
+        0.0
+    };
 
     let p50 = pct(&pooled_ns, 50.0);
     let p90 = pct(&pooled_ns, 90.0);
@@ -241,8 +248,14 @@ fn main() {
         "  rounds={} warmup={} runs={}  (instrumentation floor: {} ns / {} cyc, subtracted)",
         rounds, warmup, runs, floor_ns, floor_cyc
     );
-    println!("  {:<10} {:<10} {:<10} {:<10} {:<10}", "p50 ns", "p90 ns", "p99 ns", "min ns", "max ns");
-    println!("  {:<10} {:<10} {:<10} {:<10} {:<10}", p50, p90, p99, lo, hi);
+    println!(
+        "  {:<10} {:<10} {:<10} {:<10} {:<10}",
+        "p50 ns", "p90 ns", "p99 ns", "min ns", "max ns"
+    );
+    println!(
+        "  {:<10} {:<10} {:<10} {:<10} {:<10}",
+        p50, p90, p99, lo, hi
+    );
     println!(
         "  mean {:.1} ns | mean {:.0} cyc | derived {:.3} GHz",
         mean_ns, mean_cyc, derived_ghz
@@ -251,6 +264,18 @@ fn main() {
     // Greppable line — same spirit as SPINCSV.
     println!(
         "SWITCHCSV,{},{},{},{},{},{},{},{},{},{},{:.1},{:.0},{:.3}",
-        variant(), mode, rounds, runs, n, p50, p90, p99, lo, hi, mean_ns, mean_cyc, derived_ghz
+        variant(),
+        mode,
+        rounds,
+        runs,
+        n,
+        p50,
+        p90,
+        p99,
+        lo,
+        hi,
+        mean_ns,
+        mean_cyc,
+        derived_ghz
     );
 }

benches/tokio_favored.rs

@@ -36,7 +36,9 @@ use std::time::{Duration, Instant};
 const ITERS: u32 = 15;
 
 fn available_threads() -> usize {
-    std::thread::available_parallelism().map(|n| n.get()).unwrap_or(1)
+    std::thread::available_parallelism()
+        .map(|n| n.get())
+        .unwrap_or(1)
 }
 
 fn env_sets() -> u32 {
@@ -114,11 +116,15 @@ fn bench_storm_smarm(threads: usize) -> (u64, u128) {
                 cc.fetch_add(1, Ordering::Relaxed);
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
 
         // Tear down background.
         s2.store(true, Ordering::Relaxed);
-        for h in bg_handles { h.join().unwrap(); }
+        for h in bg_handles {
+            h.join().unwrap();
+        }
     });
     (counter.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -129,7 +135,9 @@ fn bench_storm_tokio_current() -> (u64, u128) {
     let c2 = counter.clone();
     let s2 = stop.clone();
 
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -149,9 +157,13 @@ fn bench_storm_tokio_current() -> (u64, u128) {
                 cc.fetch_add(1, Ordering::Relaxed);
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
         s2.store(true, Ordering::Relaxed);
-        for h in bg_handles { let _ = h.await; }
+        for h in bg_handles {
+            let _ = h.await;
+        }
     });
     (counter.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -184,9 +196,13 @@ fn bench_storm_tokio_multi() -> (u64, u128) {
                 cc.fetch_add(1, Ordering::Relaxed);
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
         s2.store(true, Ordering::Relaxed);
-        for h in bg_handles { let _ = h.await; }
+        for h in bg_handles {
+            let _ = h.await;
+        }
     });
     (counter.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -219,14 +235,21 @@ fn bench_mpsc_smarm(threads: usize) -> (u64, u128) {
             }
             let _ = count; // discard; run() closure must return ()
         });
-        for h in prod_handles { h.join().unwrap(); }
+        for h in prod_handles {
+            h.join().unwrap();
+        }
         let _ = consumer.join().unwrap();
     });
-    (MPSC_PRODUCERS * MPSC_PER_PRODUCER, start.elapsed().as_micros())
+    (
+        MPSC_PRODUCERS * MPSC_PER_PRODUCER,
+        start.elapsed().as_micros(),
+    )
 }
 
 fn bench_mpsc_tokio_current() -> (u64, u128) {
-    let rt = tokio::runtime::Builder::new_current_thread().build().unwrap();
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .build()
+        .unwrap();
     let start = Instant::now();
     let local = tokio::task::LocalSet::new();
     local.block_on(&rt, async move {
@@ -248,10 +271,15 @@ fn bench_mpsc_tokio_current() -> (u64, u128) {
             }
             count
         });
-        for h in prod_handles { let _ = h.await; }
+        for h in prod_handles {
+            let _ = h.await;
+        }
         let _ = consumer.await;
     });
-    (MPSC_PRODUCERS * MPSC_PER_PRODUCER, start.elapsed().as_micros())
+    (
+        MPSC_PRODUCERS * MPSC_PER_PRODUCER,
+        start.elapsed().as_micros(),
+    )
 }
 
 fn bench_mpsc_tokio_multi() -> (u64, u128) {
@@ -279,10 +307,15 @@ fn bench_mpsc_tokio_multi() -> (u64, u128) {
             }
             count
         });
-        for h in prod_handles { let _ = h.await; }
+        for h in prod_handles {
+            let _ = h.await;
+        }
         let _ = consumer.await;
     });
-    (MPSC_PRODUCERS * MPSC_PER_PRODUCER, start.elapsed().as_micros())
+    (
+        MPSC_PRODUCERS * MPSC_PER_PRODUCER,
+        start.elapsed().as_micros(),
+    )
 }
 
 // ---------------------------------------------------------------------------
@@ -308,7 +341,9 @@ fn bench_timers_smarm(threads: usize) -> (u64, u128) {
                 smarm::sleep(Duration::from_millis(ms));
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
     });
     (TIMER_ACTORS, start.elapsed().as_micros())
 }
@@ -328,7 +363,9 @@ fn bench_timers_tokio_current() -> (u64, u128) {
                 tokio::time::sleep(Duration::from_millis(ms)).await;
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
     });
     (TIMER_ACTORS, start.elapsed().as_micros())
 }
@@ -348,7 +385,9 @@ fn bench_timers_tokio_multi() -> (u64, u128) {
                 tokio::time::sleep(Duration::from_millis(ms)).await;
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
     });
     (TIMER_ACTORS, start.elapsed().as_micros())
 }
@@ -361,11 +400,22 @@ const SCALING_N: u64 = 400_000;
 const SCALING_WORKERS: u64 = 64;
 
 fn is_prime(n: u64) -> bool {
-    if n < 2 { return false; }
-    if n < 4 { return true; }
-    if n % 2 == 0 { return false; }
+    if n < 2 {
+        return false;
+    }
+    if n < 4 {
+        return true;
+    }
+    if n % 2 == 0 {
+        return false;
+    }
     let mut i = 3u64;
-    while i * i <= n { if n % i == 0 { return false; } i += 2; }
+    while i * i <= n {
+        if n % i == 0 {
+            return false;
+        }
+        i += 2;
+    }
     true
 }
 
@@ -376,7 +426,11 @@ fn count_primes(lo: u64, hi: u64) -> u64 {
 fn scaling_slice(w: u64) -> (u64, u64) {
     let per = SCALING_N / SCALING_WORKERS;
     let lo = w * per;
-    let hi = if w + 1 == SCALING_WORKERS { SCALING_N } else { lo + per };
+    let hi = if w + 1 == SCALING_WORKERS {
+        SCALING_N
+    } else {
+        lo + per
+    };
     (lo, hi)
 }
 
@@ -393,7 +447,9 @@ fn bench_scaling_smarm(threads: usize) -> (u64, u128) {
                 tc.fetch_add(count_primes(lo, hi), Ordering::Relaxed);
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
     });
     (total.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -415,7 +471,9 @@ fn bench_scaling_tokio_multi(threads: usize) -> (u64, u128) {
                 tc.fetch_add(count_primes(lo, hi), Ordering::Relaxed);
             }));
         }
-        for h in handles { let _ = h.await; }
+        for h in handles {
+            let _ = h.await;
+        }
     });
     (total.load(Ordering::Relaxed), start.elapsed().as_micros())
 }
@@ -424,7 +482,6 @@ fn bench_scaling_tokio_multi(threads: usize) -> (u64, u128) {
 // main
 // ---------------------------------------------------------------------------
 
-
 // ---------------------------------------------------------------------------
 // Knob helper — reads SMARM_ALLOC_INTERVAL / SMARM_TIMESLICE_CYCLES env vars
 // so the sweep script can override the preemption knobs without recompiling.
@@ -433,10 +490,14 @@ fn bench_scaling_tokio_multi(threads: usize) -> (u64, u128) {
 fn bench_cfg(threads: usize) -> smarm::runtime::Config {
     let mut cfg = smarm::runtime::Config::exact(threads);
     if let Ok(v) = std::env::var("SMARM_ALLOC_INTERVAL") {
-        if let Ok(n) = v.parse::<u32>() { cfg = cfg.alloc_interval(n); }
+        if let Ok(n) = v.parse::<u32>() {
+            cfg = cfg.alloc_interval(n);
+        }
     }
     if let Ok(v) = std::env::var("SMARM_TIMESLICE_CYCLES") {
-        if let Ok(n) = v.parse::<u64>() { cfg = cfg.timeslice_cycles(n); }
+        if let Ok(n) = v.parse::<u64>() {
+            cfg = cfg.timeslice_cycles(n);
+        }
     }
     cfg
 }
@@ -446,7 +507,10 @@ fn main() {
     println!("smarm tokio-favored benchmarks");
     println!("available parallelism: {n} threads");
     let sets = env_sets();
-    println!("ITERS={ITERS}×{sets} sets = {} samples (+1 warmup, discarded)", ITERS * sets);
+    println!(
+        "ITERS={ITERS}×{sets} sets = {} samples (+1 warmup, discarded)",
+        ITERS * sets
+    );
     println!(
         "STORM_BACKGROUND={STORM_BACKGROUND}, STORM_SPAWN={STORM_SPAWN}, \
          MPSC={MPSC_PRODUCERS}×{MPSC_PER_PRODUCER}, \
@@ -477,7 +541,9 @@ fn main() {
         "many_timers: {TIMER_ACTORS} actors sleeping {TIMER_MIN_MS}–{TIMER_MAX_MS} ms"
     ));
     run_n("smarm 1-thread", ITERS, || bench_timers_smarm(1));
-    run_n(&format!("smarm {n}-thread"), ITERS, || bench_timers_smarm(n));
+    run_n(&format!("smarm {n}-thread"), ITERS, || {
+        bench_timers_smarm(n)
+    });
     run_n("tokio current_thread", ITERS, bench_timers_tokio_current);
     run_n("tokio multi-thread", ITERS, bench_timers_tokio_multi);
 
@@ -487,13 +553,19 @@ fn main() {
     ));
     let sweep: Vec<usize> = {
         let mut v = vec![1usize, 2, 4];
-        if n > 4 && !v.contains(&n) { v.push(n); }
+        if n > 4 && !v.contains(&n) {
+            v.push(n);
+        }
         v.into_iter().filter(|t| *t <= n).collect()
     };
     for t in &sweep {
-        run_n(&format!("smarm {t}-thread"), ITERS, || bench_scaling_smarm(*t));
+        run_n(&format!("smarm {t}-thread"), ITERS, || {
+            bench_scaling_smarm(*t)
+        });
     }
     for t in &sweep {
-        run_n(&format!("tokio multi {t}-thread"), ITERS, || bench_scaling_tokio_multi(*t));
+        run_n(&format!("tokio multi {t}-thread"), ITERS, || {
+            bench_scaling_tokio_multi(*t)
+        });
     }
 }

build.rs

@@ -0,0 +1,11 @@
+fn main() {
+    // RFC 019 §7 test canary (agreed Q3): compiled without stack-clash
+    // protection so its 96 KiB local is a genuine one-displacement guard
+    // jumper; distro-hardened compilers would otherwise probe it page-wise
+    // and defeat the test's purpose.
+    cc::Build::new()
+        .file("canary/canary.c")
+        .flag_if_supported("-fno-stack-clash-protection")
+        .compile("smarm_canary");
+    println!("cargo:rerun-if-changed=canary/canary.c");
+}

canary/canary.c

@@ -0,0 +1,14 @@
+/* RFC 019 §7 FFI canary: an honest unprobed C frame with a 96 KiB local,
+ * touched from its LOW end first — the exact "one sub rsp steps over a small
+ * guard" pattern the RFC's motivating incident hit (a cargo-vendored gz
+ * build; cc-invoked builds do not enable -fstack-clash-protection, and this
+ * file pins that off explicitly so the canary stays a canary even on
+ * hardened-default toolchains). */
+void smarm_canary_burn(void) {
+    volatile char buf[96 * 1024];
+    buf[0] = 1; /* deepest address first */
+    for (unsigned i = 0; i < sizeof buf; i += 4096) {
+        buf[i] = (char)i;
+    }
+    buf[sizeof buf - 1] = 1;
+}

docs/BENCHMARKS_AND_TUNING.md

@@ -75,7 +75,7 @@ genuine advantage over tokio's task abort model.
 
 ### Spawn-heavy workloads (19–70×)
 
-Every smarm actor `mmap`s a 64 KiB stack with a guard page. This is
+Every smarm actor `mmap`s a 64 KiB stack reserve with a 64 KiB PROT_NONE guard below (both per-actor configurable since RFC 019; the reserve is demand-paged). This is
 a syscall. Tokio tasks are heap-allocated state machines — no stack,
 no syscall, ~100 bytes each. For workloads that spawn thousands of
 short-lived actors per second, this is a structural disadvantage.

examples/causal_attrib_probe.rs

@@ -67,7 +67,9 @@ fn main() {
     println!("calibration: {per_us} work iters/µs");
     let work_us = move |us: u64| work_iters(us * per_us);
 
-    let cores = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(1);
+    let cores = std::thread::available_parallelism()
+        .map(|n| n.get())
+        .unwrap_or(1);
     println!("cores: {cores}");
     if cores < 4 {
         println!("probe: SKIPPED (needs the stages in parallel)");

examples/gen_statem_expanded.rs

@@ -35,8 +35,8 @@
 
 #![deny(dead_code, unreachable_patterns)]
 
+use smarm::gen_statem::{spawn, Cx, GenStatemRef, Machine, Reply, Resolution, Step};
 use smarm::run;
-use smarm::gen_statem::{spawn, Cx, Machine, Reply, Resolution, Step, GenStatemRef};
 
 // === user types ============================================================
 
@@ -123,7 +123,11 @@ impl DoorSm {
     fn start(init: Door) -> GenStatemRef<DoorSm> {
         spawn(DoorSm {
             state: init,
-            data: Data { enters: 0, pushes: 0, knocks: 0 },
+            data: Data {
+                enters: 0,
+                pushes: 0,
+                knocks: 0,
+            },
         })
     }
 
@@ -193,16 +197,12 @@ impl Machine for DoorSm {
             (Door::Closed, Ev::Cast(Cast::Push | Cast::Unlock(_))) => Resolution::Unhandled,
 
             // --- Locked (branching row: handler picks within UnlockOutcome) -
-            (Door::Locked, Ev::Cast(Cast::Unlock(key))) => {
-                Resolution::To(on_unlock(key).into())
-            }
+            (Door::Locked, Ev::Cast(Cast::Unlock(key))) => Resolution::To(on_unlock(key).into()),
             // Routed out in phase 1; listed only to keep this match total.
             (Door::Locked, Ev::Cast(Cast::Knock)) => {
                 unreachable!("postponed event is replayed, not dispatched here")
             }
-            (Door::Locked, Ev::Cast(Cast::Push | Cast::Pull | Cast::Lock)) => {
-                Resolution::Unhandled
-            }
+            (Door::Locked, Ev::Cast(Cast::Push | Cast::Pull | Cast::Lock)) => Resolution::Unhandled,
 
             // --- state-independent queries (reply, then stay) ---------------
             (_, Ev::Call(Call::GetState(r))) => {

examples/gen_statem_macro.rs

@@ -18,8 +18,8 @@
                     // dispatch's own unreachable_patterns internally.
 
 use smarm::gen_statem;
-use smarm::run;
 use smarm::gen_statem::Reply;
+use smarm::run;
 
 // === user types (identical to gen_statem_expanded.rs) =========================
 
@@ -135,7 +135,14 @@ gen_statem! {
 
 fn main() {
     run(|| {
-        let door = DoorSm::start(Door::Closed, Data { enters: 0, pushes: 0, knocks: 0 });
+        let door = DoorSm::start(
+            Door::Closed,
+            Data {
+                enters: 0,
+                pushes: 0,
+                knocks: 0,
+            },
+        );
 
         door.send(Ev::Cast(Cast::Lock)).unwrap(); // Closed -> Locked
         door.send(Ev::Cast(Cast::Knock)).unwrap(); // Locked: postponed (not yet counted)

examples/named_genserver.rs

@@ -6,7 +6,9 @@
 //! every use — so the address keeps working across a supervised restart, with
 //! no stale [`GenServerRef`] to refresh.
 
-use smarm::{call, cast, run, whereis_server, GenServer, GenServerBuilder, GenServerName, GenServerRef};
+use smarm::{
+    call, cast, run, whereis_server, GenServer, GenServerBuilder, GenServerName, GenServerRef,
+};
 
 /// A counter server: synchronous `Get`, asynchronous `Inc` / `Add`.
 struct Counter {

examples/observer.rs

@@ -15,7 +15,9 @@
 //! `call`, nothing more.
 
 use smarm::observer::{self, ObserverReply, ObserverRequest};
-use smarm::{channel, register, run, spawn, ActorState, Name, RuntimeSnapshot, RuntimeTree, TreeNode};
+use smarm::{
+    channel, register, run, spawn, ActorState, Name, RuntimeSnapshot, RuntimeTree, TreeNode,
+};
 
 const ECHO: Name<u64> = Name::new("echo");
 
@@ -31,7 +33,11 @@ fn state_glyph(s: ActorState) -> &'static str {
 
 /// A `ps`-style table over the flat snapshot.
 fn print_snapshot(snap: &RuntimeSnapshot) {
-    println!("snapshot (format v{}, {} actors)", snap.format_version, snap.actors.len());
+    println!(
+        "snapshot (format v{}, {} actors)",
+        snap.format_version,
+        snap.actors.len()
+    );
     println!(
         "  {:<10} {:<9} {:<10} {:>4} {:>4} {:>4} {:>4} {:>5}  {}",
         "pid", "state", "parent", "mon", "lnk", "joi", "mbox", "msgs", "names"
@@ -52,7 +58,11 @@ fn print_snapshot(snap: &RuntimeSnapshot) {
             a.joiners,
             a.mailbox_depth,
             a.messages_received,
-            if a.names.is_empty() { "-".to_string() } else { a.names.join(",") },
+            if a.names.is_empty() {
+                "-".to_string()
+            } else {
+                a.names.join(",")
+            },
         );
     }
 }

src/causal.rs

@@ -342,8 +342,7 @@ mod inner {
             // Count the loss in would-be delta terms so the audit's columns
             // compare directly against `injected_cycles`.
             DISCARD_OVERMAX_N.fetch_add(1, Ordering::Relaxed);
-            DISCARD_OVERMAX_CYCLES
-                .fetch_add(interval.saturating_mul(pct) / 100, Ordering::Relaxed);
+            DISCARD_OVERMAX_CYCLES.fetch_add(interval.saturating_mul(pct) / 100, Ordering::Relaxed);
             return;
         }
         let delta = interval.saturating_mul(pct) / 100;
@@ -419,8 +418,7 @@ mod inner {
                 let gap = preempt::rdtsc()
                     .saturating_sub(desched_tsc)
                     .min(MAX_SAMPLE_CYCLES);
-                OFFCPU_IN_SITE_CYCLES
-                    .fetch_add(gap.saturating_mul(pct) / 100, Ordering::Relaxed);
+                OFFCPU_IN_SITE_CYCLES.fetch_add(gap.saturating_mul(pct) / 100, Ordering::Relaxed);
                 OFFCPU_IN_SITE_N.fetch_add(1, Ordering::Relaxed);
             }
         }
@@ -533,7 +531,9 @@ mod inner {
                 park_forgiven_cycles: self
                     .park_forgiven_cycles
                     .saturating_sub(before.park_forgiven_cycles),
-                drop_park_cycles: self.drop_park_cycles.saturating_sub(before.drop_park_cycles),
+                drop_park_cycles: self
+                    .drop_park_cycles
+                    .saturating_sub(before.drop_park_cycles),
                 drop_park_n: self.drop_park_n.saturating_sub(before.drop_park_n),
                 drop_yield_cycles: self
                     .drop_yield_cycles
@@ -542,12 +542,18 @@ mod inner {
                 discard_overmax_cycles: self
                     .discard_overmax_cycles
                     .saturating_sub(before.discard_overmax_cycles),
-                discard_overmax_n: self.discard_overmax_n.saturating_sub(before.discard_overmax_n),
-                discard_unarmed_n: self.discard_unarmed_n.saturating_sub(before.discard_unarmed_n),
+                discard_overmax_n: self
+                    .discard_overmax_n
+                    .saturating_sub(before.discard_overmax_n),
+                discard_unarmed_n: self
+                    .discard_unarmed_n
+                    .saturating_sub(before.discard_unarmed_n),
                 offcpu_in_site_cycles: self
                     .offcpu_in_site_cycles
                     .saturating_sub(before.offcpu_in_site_cycles),
-                offcpu_in_site_n: self.offcpu_in_site_n.saturating_sub(before.offcpu_in_site_n),
+                offcpu_in_site_n: self
+                    .offcpu_in_site_n
+                    .saturating_sub(before.offcpu_in_site_n),
             }
         }
     }
@@ -795,7 +801,9 @@ mod inner {
         let cell = results
             .iter()
             .find(|r| r.site == site && r.speedup_pct == speedup_pct)?;
-        let base = results.iter().find(|r| r.site == site && r.speedup_pct == 0)?;
+        let base = results
+            .iter()
+            .find(|r| r.site == site && r.speedup_pct == 0)?;
         let rate = normalized_rate(cell, point)?;
         let b = normalized_rate(base, point)?;
         if b <= 0.0 {
@@ -946,7 +954,9 @@ macro_rules! progress {
 macro_rules! causal_site {
     ($name:literal) => {{
         static __SMARM_SITE: ::std::sync::OnceLock<u32> = ::std::sync::OnceLock::new();
-        $crate::causal::SiteGuard::enter(*__SMARM_SITE.get_or_init(|| $crate::causal::site_id($name)))
+        $crate::causal::SiteGuard::enter(
+            *__SMARM_SITE.get_or_init(|| $crate::causal::site_id($name)),
+        )
     }};
 }
 

src/channel.rs

@@ -1,38 +1,102 @@
-//! Unbounded MPSC channels.
+//! Unbounded multi-producer, single-consumer channels: how actors talk to
+//! each other.
 //!
-//! Inner state is `Arc<RawMutex<Inner<T>>>` so channels can be sent across OS
-//! threads (required for the multi-scheduler runtime where a sender and
-//! receiver may run on different scheduler threads simultaneously).
+//! A channel is a queue with a typed [`Sender`] on one end and a typed
+//! [`Receiver`] on the other. Any number of actors can hold a clone of the
+//! `Sender` and push messages onto the same queue; exactly one [`Receiver`]
+//! reads them back out, in the order they arrived. This is the basic wiring
+//! smarm's other actor primitives (`gen_server`, `pg`, the registry) are all
+//! built out of, and it is directly usable on its own for a worker that just
+//! needs an inbox.
 //!
-//! ## Why `RawMutex` (Channel class), not `std::sync::Mutex`
+//! ## A first channel
 //!
-//! An actor holding a guard with preemption *enabled* can be timesliced
-//! inside the critical section and resume on a different OS thread — the
-//! pthread mutex would then be released from a thread that didn't lock it,
-//! which is UB (Linux futexes happen to tolerate it, but it's not
-//! guaranteed). `RawMutex` disables preemption for the guard's span and is
-//! cross-thread-release sound by construction, closing the hole. It also
-//! cannot poison. Channel locks form their own [`LockClass::Channel`]
-//! (raw_mutex.rs): they may be taken under a cold (Leaf) lock — finalize and
-//! `monitor()` clone senders that live in slots — but nothing may be locked
-//! under them, which the debug build enforces. `recv_match` runs its user
-//! predicate under this lock: keep it cheap, pure, and channel-free.
+//! ```
+//! use smarm::{channel, run, spawn};
 //!
-//! Semantics:
-//!   - Senders are clonable; the last sender drop closes the channel.
-//!   - `Receiver::recv` on an empty open channel parks the receiver.
-//!   - `Receiver::recv` on an empty closed channel returns `Err(RecvError)`.
-//!   - `Sender::send` on an open channel always succeeds.
-//!   - `Sender::send` on a closed channel (receiver dropped) returns
-//!     `Err(SendError(value))`.
-//!   - When a send pushes to a previously empty queue and a receiver is
-//!     parked, the receiver is unparked.
+//! run(|| {
+//!     let (tx, rx) = channel::<u64>();
+//!
+//!     let worker = spawn(move || {
+//!         // Blocks until a message arrives.
+//!         let n = rx.recv().unwrap();
+//!         assert_eq!(n, 42);
+//!
+//!         // Once every Sender is dropped, recv() reports the channel closed
+//!         // instead of blocking forever.
+//!         assert!(rx.recv().is_err());
+//!     });
+//!
+//!     tx.send(42).unwrap();
+//!     drop(tx); // last sender gone: the channel is now closed
+//!     worker.join().unwrap();
+//! });
+//! ```
+//!
+//! ## Sending
+//!
+//! [`Sender`] is cheaply clonable: hand a clone to every actor that needs to
+//! push messages into this queue. The channel stays open as long as at least
+//! one clone exists; [`Sender::send`] never blocks and always succeeds while
+//! the channel is open, since the queue is unbounded. Once the [`Receiver`]
+//! has been dropped, `send` returns the message back to you in
+//! [`SendError`] instead of delivering it.
+//!
+//! ## Receiving
+//!
+//! There is exactly one [`Receiver`] per channel (it is not clonable).
+//! [`Receiver::recv`] returns the next message in arrival order, parking the
+//! calling actor if the queue is currently empty. Once every `Sender` has
+//! been dropped and the queue has been drained, `recv` stops parking and
+//! returns [`RecvError`] instead, so a receiver never blocks forever waiting
+//! on senders that are never coming back.
+//!
+//! Beyond plain `recv`, three variants cover the common needs:
+//!
+//! - [`Receiver::try_recv`]: never parks: reports an empty-but-open channel
+//!   as `Ok(None)` instead of waiting.
+//! - [`Receiver::recv_timeout`]: parks, but gives up and returns
+//!   [`RecvTimeoutError::Timeout`] if no message arrives before a deadline.
+//! - [`Receiver::recv_match`] / [`Receiver::try_recv_match`]: selective
+//!   receive. Instead of taking whatever is at the front of the queue, pick
+//!   out the first message matching a predicate, leaving the rest queued in
+//!   order. Handy for an actor that wants to prioritise one kind of message
+//!   over others already waiting.
+//!
+//! ## Waiting on several channels: `select`
+//!
+//! [`select`] parks an actor across several receivers at once and reports
+//! the index of the first one that is ready (has a message queued, or has
+//! been closed). [`select_timeout`] adds a deadline, the way `recv_timeout`
+//! does for a single channel. See their docs for the full contract,
+//! including the priority-order and no-fairness guarantee.
+//!
+//! ## Implementation notes
+//!
+//! The queue and its bookkeeping live behind `Arc<RawMutex<Inner<T>>>`
+//! rather than a `std::sync::Mutex`, so that a channel can be freely shared
+//! and sent across the OS threads backing the multi-scheduler runtime.
+//! `RawMutex` matters here for a subtler reason too: an ordinary pthread
+//! mutex can be released from a different OS thread than the one that took
+//! it (smarm's preemption can migrate a timesliced actor between scheduler
+//! threads mid-critical-section), and doing that to a `std::sync::Mutex` is
+//! undefined behavior. `RawMutex` disables preemption for the guard's short
+//! lifetime instead, so the release always happens on the thread that
+//! acquired it, and it has no poisoning to worry about besides. Channel
+//! locks are cheap and are never held across another lock acquisition or a
+//! blocking call; the predicate passed to `recv_match` runs under this lock,
+//! which is why it needs to stay cheap, pure, and must not call back into
+//! the same channel.
 
 use crate::pid::Pid;
 use crate::raw_mutex::RawMutex;
 use std::collections::VecDeque;
 use std::sync::Arc;
 
+/// Create a new channel and return its `(Sender, Receiver)` halves.
+///
+/// The channel is unbounded (no capacity limit) and open until every
+/// `Sender` has been dropped.
 pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
     let inner = Arc::new(RawMutex::new_channel(Inner {
         queue: VecDeque::new(),
@@ -40,34 +104,50 @@ pub fn channel<T>() -> (Sender<T>, Receiver<T>) {
         senders: 1,
         receiver_alive: true,
     }));
-    (Sender { inner: inner.clone() }, Receiver { inner })
+    (
+        Sender {
+            inner: inner.clone(),
+        },
+        Receiver { inner },
+    )
 }
 
 struct Inner<T> {
     queue: VecDeque<T>,
-    /// The parked receiver's `(pid, park-epoch)`. The epoch is the slot
-    /// word's runtime-wide wait identity (see slot_state.rs): wakers call
-    /// `unpark_at(pid, epoch)`, so an entry left over from an already-woken
-    /// wait — a `select` loser arm, a satisfied `recv_timeout`'s timer — is
-    /// inert: the wake fails the word's epoch CAS and no-ops. This replaces
-    /// the old per-channel `cur_wait`/`next_wait_seq`/`timed_out` trio: wait
-    /// identity now exists exactly once, in the slot word.
+    /// The parked receiver's `(pid, park-epoch)`, if one is currently
+    /// waiting. The epoch identifies exactly which wait this is, so a waker
+    /// left over from a wait that already ended (a losing `select` arm, a
+    /// `recv_timeout` whose timer fired after it was already satisfied) is
+    /// inert and does nothing when it fires.
     parked_receiver: Option<(Pid, u32)>,
     senders: usize,
     receiver_alive: bool,
 }
 
+/// The sending half of a channel, created by [`channel`]. Clonable: every
+/// clone pushes onto the same queue, and the channel stays open as long as
+/// any clone is alive. Dropping the last `Sender` closes the channel, which
+/// wakes a parked [`Receiver`] so it can observe the closure.
 pub struct Sender<T> {
     inner: Arc<RawMutex<Inner<T>>>,
 }
 
+/// The receiving half of a channel, created by [`channel`]. Not clonable:
+/// a channel has exactly one receiver. Reads messages in the order they
+/// were sent, via [`recv`](Receiver::recv) and its variants.
 pub struct Receiver<T> {
     inner: Arc<RawMutex<Inner<T>>>,
 }
 
+/// Returned by [`Sender::send`] when the channel's [`Receiver`] has already
+/// been dropped. Carries the message back so it is never silently lost;
+/// recover it with `.0` or by matching.
 #[derive(Debug, PartialEq, Eq)]
 pub struct SendError<T>(pub T);
 
+/// Returned by [`Receiver::recv`] (and the other receive methods, in their
+/// own error types) when the channel is closed: every `Sender` has been
+/// dropped and no message is left queued.
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub struct RecvError;
 
@@ -84,8 +164,8 @@ impl std::error::Error for RecvError {}
 pub enum RecvTimeoutError {
     /// The deadline passed with no message available.
     Timeout,
-    /// All senders dropped with no message available — the bounded analogue
-    /// of [`RecvError`].
+    /// Every sender was dropped with no message available. The
+    /// timeout-aware counterpart of plain [`RecvError`].
     Disconnected,
 }
 
@@ -103,7 +183,9 @@ impl std::error::Error for RecvTimeoutError {}
 impl<T> Clone for Sender<T> {
     fn clone(&self) -> Self {
         self.inner.lock().senders += 1;
-        Sender { inner: self.inner.clone() }
+        Sender {
+            inner: self.inner.clone(),
+        }
     }
 }
 
@@ -115,8 +197,8 @@ impl<T> Drop for Sender<T> {
             // Wake the parked receiver on the last sender drop regardless of
             // whether the queue is empty. A plain `recv` only ever parks on an
             // empty queue (so this is unchanged for it), but a selective
-            // `recv_match` may be parked on a *non-empty* queue holding only
-            // non-matching messages — it must wake to observe closure and
+            // `recv_match` may be parked on a non-empty queue holding only
+            // non-matching messages. It must wake to observe closure and
             // return Err rather than sleep forever.
             if g.senders == 0 {
                 g.parked_receiver.take()
@@ -133,14 +215,15 @@ impl<T> Drop for Sender<T> {
 impl<T> Drop for Receiver<T> {
     fn drop(&mut self) {
         // The only consumer is gone: queued messages can never be delivered.
-        // Drop them now instead of stranding them until the last Sender goes
-        // away (a registry entry under lazy prune can keep a Sender — and thus
-        // the Arc<Inner> — alive long after the server exits). Dropping a queued
-        // Envelope::Call drops its reply_tx, waking any caller parked in `call`
-        // with ServerDown, so the documented guarantee holds on *every* teardown
-        // path, not only the all-senders-drop one. Drain under the lock, then
-        // run item destructors after releasing it (a reply_tx drop reaches into
-        // a *different* channel's lock + the scheduler, so it must not nest).
+        // Drop them now instead of leaving them queued until the last Sender
+        // happens to go away, which can be long after this receiver's owner
+        // has exited if some other part of the runtime is still holding a
+        // clone of the Sender. Draining runs each queued message's own drop
+        // glue, which matters for a gen_server call: dropping a queued call
+        // envelope drops its reply channel too, which wakes the caller with
+        // an error instead of leaving it parked forever. Drain under the
+        // lock, then run the drops after releasing it, since a message's
+        // drop glue may itself touch a different channel or the scheduler.
         let drained = {
             let mut g = self.inner.lock();
             g.receiver_alive = false;
@@ -151,14 +234,17 @@ impl<T> Drop for Receiver<T> {
 }
 
 impl<T> Sender<T> {
-    /// Number of messages currently queued behind this channel. Introspection
-    /// only (RFC 016 mailbox depth); takes the channel lock, so callers reach
-    /// it under the registry Leaf (Leaf → Channel) via the erased probe in
-    /// `registry.rs`, never on a hot path.
+    /// Number of messages currently queued and not yet received. For
+    /// introspection and monitoring; takes the channel's internal lock, so
+    /// avoid calling it from a hot path.
     pub(crate) fn queued_len(&self) -> usize {
         self.inner.lock().queue.len()
     }
 
+    /// Push `value` onto the channel. Succeeds unconditionally as long as
+    /// the [`Receiver`] is still alive: the queue has no capacity limit, so
+    /// this never blocks and never fails except when the channel is closed,
+    /// in which case `value` comes back in [`SendError`].
     pub fn send(&self, value: T) -> Result<(), SendError<T>> {
         let unpark = {
             let mut g = self.inner.lock();
@@ -169,16 +255,28 @@ impl<T> Sender<T> {
             g.parked_receiver.take()
         };
         if let Some((pid, epoch)) = unpark {
-            crate::te!(crate::trace::Event::Send { sender: crate::actor::current_pid().unwrap_or(crate::pid::Pid::new(u32::MAX, u32::MAX)), receiver: Some(pid) });
+            crate::te!(crate::trace::Event::Send {
+                sender: crate::actor::current_pid()
+                    .unwrap_or(crate::pid::Pid::new(u32::MAX, u32::MAX)),
+                receiver: Some(pid)
+            });
             crate::scheduler::unpark_at(pid, epoch);
         } else {
-            crate::te!(crate::trace::Event::Send { sender: crate::actor::current_pid().unwrap_or(crate::pid::Pid::new(u32::MAX, u32::MAX)), receiver: None });
+            crate::te!(crate::trace::Event::Send {
+                sender: crate::actor::current_pid()
+                    .unwrap_or(crate::pid::Pid::new(u32::MAX, u32::MAX)),
+                receiver: None
+            });
         }
         Ok(())
     }
 }
 
 impl<T> Receiver<T> {
+    /// Block until a message is available and return it. Messages come back
+    /// in the order they were sent. If the queue is empty and every
+    /// [`Sender`] has already been dropped, returns [`RecvError`] instead of
+    /// blocking forever.
     pub fn recv(&self) -> Result<T, RecvError> {
         loop {
             {
@@ -198,45 +296,36 @@ impl<T> Receiver<T> {
                     g.parked_receiver.is_none_or(|(p, _)| p == me),
                     "channel has more than one receiver"
                 );
-                // begin_wait is lock-free — legal under the Channel lock;
+                // begin_wait is lock-free, so it's legal under the Channel lock;
                 // registering in the same critical section makes the epoch
                 // atomic with the senders' view of the registration.
                 g.parked_receiver = Some((me, crate::scheduler::begin_wait()));
                 crate::te!(crate::trace::Event::RecvPark(me));
             }
-            // Release the lock before parking — the unparker will need it.
+            // Release the lock before parking: the unparker will need it.
             crate::scheduler::park_current();
-            // Woken up — record it before looping to check the queue.
-            crate::te!(crate::trace::Event::RecvWake(match crate::actor::current_pid() {
+            // Woken up. Record it before looping to check the queue.
+            crate::te!(crate::trace::Event::RecvWake(
+                match crate::actor::current_pid() {
                     Some(p) => p,
                     None => panic!("smarm: RecvWake outside an actor (core corrupt)"),
-            }));
+                }
+            ));
         }
     }
 
-    /// Bounded receive: like [`recv`](Self::recv), but gives up once
-    /// `timeout` has elapsed, returning [`RecvTimeoutError::Timeout`].
+    /// Like [`recv`](Self::recv), but gives up and returns
+    /// [`RecvTimeoutError::Timeout`] if no message has arrived by the time
+    /// `timeout` elapses.
     ///
-    /// Built on the same timer machinery as `Mutex::lock_timeout`: the wait
-    /// registers a `WaitTimeout` entry stamped with the wait's park-epoch;
-    /// on expiry the channel (as the
-    /// [`TimerTarget`](crate::timer::TimerTarget)) checks whether *this*
-    /// wait is still parked and, only then, cancels it. A wake that races
-    /// the deadline resolves message-first: if a message is available when
-    /// the receiver runs, it is delivered even if the timer had already
-    /// fired. A satisfied or abandoned wait leaves its timer entry to expire
-    /// as a no-op (registration gone; epoch consumed), per the
-    /// no-cancellation convention in `timer.rs`.
+    /// If a message arrives at essentially the same moment the deadline
+    /// passes, the message wins: you get `Ok` rather than `Timeout`. If
+    /// every sender is dropped before a message arrives or the deadline
+    /// passes, you get [`RecvTimeoutError::Disconnected`].
     ///
-    /// The wake is classified from state alone — wakes are precise (the only
-    /// stamped wakers of this wait are a send, the last-sender drop, and the
-    /// timer; a stop wake unwinds out of `park_current` and never reaches
-    /// the classification), so: message queued → `Ok`; `senders == 0` →
-    /// `Disconnected`; neither → it was the timer → `Timeout`.
-    ///
-    /// `Duration::ZERO` is a valid timeout: it parks until the immediately-
-    /// due timer is drained, then reports `Timeout` unless a message was
-    /// already queued.
+    /// `Duration::ZERO` is a valid timeout: it still gives any
+    /// already-queued message a chance to be returned, and only then
+    /// reports `Timeout`.
     pub fn recv_timeout(&self, timeout: std::time::Duration) -> Result<T, RecvTimeoutError>
     where
         T: Send + 'static,
@@ -268,17 +357,19 @@ impl<T> Receiver<T> {
 
         // Arm the timer after releasing the channel lock (insert takes the
         // timers lock; never nest under a Channel lock). A send or even the
-        // timer itself may unpark us before we park — the RunningNotified
+        // timer itself may unpark us before we park; the runtime's wake
         // protocol makes the park below return immediately in that case.
         let deadline = crate::timer::deadline_from_now(timeout);
         let target: std::sync::Arc<dyn crate::timer::TimerTarget> = self.inner.clone();
         crate::scheduler::insert_wait_timer(deadline, me, target, epoch);
 
         crate::scheduler::park_current();
-        crate::te!(crate::trace::Event::RecvWake(match crate::actor::current_pid() {
+        crate::te!(crate::trace::Event::RecvWake(
+            match crate::actor::current_pid() {
                 Some(p) => p,
                 None => panic!("smarm: RecvWake outside an actor (core corrupt)"),
-        }));
+            }
+        ));
         let mut g = self.inner.lock();
         if let Some(v) = g.queue.pop_front() {
             crate::preempt::note_message_received();
@@ -290,16 +381,23 @@ impl<T> Receiver<T> {
         Err(RecvTimeoutError::Timeout)
     }
 
-    /// Selective receive: remove and return the first queued message for which
-    /// `pred` holds, leaving the rest in arrival order. If no queued message
-    /// matches, parks and re-scans on every send (a selective receiver may park
-    /// on a *non-empty* queue). Returns `Err(RecvError)` only once the channel
-    /// is closed and no queued message matches.
+    /// Selective receive: find and return the first queued message for
+    /// which `pred` returns `true`, leaving every other message in the
+    /// queue untouched and in order. Useful when an actor's inbox mixes
+    /// message kinds and it wants to handle one kind out of turn, without
+    /// discarding the rest.
     ///
-    /// `pred` is run while the channel lock is held: keep it cheap and pure,
-    /// and do not call back into this channel from inside it. It is modelled as
-    /// `Fn` (not `FnMut`) deliberately — it is re-run from scratch on every
-    /// scan, so a stateful predicate would observe surprising re-counting.
+    /// If nothing queued matches, this blocks and re-checks every time a new
+    /// message arrives, the same way [`recv`](Self::recv) blocks on an empty
+    /// queue: a selective receiver can be waiting even while the queue holds
+    /// messages, just none that match yet. Returns [`RecvError`] only once
+    /// the channel is closed and still nothing matches.
+    ///
+    /// `pred` runs while the channel is locked, so keep it cheap, side
+    /// effect free, and make sure it never calls back into this same
+    /// channel. It takes `&T` and is called fresh on every scan (not `FnMut`
+    /// with running state), so it should judge each message purely on its
+    /// own content.
     pub fn recv_match<F>(&self, pred: F) -> Result<T, RecvError>
     where
         F: Fn(&T) -> bool,
@@ -331,19 +429,23 @@ impl<T> Receiver<T> {
                 g.parked_receiver = Some((me, crate::scheduler::begin_wait()));
                 crate::te!(crate::trace::Event::RecvPark(me));
             }
-            // Release the lock before parking — the unparker will need it.
+            // Release the lock before parking: the unparker will need it.
             crate::scheduler::park_current();
-            crate::te!(crate::trace::Event::RecvWake(match crate::actor::current_pid() {
+            crate::te!(crate::trace::Event::RecvWake(
+                match crate::actor::current_pid() {
                     Some(p) => p,
                     None => panic!("smarm: RecvWake outside an actor (core corrupt)"),
-            }));
+                }
+            ));
         }
     }
 
-    /// Non-blocking selective receive. `Ok(Some(v))` if a queued message
-    /// matched `pred` (removed, rest left in order), `Ok(None)` if the channel
-    /// is open but nothing matched, `Err(RecvError)` if closed and nothing
-    /// matched. Same predicate contract as [`recv_match`](Self::recv_match).
+    /// The non-blocking counterpart of [`recv_match`](Self::recv_match):
+    /// returns immediately either way. `Ok(Some(v))` if a queued message
+    /// matched `pred` (removed; the rest stay queued in order), `Ok(None)`
+    /// if the channel is open but nothing currently matches, `Err(RecvError)`
+    /// if the channel is closed and nothing matches. Same predicate contract
+    /// as `recv_match`.
     pub fn try_recv_match<F>(&self, pred: F) -> Result<Option<T>, RecvError>
     where
         F: Fn(&T) -> bool,
@@ -363,8 +465,10 @@ impl<T> Receiver<T> {
         Ok(None)
     }
 
-    /// Non-blocking. `Ok(Some(v))` if a message was available, `Ok(None)` if
-    /// the channel is empty but open, `Err(RecvError)` if closed and drained.
+    /// The non-blocking counterpart of [`recv`](Self::recv): returns
+    /// immediately either way. `Ok(Some(v))` if a message was queued,
+    /// `Ok(None)` if the channel is open but currently empty, `Err(RecvError)`
+    /// if the channel is closed and the queue is drained.
     pub fn try_recv(&self) -> Result<Option<T>, RecvError> {
         let mut g = self.inner.lock();
         if let Some(v) = g.queue.pop_front() {
@@ -379,18 +483,18 @@ impl<T> Receiver<T> {
 }
 
 // ---------------------------------------------------------------------------
-// TimerTarget — the expiry half of recv_timeout
+// TimerTarget: the expiry half of recv_timeout
 // ---------------------------------------------------------------------------
 
 impl<T: Send + 'static> crate::timer::TimerTarget for RawMutex<Inner<T>> {
     fn on_timeout(&self, pid: Pid, epoch: u32) {
         // Cancel the wait only if THIS wait (epoch match) is still
         // registered. If a sender already took `parked_receiver`, the
-        // receiver is waking with a message — message wins, the timer
+        // receiver is waking with a message: message wins, the timer
         // no-ops. If a later wait by the same receiver is registered, the
-        // epoch mismatches — stale entry, no-op. (The unpark_at would fail
-        // its word CAS in either case anyway; checking under the lock keeps
-        // the registration bookkeeping exact.)
+        // epoch mismatches: stale entry, no-op. (unpark_at would fail its
+        // internal check in either case anyway; checking under the lock
+        // keeps the registration bookkeeping exact.)
         let unpark = {
             let mut g = self.lock();
             if g.parked_receiver == Some((pid, epoch)) {
@@ -400,7 +504,7 @@ impl<T: Send + 'static> crate::timer::TimerTarget for RawMutex<Inner<T>> {
                 false
             }
         };
-        // Unpark outside the channel lock — it may take the run-queue lock;
+        // Unpark outside the channel lock: it may take the run-queue lock;
         // legal under a Channel lock, but pointless to nest.
         if unpark {
             crate::scheduler::unpark_at(pid, epoch);
@@ -409,7 +513,7 @@ impl<T: Send + 'static> crate::timer::TimerTarget for RawMutex<Inner<T>> {
 }
 
 // ---------------------------------------------------------------------------
-// select — ready-index wait over multiple receivers
+// select: ready-index wait over multiple receivers
 // ---------------------------------------------------------------------------
 
 pub(crate) mod sealed {
@@ -417,33 +521,34 @@ pub(crate) mod sealed {
 }
 impl<T> sealed::Sealed for Receiver<T> {}
 
-/// An arm of a [`select`]. Implemented by [`Receiver`]; sealed, because the
-/// registration contract below is part of the runtime's wake protocol.
+/// An arm of a [`select`]: something you can wait on alongside other arms
+/// and be told when it becomes ready. Implemented by [`Receiver`]; sealed
+/// (cannot be implemented outside this crate), since the registration
+/// contract below is part of the runtime's internal wake protocol.
 ///
 /// Contract (all under the arm's own lock): `sel_register` checks-or-
-/// registers atomically — if the arm is ready it does NOT register and
+/// registers atomically. If the arm is ready it does not register and
 /// returns `Ok(false)`; otherwise it publishes `(pid, epoch)` where its
 /// wakers will find it and returns `Ok(true)`. "Ready" means a receive
-/// would not park: a message is queued, or the arm is closed. `Err` means
+/// would not block: a message is queued, or the arm is closed. `Err` means
 /// the arm could not register at all (only fd arms can fail; channel
-/// registration is infallible) — the wait must be retired and earlier
+/// registration always succeeds), and the wait must be retired and earlier
 /// eager-cleanup arms unregistered.
 pub trait Selectable: sealed::Sealed {
     #[doc(hidden)]
     fn sel_register(&self, pid: Pid, epoch: u32) -> std::io::Result<bool>;
     #[doc(hidden)]
     fn sel_ready(&self) -> bool;
-    /// Remove this arm's `(pid, epoch)` registration if — and only if — it
-    /// is still in place. Default no-op: a losing channel arm's stale
-    /// registration is inert (its wakers die at the epoch CAS; the next
-    /// wait overwrites the slot). Fd arms override this: their staleness
-    /// poisons the fd (waiters entry + kernel-side ONESHOT registration)
-    /// and needs an eager cleanup pass.
+    /// Remove this arm's `(pid, epoch)` registration if, and only if, it is
+    /// still in place. Default no-op: a losing channel arm's stale
+    /// registration is harmless and self-cleans. Fd arms override this:
+    /// their staleness would otherwise leave the fd unusable for future
+    /// selects, so they need an eager cleanup pass.
     #[doc(hidden)]
     fn sel_unregister(&self, _pid: Pid, _epoch: u32) {}
     /// Whether this arm requires the eager cleanup pass at all. Gates the
-    /// post-wake `sel_unregister` sweep so channel-only selects keep
-    /// today's zero-cancellation hot path.
+    /// post-wake `sel_unregister` sweep so channel-only selects keep their
+    /// cheap, cleanup-free path.
     #[doc(hidden)]
     fn sel_eager_cleanup(&self) -> bool {
         false
@@ -470,38 +575,35 @@ impl<T> Selectable for Receiver<T> {
     }
 }
 
-/// Park on every arm at once; return the index of the first ready one.
+/// Wait on several channels at once and return the index of the first one
+/// that is ready, instead of blocking on just one with [`Receiver::recv`].
 ///
-/// "Ready" means a receive on that arm would not park: a message is queued,
-/// or the arm is **closed** (so the caller's `try_recv` observes the
-/// disconnect — a dead arm is an event, not a hang). The caller consumes the
-/// arm itself, typically via [`Receiver::try_recv`]; single-receiver
-/// channels guarantee nothing can steal the message in between.
+/// "Ready" means a receive on that arm would not block: a message is
+/// queued, or the arm is closed (so the caller's own `try_recv` observes
+/// the disconnect: a dead arm is something to react to, not something to
+/// hang on). `select` only tells you which arm is ready; read the actual
+/// message yourself, typically with [`Receiver::try_recv`] on that arm.
 ///
-/// A closed arm stays ready *forever*: once its disconnect has been
-/// observed, drop it from the arm set — under priority order it would
-/// otherwise win every subsequent call and starve every higher-indexed arm.
+/// A closed arm stays ready forever. Once you have observed its disconnect,
+/// drop it from the arm set you pass in next time: otherwise, under the
+/// priority order below, it would win every subsequent call and starve
+/// every arm listed after it.
 ///
-/// Arms are scanned **in order**: index 0 is the highest priority, both on
-/// the immediate-ready path and after a wake. This is a documented
-/// guarantee (compose like BEAM receive clauses: put control channels
-/// first), not an accident — and therefore there is NO fairness promise; a
-/// saturated arm 0 starves arm 1 by design.
+/// Arms are checked **in order**: index 0 is the highest priority, both
+/// when checking immediately and after being woken. This is a deliberate,
+/// documented guarantee, not an accident of implementation: put a control
+/// or shutdown channel first so it is always noticed promptly. The
+/// flip side is that there is **no fairness guarantee**: a busy arm 0 can
+/// starve arm 1 indefinitely by design.
 ///
-/// One actor may select on a channel and later `recv` on it (or select on
-/// overlapping sets) freely. What stays illegal is what was always illegal:
-/// two *different* actors receiving on one channel.
+/// One actor can `select` on a channel and later plain `recv` on it (or
+/// `select` again on an overlapping set of arms) with no restriction. What
+/// stays illegal is what was always illegal for a channel: two *different*
+/// actors receiving on the same one.
 ///
-/// Built on the consuming-wake protocol (see slot_state.rs): all arms are
-/// registered under one wait epoch; the winning wake consumes it, so losing
-/// arms' registrations are inert and need no cancellation pass — they
-/// self-clean at their wakers' failed CAS, or get overwritten by this
-/// receiver's next wait on that channel.
-///
-/// Panics if `arms` is empty, when called outside an actor, or if an fd
-/// arm fails to register (EBADF, EMFILE, a second waiter on one fd —
-/// see [`try_select`] for the fallible form; channel-only selects cannot
-/// fail).
+/// Panics if `arms` is empty, if called outside an actor, or if an fd arm
+/// fails to register (see [`try_select`] for the fallible form; a
+/// channel-only `select` can never fail).
 pub fn select(arms: &[&dyn Selectable]) -> usize {
     match try_select(arms) {
         Ok(i) => i,
@@ -509,9 +611,10 @@ pub fn select(arms: &[&dyn Selectable]) -> usize {
     }
 }
 
-/// [`select`], fallible: `Err` when an arm fails to register (only fd
-/// arms can — EBADF, EMFILE on the epoll set, or a second waiter on an
-/// fd that already has one). On `Err` the wait is fully retired and no
+/// The fallible form of [`select`]: `Err` when an arm fails to register.
+/// Only fd arms can fail this way (for example, the file descriptor is
+/// invalid, or something else is already waiting on it); a channel-only
+/// select can never fail. On `Err` the wait is fully retired and no
 /// registration is left behind: every arm registered before the failing
 /// one has been unregistered.
 pub fn try_select(arms: &[&dyn Selectable]) -> std::io::Result<usize> {
@@ -527,15 +630,19 @@ pub fn try_select(arms: &[&dyn Selectable]) -> std::io::Result<usize> {
         }
 
         // Stale fd registrations are not harmless (a losing fd arm's
-        // waiters entry poisons the fd with AlreadyExists and its
-        // kernel-side ONESHOT registration can fire arbitrarily late), so
-        // selects containing fd arms run an eager cleanup pass after the
-        // park — including when a terminal stop unwinds out of it, via
-        // the guard. Channel-only selects skip all of it: `eager` is
-        // false, the guard is disarmed, and the loser-arm self-cleaning
-        // story is unchanged.
+        // leftover registration can make the fd unusable for the next
+        // select until a kernel event happens to clear it), so selects
+        // containing fd arms run an eager cleanup pass after the park,
+        // including when a terminal stop unwinds out of it, via the guard.
+        // Channel-only selects skip all of it: `eager` is false, the guard
+        // is disarmed, and the loser-arm self-cleaning story is unchanged.
         let eager = arms.iter().any(|a| a.sel_eager_cleanup());
-        let mut guard = UnregisterGuard { arms, me, epoch, armed: eager };
+        let mut guard = UnregisterGuard {
+            arms,
+            me,
+            epoch,
+            armed: eager,
+        };
 
         crate::scheduler::park_current();
 
@@ -546,22 +653,22 @@ pub fn try_select(arms: &[&dyn Selectable]) -> std::io::Result<usize> {
         drop(guard);
 
         // Woken precisely: an arm's send (message) or last-sender drop
-        // (closure) consumed our epoch, and both leave their arm ready —
-        // return the first one, in priority order (which may be a
+        // (closure) is what woke us, and both leave their arm ready.
+        // Return the first ready one, in priority order (which may be a
         // different, higher-priority arm than the one that woke us; its
         // message stays queued and re-reports ready on the next call).
         // Fd arms classify by a fresh zero-timeout poll, so they too are
-        // a pure function of state — independent of the registration the
-        // cleanup pass just removed.
+        // a pure function of current state, independent of the
+        // registration the cleanup pass just removed.
         for (i, arm) in arms.iter().enumerate() {
             if arm.sel_ready() {
                 return Ok(i);
             }
         }
-        // Unreachable by protocol (a stop wake unwinds out of
-        // park_current). Defensive: re-open the wait and re-register —
-        // stale own-registrations are overwritten (channels) or were
-        // removed by the cleanup pass above (fds).
+        // Unreachable in practice (a stop wake unwinds out of
+        // park_current before we get here). Defensive: re-open the wait
+        // and re-register; stale own-registrations are overwritten
+        // (channels) or were removed by the cleanup pass above (fds).
     }
 }
 
@@ -576,11 +683,11 @@ fn unregister_arms(arms: &[&dyn Selectable], me: Pid, epoch: u32) {
     }
 }
 
-/// Stop-unwind twin of the explicit cleanup pass: a terminal stop unwinds
-/// out of `park_current`, and a registered fd arm must not outlive its
-/// actor (the generalization of `wait_fd`'s `Dereg`). Disarmed on the
-/// normal path after the explicit pass runs; never armed when no fd arm
-/// registered, keeping the channel-only path guard-free in effect.
+// Stop-unwind twin of the explicit cleanup pass: a terminal stop unwinds
+// out of `park_current`, and a registered fd arm must not outlive its
+// actor. Disarmed on the normal path after the explicit pass runs; never
+// armed when no fd arm is registered, keeping the channel-only path
+// guard-free in effect.
 struct UnregisterGuard<'a> {
     arms: &'a [&'a dyn Selectable],
     me: Pid,
@@ -596,25 +703,17 @@ impl Drop for UnregisterGuard<'_> {
     }
 }
 
-/// The registration pass shared by [`select`] and [`select_timeout`]:
-/// check-or-register each arm, in priority order, each atomically under its
-/// own lock. Cross-arm atomicity is unnecessary: an arm becoming ready
-/// right after its registration wakes the caller through the protocol (the
-/// prep-to-park window is closed by RunningNotified).
-///
-/// `Ok(Some(i))` = arm `i` was ready, the pass stopped, and the wait has
-/// been RETIRED (no park may follow): earlier arms hold live-epoch
-/// registrations, so earlier *fd* arms are unregistered eagerly, then the
-/// epoch is bumped, a landed notification eaten, and a pending stop
-/// re-observed — without which a stale arm wake could fault a later
-/// one-shot park. `Err` = an arm failed to register; identical unwind
-/// (earlier fd arms unregistered, wait retired). `Ok(None)` = every arm
-/// registered; the caller parks.
-fn register_arms(
-    me: Pid,
-    epoch: u32,
-    arms: &[&dyn Selectable],
-) -> std::io::Result<Option<usize>> {
+// The registration pass shared by `select` and `select_timeout`: check-or-
+// register each arm, in priority order, each atomically under its own lock.
+// Cross-arm atomicity is unnecessary: an arm becoming ready right after its
+// registration still wakes the caller through the normal wake path.
+//
+// `Ok(Some(i))` = arm `i` was already ready, the pass stopped, and the wait
+// has been fully retired (no park may follow): earlier fd arms are
+// unregistered eagerly so none are left dangling. `Err` = an arm failed to
+// register; same unwind (earlier fd arms unregistered, wait retired).
+// `Ok(None)` = every arm registered successfully; the caller parks.
+fn register_arms(me: Pid, epoch: u32, arms: &[&dyn Selectable]) -> std::io::Result<Option<usize>> {
     for (i, arm) in arms.iter().enumerate() {
         let registered = match arm.sel_register(me, epoch) {
             Ok(r) => r,
@@ -633,10 +732,10 @@ fn register_arms(
     Ok(None)
 }
 
-/// The [`select_timeout`] timer target: stateless, because precise wakes
-/// make classification a pure function of channel state. The entry is
-/// stamped with the select's epoch; if an arm already won, this unpark dies
-/// at the word's epoch CAS (the no-cancellation convention in `timer.rs`).
+// The `select_timeout` timer target: stateless, because a wake's cause can
+// always be read back off plain channel state (an arm ready, or not). If
+// an arm already won before the deadline, this timer's fire is simply
+// ignored, the way any other stale wakeup is.
 struct SelectTimeout;
 impl crate::timer::TimerTarget for SelectTimeout {
     fn on_timeout(&self, pid: Pid, epoch: u32) {
@@ -644,31 +743,22 @@ impl crate::timer::TimerTarget for SelectTimeout {
     }
 }
 
-/// [`select`] with a deadline: returns `Some(index)` like `select`, or
-/// `None` once `timeout` elapses with no arm ready.
+/// Like [`select`], but gives up and returns `None` if no arm becomes
+/// ready before `timeout` elapses.
 ///
-/// All of `select`'s semantics carry over (priority order, closed arms
-/// permanently ready, no fairness promise). The timeout is one more stamped
-/// waker on the same wait epoch — nothing is registered in any arm for it,
-/// so there is nothing to cancel or leak: an arm winning leaves the timer
-/// entry to expire as a stale-epoch no-op; the timer winning leaves the
-/// arms' registrations to self-clean exactly as a `select` loser's would.
+/// All of `select`'s semantics carry over: arms are still checked in
+/// priority order, a closed arm is still permanently ready, and there is
+/// still no fairness guarantee across arms. A message that arrives at
+/// essentially the same moment the deadline passes still wins, the same
+/// way [`Receiver::recv_timeout`] resolves that race.
 ///
-/// The wake is classified from state alone (wakes are precise): some arm
-/// ready → `Some` of the first, in priority order; none ready → the timer
-/// was the only remaining stamped waker → `None`. A message that races the
-/// deadline resolves message-first, as `recv_timeout` does.
+/// `Duration::ZERO` is a valid timeout: it still gives an already-ready arm
+/// a chance to be reported before falling through to `None`.
 ///
-/// `Duration::ZERO` is a valid timeout: it parks until the immediately-due
-/// timer is drained, then reports `None` unless an arm was already ready.
-///
-/// Panics if `arms` is empty, when called outside an actor, or if an fd
-/// arm fails to register (see [`try_select_timeout`] for the fallible
-/// form; channel-only selects cannot fail).
-pub fn select_timeout(
-    arms: &[&dyn Selectable],
-    timeout: std::time::Duration,
-) -> Option<usize> {
+/// Panics if `arms` is empty, if called outside an actor, or if an fd arm
+/// fails to register (see [`try_select_timeout`] for the fallible form; a
+/// channel-only select can never fail).
+pub fn select_timeout(arms: &[&dyn Selectable], timeout: std::time::Duration) -> Option<usize> {
     match try_select_timeout(arms, timeout) {
         Ok(r) => r,
         Err(e) => panic!(
@@ -677,9 +767,9 @@ pub fn select_timeout(
     }
 }
 
-/// [`select_timeout`], fallible: `Err` when an arm fails to register
-/// (only fd arms can). On `Err` the wait is fully retired and no
-/// registration — arm-side or kernel-side — is left behind.
+/// The fallible form of [`select_timeout`]: `Err` when an arm fails to
+/// register (only fd arms can). On `Err` the wait is fully retired and no
+/// registration is left behind on any arm.
 pub fn try_select_timeout(
     arms: &[&dyn Selectable],
     timeout: std::time::Duration,
@@ -694,19 +784,23 @@ pub fn try_select_timeout(
         return Ok(Some(i)); // ready now: the timer was never armed
     }
 
-    // Arm the timer after the registration pass, outside every Channel
-    // lock (insert takes the timers lock).
+    // Arm the timer after the registration pass, outside every channel
+    // lock (inserting a timer takes the timers lock).
     let deadline = crate::timer::deadline_from_now(timeout);
     let target: std::sync::Arc<dyn crate::timer::TimerTarget> = std::sync::Arc::new(SelectTimeout);
     crate::scheduler::insert_wait_timer(deadline, me, target, epoch);
 
-    // Same eager-cleanup story as `try_select`: the timer arm needs none
-    // (stateless, stale entries die at the epoch CAS), channel arms need
-    // none, fd arms do — and a timer win in particular leaves every fd
-    // arm's registration behind, which without this pass would poison
-    // those fds until a kernel event happened to fire.
+    // Same eager-cleanup story as `try_select`: a timer win in particular
+    // leaves every fd arm's registration behind, which without this pass
+    // would leave those fds unusable until a kernel event happened to
+    // clear them.
     let eager = arms.iter().any(|a| a.sel_eager_cleanup());
-    let mut guard = UnregisterGuard { arms, me, epoch, armed: eager };
+    let mut guard = UnregisterGuard {
+        arms,
+        me,
+        epoch,
+        armed: eager,
+    };
 
     crate::scheduler::park_current();
 

src/context.rs

@@ -16,10 +16,18 @@ thread_local! {
     static ACTOR_SP:     Cell<usize> = const { Cell::new(0) };
 }
 
-fn  get_scheduler_sp() -> usize     { SCHEDULER_SP.with(|c| c.get()) }
-fn  set_scheduler_sp(v: usize)      { SCHEDULER_SP.with(|c| c.set(v)) }
-pub fn get_actor_sp() -> usize      { ACTOR_SP.with(|c| c.get()) }
-pub fn set_actor_sp(v: usize)       { ACTOR_SP.with(|c| c.set(v)) }
+fn get_scheduler_sp() -> usize {
+    SCHEDULER_SP.with(|c| c.get())
+}
+fn set_scheduler_sp(v: usize) {
+    SCHEDULER_SP.with(|c| c.set(v))
+}
+pub fn get_actor_sp() -> usize {
+    ACTOR_SP.with(|c| c.get())
+}
+pub fn set_actor_sp(v: usize) {
+    ACTOR_SP.with(|c| c.set(v))
+}
 
 // ---------------------------------------------------------------------------
 // Initial stack layout
@@ -49,13 +57,20 @@ pub fn set_actor_sp(v: usize)       { ACTOR_SP.with(|c| c.set(v)) }
 pub fn init_actor_stack(top: *mut u8, entry: extern "C-unwind" fn()) -> usize {
     unsafe {
         let mut sp = (top as usize & !15) - 8;
-        sp -= 8; (sp as *mut usize).write(entry as usize); // ret target
-        sp -= 8; (sp as *mut usize).write(0); // rbx
-        sp -= 8; (sp as *mut usize).write(0); // rbp
-        sp -= 8; (sp as *mut usize).write(0); // r12
-        sp -= 8; (sp as *mut usize).write(0); // r13
-        sp -= 8; (sp as *mut usize).write(0); // r14
-        sp -= 8; (sp as *mut usize).write(0); // r15
+        sp -= 8;
+        (sp as *mut usize).write(entry as usize); // ret target
+        sp -= 8;
+        (sp as *mut usize).write(0); // rbx
+        sp -= 8;
+        (sp as *mut usize).write(0); // rbp
+        sp -= 8;
+        (sp as *mut usize).write(0); // r12
+        sp -= 8;
+        (sp as *mut usize).write(0); // r13
+        sp -= 8;
+        (sp as *mut usize).write(0); // r14
+        sp -= 8;
+        (sp as *mut usize).write(0); // r15
         sp
     }
 }

src/gen_server.rs

@@ -178,11 +178,13 @@
 //! from any handler via [`Watcher::watch`]) because monitors are inherently
 //! created at runtime. The idle window is set once, in `init`.
 
-use crate::channel::{channel, select, select_timeout, Receiver, RecvTimeoutError, Selectable, Sender};
+use crate::channel::{
+    channel, select, select_timeout, Receiver, RecvTimeoutError, Selectable, Sender,
+};
 use crate::monitor::{demonitor, monitor, Down, Monitor};
 use crate::pid::Pid;
 use crate::registry::{register_with, resolve_named_sender, RegisterError};
-use crate::scheduler::{cancel_timer, request_stop, send_after_to, spawn, spawn_under};
+use crate::scheduler::{cancel_timer, request_stop, send_after_to};
 use crate::timer::TimerId;
 use std::cell::Cell;
 use std::collections::HashMap;
@@ -273,7 +275,10 @@ pub struct GenServerRef<G: GenServer> {
 
 impl<G: GenServer> Clone for GenServerRef<G> {
     fn clone(&self) -> Self {
-        GenServerRef { tx: self.tx.clone(), pid: self.pid }
+        GenServerRef {
+            tx: self.tx.clone(),
+            pid: self.pid,
+        }
     }
 }
 
@@ -412,7 +417,9 @@ impl<G: GenServer> GenServerCtx<G> {
     /// A clonable handle to the loop's monitor intake. Store it in the state
     /// during `init` to watch monitors from later handlers.
     pub fn watcher(&self) -> Watcher<G> {
-        Watcher { tx: self.sys_tx.clone() }
+        Watcher {
+            tx: self.sys_tx.clone(),
+        }
     }
 
     /// Shorthand for `ctx.watcher().watch(m)` when watching during `init`.
@@ -426,7 +433,10 @@ impl<G: GenServer> GenServerCtx<G> {
     /// [`tick_every`](TimerHandle::tick_every) /
     /// [`cancel`](TimerHandle::cancel) from any later handler.
     pub fn timer(&self) -> TimerHandle<G> {
-        TimerHandle { sys_tx: self.sys_tx.clone(), reg: self.reg.clone() }
+        TimerHandle {
+            sys_tx: self.sys_tx.clone(),
+            reg: self.reg.clone(),
+        }
     }
 
     /// Set a quiet-period window: if the loop goes `after` without dispatching
@@ -518,7 +528,10 @@ pub struct TimerHandle<G: GenServer> {
 // Manual Clone for the same reason as `Watcher`: no `G: Clone` needed.
 impl<G: GenServer> Clone for TimerHandle<G> {
     fn clone(&self) -> Self {
-        TimerHandle { sys_tx: self.sys_tx.clone(), reg: self.reg.clone() }
+        TimerHandle {
+            sys_tx: self.sys_tx.clone(),
+            reg: self.reg.clone(),
+        }
     }
 }
 
@@ -569,8 +582,18 @@ impl<G: GenServer> TimerHandle<G> {
         // First instance fires after `every`; the payload is produced loop-side
         // from `make` on fire, so the tick carries only the stable id.
         let sub = send_after_to(every, self.sys_tx.clone(), Sys::Tick(local));
-        reg.periodics.insert(local, Periodic { every, live: sub, make });
-        debug_assert!(reg.rearm_tx.is_some(), "rearm_tx must be Some while periodics is non-empty");
+        reg.periodics.insert(
+            local,
+            Periodic {
+                every,
+                live: sub,
+                make,
+            },
+        );
+        debug_assert!(
+            reg.rearm_tx.is_some(),
+            "rearm_tx must be Some while periodics is non-empty"
+        );
         local
     }
 
@@ -617,7 +640,9 @@ pub struct Watcher<G: GenServer> {
 // regardless of the server type (it clones only the inner sender).
 impl<G: GenServer> Clone for Watcher<G> {
     fn clone(&self) -> Self {
-        Watcher { tx: self.tx.clone() }
+        Watcher {
+            tx: self.tx.clone(),
+        }
     }
 }
 
@@ -643,11 +668,17 @@ pub struct GenServerBuilder<G: GenServer> {
     state: G,
     infos: Vec<Receiver<G::Info>>,
     supervisor: Option<Pid>,
+    stack_opts: crate::scheduler::SpawnOpts,
 }
 
 impl<G: GenServer> GenServerBuilder<G> {
     pub fn new(state: G) -> Self {
-        GenServerBuilder { state, infos: Vec::new(), supervisor: None }
+        GenServerBuilder {
+            state,
+            infos: Vec::new(),
+            supervisor: None,
+            stack_opts: crate::scheduler::SpawnOpts::default(),
+        }
     }
 
     /// Add an out-of-band channel; messages arriving on it are dispatched to
@@ -665,6 +696,14 @@ impl<G: GenServer> GenServerBuilder<G> {
         self
     }
 
+    /// Stack shape for the server actor (RFC 019) — see
+    /// [`SpawnOpts`](crate::SpawnOpts). Useful for servers that recurse
+    /// deeply or call into FFI with large C frames.
+    pub fn stack_opts(mut self, opts: crate::scheduler::SpawnOpts) -> Self {
+        self.stack_opts = opts;
+        self
+    }
+
     /// Spawn the server actor and hand back its [`GenServerRef`]. The server's
     /// lifetime is governed by its refs, not by joining, so the backing join
     /// handle is dropped.
@@ -677,7 +716,10 @@ impl<G: GenServer> GenServerBuilder<G> {
     /// live server). Consumes the builder, carrying its `with_info` / `under`
     /// configuration through.
     pub fn named(self, name: GenServerName<G>) -> NamedGenServerBuilder<G> {
-        NamedGenServerBuilder { builder: self, name: name.as_str() }
+        NamedGenServerBuilder {
+            builder: self,
+            name: name.as_str(),
+        }
     }
 
     /// Private shared body behind [`start`](Self::start) and
@@ -686,12 +728,24 @@ impl<G: GenServer> GenServerBuilder<G> {
     /// under the name before returning.
     fn spawn_server(self) -> GenServerRef<G> {
         let (tx, rx) = channel::<Envelope<G>>();
-        let GenServerBuilder { state, infos, supervisor } = self;
+        let GenServerBuilder {
+            state,
+            infos,
+            supervisor,
+            stack_opts,
+        } = self;
         let handle = match supervisor {
-            Some(sup) => spawn_under(sup, move || server_loop::<G>(rx, state, infos)),
-            None => spawn(move || server_loop::<G>(rx, state, infos)),
+            Some(sup) => crate::scheduler::spawn_under_with(sup, stack_opts, move || {
+                server_loop::<G>(rx, state, infos)
+            }),
+            None => {
+                crate::scheduler::spawn_with(stack_opts, move || server_loop::<G>(rx, state, infos))
+            }
         };
-        GenServerRef { tx, pid: handle.pid() }
+        GenServerRef {
+            tx,
+            pid: handle.pid(),
+        }
     }
 }
 
@@ -719,7 +773,10 @@ impl<G> GenServerName<G> {
     /// associated constants at call sites.
     #[inline]
     pub const fn new(name: &'static str) -> Self {
-        Self { name, _marker: PhantomData }
+        Self {
+            name,
+            _marker: PhantomData,
+        }
     }
 
     /// The underlying registry key.
@@ -758,6 +815,12 @@ impl<G: GenServer> NamedGenServerBuilder<G> {
         self
     }
 
+    /// Stack shape for the server actor (see [`GenServerBuilder::stack_opts`]).
+    pub fn stack_opts(mut self, opts: crate::scheduler::SpawnOpts) -> Self {
+        self.builder = self.builder.stack_opts(opts);
+        self
+    }
+
     /// Spawn the server and bind its name in one step. Fallible: returns
     /// [`RegisterError::NameTaken`] if the name is already held by a different
     /// live server.
@@ -907,7 +970,11 @@ fn server_loop<G: GenServer>(
     // Bind the ctx so the idle window set during init can be read back, then
     // drop it — that drops the loop's own Sys sender, so a state that cloned no
     // Watcher/TimerHandle lets the arm auto-close (the unused-ctx behaviour).
-    let ctx = GenServerCtx { sys_tx, reg: reg.clone(), idle: Cell::new(None) };
+    let ctx = GenServerCtx {
+        sys_tx,
+        reg: reg.clone(),
+        idle: Cell::new(None),
+    };
     guard.0.init(&ctx);
     let idle = ctx.idle.get();
     drop(ctx);
@@ -962,8 +1029,7 @@ fn server_loop<G: GenServer>(
                                         // info band:     [nd+nw,  nd+nw+ni)
                                         // inbox arm:     [nd+nw+ni]
             let sel = {
-                let mut arms: Vec<&dyn Selectable> =
-                    Vec::with_capacity(nd + nw + infos.len() + 1);
+                let mut arms: Vec<&dyn Selectable> = Vec::with_capacity(nd + nw + infos.len() + 1);
                 for m in &monitors {
                     arms.push(&m.rx);
                 }
@@ -975,9 +1041,7 @@ fn server_loop<G: GenServer>(
                 }
                 arms.push(&rx);
                 match idle_deadline {
-                    Some(dl) => {
-                        select_timeout(&arms, dl.saturating_duration_since(Instant::now()))
-                    }
+                    Some(dl) => select_timeout(&arms, dl.saturating_duration_since(Instant::now())),
                     None => Some(select(&arms)),
                 }
             };
@@ -1008,8 +1072,12 @@ fn server_loop<G: GenServer>(
                         // live set tracks only still-pending timers, then
                         // dispatch.
                         match reg.lock() {
-                            Ok(mut g) => { g.oneshots.remove(&id); }
-                            Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
+                            Ok(mut g) => {
+                                g.oneshots.remove(&id);
+                            }
+                            Err(e) => {
+                                panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}")
+                            }
                         }
                         guard.0.handle_timer(msg);
                         reset_idle(&mut idle_deadline);
@@ -1023,7 +1091,9 @@ fn server_loop<G: GenServer>(
                         let msg = {
                             let mut g = match reg.lock() {
                                 Ok(g) => g,
-                                Err(e) => panic!("smarm: gen_server reg lock poisoned (core corrupt): {e}"),
+                                Err(e) => panic!(
+                                    "smarm: gen_server reg lock poisoned (core corrupt): {e}"
+                                ),
                             };
                             let r = &mut *g;
                             if let Some(p) = r.periodics.get_mut(&id) {
@@ -1031,9 +1101,9 @@ fn server_loop<G: GenServer>(
                                 let msg = (p.make)();
                                 let tx = match r.rearm_tx.as_ref() {
                                     Some(tx) => tx.clone(),
-                                    None => panic!(
-                                        "smarm: live periodic without rearm_tx (logic bug)"
-                                    ),
+                                    None => {
+                                        panic!("smarm: live periodic without rearm_tx (logic bug)")
+                                    }
                                 };
                                 p.live = send_after_to(every, tx, Sys::Tick(id));
                                 Some(msg)

src/gen_statem.rs

@@ -71,7 +71,7 @@
 
 use crate::channel::{channel, select, Receiver, Sender};
 use crate::pid::Pid;
-use crate::scheduler::{cancel_timer, send_after_to, spawn as spawn_actor};
+use crate::scheduler::{cancel_timer, send_after_to};
 use crate::timer::TimerId;
 use std::collections::{HashMap, VecDeque};
 use std::marker::PhantomData;
@@ -219,7 +219,11 @@ struct Timers {
 
 impl Timers {
     fn new() -> Self {
-        Timers { next_local: 0, state: None, named: HashMap::new() }
+        Timers {
+            next_local: 0,
+            state: None,
+            named: HashMap::new(),
+        }
     }
 
     fn mint(&mut self) -> u64 {
@@ -248,7 +252,11 @@ pub struct Cx<Ev> {
 
 impl<Ev> Cx<Ev> {
     fn new(sys_tx: Sender<Sys>, reg: Arc<Mutex<Timers>>) -> Self {
-        Cx { sys_tx, reg, _ev: PhantomData }
+        Cx {
+            sys_tx,
+            reg,
+            _ev: PhantomData,
+        }
     }
 
     /// Arm the **state timeout**: fire a `state_timeout` event after `after` in
@@ -387,7 +395,10 @@ pub struct GenStatemRef<M: Machine> {
 
 impl<M: Machine> Clone for GenStatemRef<M> {
     fn clone(&self) -> Self {
-        GenStatemRef { tx: self.tx.clone(), pid: self.pid }
+        GenStatemRef {
+            tx: self.tx.clone(),
+            pid: self.pid,
+        }
     }
 }
 
@@ -434,9 +445,22 @@ impl<M: Machine> GenStatemRef<M> {
 ///
 /// Panics if called outside `Runtime::run()`.
 pub fn spawn<M: Machine>(machine: M) -> GenStatemRef<M> {
+    spawn_with(crate::scheduler::SpawnOpts::default(), machine)
+}
+
+/// [`spawn`] with per-actor stack shape overrides (RFC 019) for the machine's
+/// actor — see [`SpawnOpts`](crate::SpawnOpts). gen_statem has no builder
+/// (its one-shot `spawn(machine)` shape predates RFC 019), so the opts ride
+/// a `_with` variant like the scheduler's own spawns.
+///
+/// Panics if called outside `Runtime::run()`.
+pub fn spawn_with<M: Machine>(opts: crate::scheduler::SpawnOpts, machine: M) -> GenStatemRef<M> {
     let (tx, rx) = channel::<M::Ev>();
-    let handle = spawn_actor(move || statem_loop(rx, machine));
-    GenStatemRef { tx, pid: handle.pid() }
+    let handle = crate::scheduler::spawn_with(opts, move || statem_loop(rx, machine));
+    GenStatemRef {
+        tx,
+        pid: handle.pid(),
+    }
 }
 
 /// The machine actor body: `on_start`, then one `handle` per event until the
@@ -475,7 +499,9 @@ fn statem_loop<M: Machine>(rx: Receiver<M::Ev>, mut machine: M) {
                         Sys::StateTimeout(local) => {
                             let mut t = match reg.lock() {
                                 Ok(g) => g,
-                                Err(e) => panic!("smarm: gen_statem reg lock poisoned (core corrupt): {e}"),
+                                Err(e) => panic!(
+                                    "smarm: gen_statem reg lock poisoned (core corrupt): {e}"
+                                ),
                             };
                             match t.state {
                                 Some((live, _)) if live == local => {
@@ -488,7 +514,9 @@ fn statem_loop<M: Machine>(rx: Receiver<M::Ev>, mut machine: M) {
                         Sys::Timeout(name, local) => {
                             let mut t = match reg.lock() {
                                 Ok(g) => g,
-                                Err(e) => panic!("smarm: gen_statem reg lock poisoned (core corrupt): {e}"),
+                                Err(e) => panic!(
+                                    "smarm: gen_statem reg lock poisoned (core corrupt): {e}"
+                                ),
                             };
                             match t.named.get(name) {
                                 Some(&(live, _)) if live == local => {

src/introspect.rs

@@ -1,26 +1,85 @@
-//! RFC 016 — runtime introspection (Chunk 1: the read primitive).
+//! Inspect what is running right now: which actors exist, what state each one
+//! is in, and how they are related.
 //!
-//! A synchronous, internal read of the slab that returns *owned* data. This is
-//! the mechanism the whole RFC hangs off: tests, the future observer
-//! gen_server (Chunk 4), and a later control plane (RFC 003) are all consumers
-//! of [`snapshot`] / [`actor_info`], never of the runtime internals directly.
+//! This is the tool for questions like "is my server still alive", "how many
+//! actors are currently parked waiting on something", or "what does the spawn
+//! tree look like". It is meant for debugging, test assertions, a health check
+//! endpoint, or a monitoring dashboard: anywhere you want to look at the
+//! runtime from the outside without stopping it or coupling your code to its
+//! internals.
 //!
-//! ## Consistency (DECISION D2 — per-slot tearing, `ps` semantics)
+//! Three entry points, in order of scope:
 //!
-//! [`snapshot`] is point-in-time and mildly racy *across* actors: each slot's
-//! scheduling state is a lock-free word load, so an actor reported `Running`
-//! may already be `Parked`, and an actor can die mid-scan. This is the cheap,
-//! useful model (a coherent stop-the-world cut is expensive and rarely wanted).
-//! [`actor_info`] is coherent for the single actor it names.
+//! - [`snapshot`] returns every actor that currently exists, as a plain
+//!   owned `Vec`, so you can filter, count, or search it however you like.
+//! - [`actor_info`] returns a coherent view of exactly one actor, by pid.
+//!   Cheaper than filtering a whole snapshot down to one entry, and more
+//!   precise (see "Consistency" below).
+//! - [`tree`] returns the same actors as [`snapshot`], folded into a
+//!   parent/child forest that mirrors who spawned whom.
 //!
-//! ## Locking
+//! ```
+//! use smarm::{actor_info, channel, run, snapshot, spawn, ActorState};
 //!
-//! The lock order is **Leaf → Channel, at most one of each** (`raw_mutex.rs`);
-//! cold locks, the registry, and the free list are all Leaves, so we may never
-//! hold two at once. The read is therefore phased: first a single registry-leaf
-//! pass for names and mailbox depth (the per-channel length read is a Channel
-//! lock taken under that Leaf — legal), released before the slab scan takes any
-//! per-slot cold Leaf.
+//! run(|| {
+//!     let (ready_tx, ready_rx) = channel::<()>();
+//!     let (gate_tx, gate_rx) = channel::<()>();
+//!
+//!     let worker = spawn(move || {
+//!         ready_tx.send(()).unwrap();
+//!         gate_rx.recv().unwrap(); // blocks here until released
+//!     });
+//!     ready_rx.recv().unwrap();
+//!
+//!     // `snapshot` sees every actor, including this one and the worker.
+//!     let snap = snapshot();
+//!     assert!(snap.actors.len() >= 2);
+//!
+//!     // `actor_info` gives a coherent view of just the worker. It is
+//!     // blocked on the gate channel, so it must be Parked.
+//!     let pid = worker.pid();
+//!     let info = actor_info(pid).expect("worker is still alive");
+//!     assert_eq!(info.state, ActorState::Parked);
+//!
+//!     gate_tx.send(()).unwrap();
+//!     worker.join().unwrap();
+//!
+//!     // Once joined, the pid no longer names a live actor.
+//!     assert!(actor_info(pid).is_none());
+//! });
+//! ```
+//!
+//! ## Consistency
+//!
+//! [`snapshot`] is not a single atomic pause-the-world freeze: it walks every
+//! actor's state one after another, so it is a series of independent,
+//! cheap, lock-free reads rather than one coherent moment in time. Between
+//! reading actor A and actor B, either one can change state, and an actor can
+//! even finish and disappear mid-scan. In practice this is exactly what you
+//! want: a coherent stop-the-world snapshot would mean pausing every actor in
+//! the runtime just to look at it, which is expensive and rarely necessary
+//! for a dashboard, a test assertion, or a debugging session.
+//!
+//! [`actor_info`], in contrast, is coherent for the one actor it names: all of
+//! its fields describe the same instant for that actor, because a single
+//! actor's data cannot tear the way a scan across many actors can.
+//!
+//! ## Implementation notes
+//!
+//! These details matter if you are working on smarm itself; they are not part
+//! of the public contract.
+//!
+//! The read never stops the scheduler and never holds a lock across the whole
+//! scan. Each actor's scheduling state is a single lock-free word load
+//! (hence the possible tearing described above). Reading the rest of an
+//! actor's cold data (its supervisor, monitors, links, and so on) takes a
+//! brief per-actor lock, just long enough to copy those fields out; nothing
+//! is held across actors. Locking follows the crate-wide rule that at most
+//! one "leaf" lock (a per-actor lock, the registry lock, or the free list
+//! lock) is held at a time, with no leaf lock held while acquiring another.
+//! The read is phased accordingly: first one pass over the registry to
+//! collect every actor's registered names and mailbox depth, released before
+//! the per-actor scan begins.
 
 use crate::pid::Pid;
 use crate::registry::MailboxInfo;
@@ -31,15 +90,28 @@ use crate::slot_state::{
 };
 use std::collections::HashMap;
 
-/// Snapshot wire-format version (DECISION D1). [`RuntimeSnapshot`] is treated as
-/// a stable type from day one: it becomes the observer protocol (Chunk 4) and
-/// crosses a version boundary the moment a remote observer attaches (RFC 011),
-/// so the version travels with the data from the start.
+/// The format version carried by every [`RuntimeSnapshot`] and
+/// [`RuntimeTree`], as [`RuntimeSnapshot::format_version`] /
+/// [`RuntimeTree::format_version`]. If you serialize a snapshot (for example
+/// to send it somewhere else, or to compare snapshots taken with different
+/// versions of smarm) check this field: a change in its value means the shape
+/// of [`ActorInfo`] or its neighbors has changed and old and new snapshots
+/// should not be assumed compatible. If you only ever read a snapshot
+/// in-process in the same version of smarm that produced it, you can ignore
+/// this field.
 pub const SNAPSHOT_FORMAT_VERSION: u16 = 1;
 
-/// Fine-grained scheduling state, mapped from the packed slot word with no new
-/// storage. `RunningNotified` collapses into `Notified` — a wake landed while
-/// the actor was on-CPU and it will re-queue when it yields.
+/// What an actor is doing right now, from the scheduler's point of view.
+///
+/// - `Queued`: runnable, waiting for a scheduler thread to pick it up.
+/// - `Running`: currently executing on a scheduler thread.
+/// - `Notified`: was running and got woken up (for example, a message
+///   arrived) before it had a chance to yield or park; it will be re-queued
+///   as soon as it does.
+/// - `Parked`: blocked, waiting on something such as a channel receive, a
+///   mutex, a timer, or an IO event.
+/// - `Done`: has finished (returned or panicked) but its slot has not been
+///   reclaimed for reuse yet, so it is still visible to introspection.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum ActorState {
     Queued,
@@ -49,8 +121,8 @@ pub enum ActorState {
     Done,
 }
 
-/// Classify a packed state word. `None` for a Vacant slot (skipped by the scan)
-/// — the only state that is not an actor.
+/// Classify a packed state word. `None` for a Vacant slot (skipped by the
+/// scan): a vacant slot holds no actor at all, live or done.
 fn classify(w: u64) -> Option<ActorState> {
     Some(match word_state(w) {
         ST_QUEUED => ActorState::Queued,
@@ -62,61 +134,104 @@ fn classify(w: u64) -> Option<ActorState> {
     })
 }
 
-/// Owned, point-in-time view of one actor — no borrows of runtime internals, so
-/// it is safe to hand to any consumer.
+/// An owned, self-contained view of one actor at (approximately) one moment.
+/// It borrows nothing from the runtime, so you can keep it, send it
+/// elsewhere, or print it long after the actor it describes has changed
+/// state or even exited.
 #[derive(Debug, Clone)]
 pub struct ActorInfo {
     pub pid: Pid,
-    /// Registered names, inverted from the registry (usually 0 or 1).
+    /// Names this actor is currently registered under (see the
+    /// [`registry`](crate::registry) module). Usually empty or one name;
+    /// an actor can have more if it registered several.
     pub names: Vec<&'static str>,
     pub state: ActorState,
-    /// Spawn-time parent edge (DECISION D9): `spawn_under` sets it to the
-    /// supervisor, plain `spawn` to the spawning actor — so it is parentage,
-    /// not necessarily a supervision relationship. `ROOT_PID` for the run's
-    /// root actor and for `Done` tombstones (whose `Actor` is already gone).
+    /// The actor that spawned this one: whoever called `spawn` or
+    /// `spawn_under` to create it. This is a parentage record, not
+    /// necessarily a supervision relationship: `spawn_under` records the
+    /// supervisor you asked for, while plain `spawn` records the spawning
+    /// actor itself, whether or not it supervises anything. It is the
+    /// runtime's root pid for the run's own root actor, and for a `Done`
+    /// actor whose bookkeeping has already been cleared.
     pub supervisor: Pid,
     pub trap_exit: bool,
     pub monitors: u32,
     pub links: u32,
     pub joiners: u32,
-    /// Queued messages summed over the actor's *published* channels (register /
-    /// install / spawn_addr / gen_server). 0 for an actor that holds only a
-    /// private `channel()` receiver — those are invisible to the registry.
+    /// Messages currently queued and not yet delivered, summed across every
+    /// channel this actor has published (via `register`, `install`,
+    /// `spawn_addr`, or starting a gen_server). This is 0 for an actor that
+    /// only holds a private, unpublished `channel()` receiver, since nothing
+    /// outside the actor can see that channel exists.
     pub mailbox_depth: u32,
-    /// Timeslice overruns tallied for this incarnation (RFC 016 Chunk 2): how
-    /// many times the actor was preempted for exceeding its slice. Resets on
-    /// restart (per-incarnation, D7).
+    /// How many times this actor has been preempted for running past its
+    /// scheduling timeslice. Counts only since the actor's current start (a
+    /// supervisor restart begins a fresh count).
     pub overruns: u64,
-    /// Messages this actor has received (dequeued) this incarnation (RFC 016
-    /// Chunk 2) — answers "is this actor a hotspot / draining slower than its
-    /// mailbox fills." Counts received, not sent (D4). Per-incarnation (D7).
+    /// How many messages this actor has received (taken off its inbox), since
+    /// its current start. Useful for spotting an actor whose mailbox is
+    /// filling up faster than it can drain it: compare this against
+    /// `mailbox_depth` over time.
     pub messages_received: u64,
-    /// Approximate on-CPU cycles this incarnation has consumed (RFC 016 Chunk 2)
-    /// — a reductions-like work metric for relative comparison. Always 0 unless
-    /// the `budget-accounting` feature is enabled (it costs an RDTSC per resume,
-    /// D6). Per-incarnation (D7).
+    /// Approximate CPU cycles this actor has spent running, since its current
+    /// start. A relative measure for comparing actors against each other, not
+    /// an absolute or wall-clock figure. Always 0 unless the crate's
+    /// `budget-accounting` feature is enabled, since measuring it costs a
+    /// timestamp read on every resume.
     pub budget_cycles: u64,
+    /// RFC 019 §8 — this actor's stack, as the runtime sees it. All fields
+    /// are lock-free atomic reads, coherent for this incarnation via the
+    /// same generation check as the counters above. Exact RSS is
+    /// deliberately absent: `mincore` is debug tooling, never a runtime
+    /// path.
+    pub stack: StackInfo,
 }
 
-/// A whole-runtime snapshot. See the module docs for the D2 tearing model.
+/// RFC 019 §8 — per-actor stack introspection. Sizes are page-rounded, as
+/// [`Stack::new`](crate::stack::Stack::new) rounds them.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub struct StackInfo {
+    /// Usable stack size ([`SpawnOpts::stack_reserve`]
+    /// (crate::SpawnOpts::stack_reserve) or the Config/default).
+    pub reserve: usize,
+    /// PROT_NONE guard below the usable region.
+    pub guard: usize,
+    /// Sampled high-water depth in bytes: `top − lowest saved sp`. Sampled,
+    /// not exact — the context save at yields/parks/preemptions is the
+    /// sampler (RFC 019 §2), so a spike the actor never yielded inside is
+    /// invisible. 0 depth means "never descheduled at any depth", not
+    /// "never ran".
+    pub depth_high_water: usize,
+    /// Parks on this incarnation since its last shrink (or since install if
+    /// it has never shrunk) — the §3 cooldown counter, live.
+    pub parks_since_shrink: u32,
+    /// §3 shrinks performed on this incarnation.
+    pub shrinks: u32,
+}
+
+/// A snapshot of every actor in the runtime at (approximately) one moment.
+/// See the module docs' "Consistency" section for what "approximately" means
+/// here.
 #[derive(Debug, Clone)]
 pub struct RuntimeSnapshot {
     pub format_version: u16,
     pub actors: Vec<ActorInfo>,
 }
 
-/// Snapshot every live (and `Done`-but-not-yet-reclaimed) actor on the slab.
-/// O(n) over the slot table, running with preemption disabled (like every
-/// runtime primitive) but holding no lock across the scan. Panics outside
-/// `Runtime::run()`; callable from actor code and the run thread.
+/// Every actor that currently exists: running, queued, parked, or finished
+/// but not yet cleaned up. Cheap and lock-free per actor; see the module
+/// docs for what "approximately one moment" means for the result as a whole.
+/// Panics if called outside [`run`](crate::run).
 pub fn snapshot() -> RuntimeSnapshot {
     with_runtime(|inner| {
-        // Phase A: one registry-leaf pass for names + mailbox depth, released
-        // before any cold leaf (no two Leaves at once).
+        // First pass: one registry lock to collect names + mailbox depth for
+        // every actor, released before touching any per-actor lock below.
         let mail = inner.registry.lock().introspect_map();
 
-        // Phase B: lock-free slab scan; per-slot cold leaf only to copy cold
-        // fields. Tearing across slots is intentional (D2).
+        // Second pass: walk the actor table. Each actor's scheduling state is
+        // a lock-free word load; only copying its other fields takes a brief
+        // per-actor lock. Tearing across actors is expected here (see the
+        // module docs' "Consistency" section).
         let mut actors = Vec::new();
         for (idx, slot) in inner.slots.iter().enumerate() {
             let idx = idx as u32;
@@ -124,12 +239,34 @@ pub fn snapshot() -> RuntimeSnapshot {
                 actors.push(info);
             }
         }
-        RuntimeSnapshot { format_version: SNAPSHOT_FORMAT_VERSION, actors }
+        RuntimeSnapshot {
+            format_version: SNAPSHOT_FORMAT_VERSION,
+            actors,
+        }
+    })
+}
+
+/// A coherent view of exactly one actor, or `None` if `pid` does not name a
+/// currently-live entry: it is stale (that actor has already exited and its
+/// slot was reused by another), out of range, or was never a real pid at
+/// all. Unlike [`snapshot`], every field of the result describes the same
+/// instant, since there is only one actor to read.
+/// The stack shape `(reserve, guard)` of a live actor, page-rounded — the
+/// RFC 019 introspection surface's first field (depth sampling and shrink
+/// counters land with the shrink machinery). `None` if `pid` no longer names
+/// a live actor. Takes the actor's cold lock briefly; debugging/assertion
+/// use, not a hot-path call.
+pub fn stack_shape(pid: Pid) -> Option<(usize, usize)> {
+    with_runtime(|inner| {
+        let slot = inner.slot_at(pid)?;
+        let cold = slot.cold.lock();
+        if slot.generation() != pid.generation() {
+            return None;
+        }
+        cold.actor.as_ref().map(|a| a.stack.shape())
     })
 }
 
-/// Coherent view of a single actor, or `None` if the pid is stale, out of
-/// range, or names a Vacant slot.
 pub fn actor_info(pid: Pid) -> Option<ActorInfo> {
     with_runtime(|inner| {
         let slot = inner.slot_at(pid)?;
@@ -141,11 +278,12 @@ pub fn actor_info(pid: Pid) -> Option<ActorInfo> {
     })
 }
 
-/// Build one `ActorInfo` for slot `idx`, or `None` if Vacant or
-/// racing-reclaimed. State is classified from a lock-free word load (the torn
-/// read); the cold lock then pins the generation (reclaim bumps it under that
-/// same lock) so the cold fields are coherent for this incarnation. `mail` is
-/// this slot's registry entry, if any.
+/// Build one `ActorInfo` for slot `idx`, or `None` if the slot is empty or
+/// was reclaimed while this read was in progress. The scheduling state comes
+/// from a lock-free word load (the source of the tearing described in the
+/// module docs); the per-actor lock then confirms the actor has not since
+/// exited and been replaced, so the rest of the fields are coherent for this
+/// exact actor. `mail` is this slot's registry entry, if any.
 fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorInfo> {
     let w = slot.state_word();
     let state = classify(w)?;
@@ -153,10 +291,11 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
     let pid = Pid::new(idx, gen);
 
     let cold = slot.cold.lock();
-    // If the generation moved between the lock-free load and acquiring the cold
-    // lock, the slot was reclaimed (and maybe reused) — drop it rather than mix
-    // one incarnation's state with another's cold data. (ps semantics: a racing
-    // actor may simply be missed mid-scan.)
+    // If the generation moved between the lock-free load and acquiring the
+    // per-actor lock, this actor exited (and the slot may already hold a new
+    // one). Drop it rather than mix one actor's state with another's data; a
+    // racing actor may simply be missed by this scan, which is expected (see
+    // the module docs' "Consistency" section).
     if word_gen(slot.state_word()) != gen {
         return None;
     }
@@ -172,7 +311,15 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
     let joiners = cold.waiters.len() as u32;
     drop(cold);
 
-    // Counters are hot-region atomics, read lock-free (RFC 016 Chunk 2).
+    // Counters are plain atomics, read lock-free.
+    let (reserve, guard, top, hwm, parks_since_shrink, shrinks) = slot.stack_introspect();
+    let stack = StackInfo {
+        reserve,
+        guard,
+        depth_high_water: top.saturating_sub(hwm),
+        parks_since_shrink,
+        shrinks,
+    };
     let overruns = slot.overruns();
     let messages_received = slot.messages_received();
     let budget_cycles = slot.budget_cycles();
@@ -198,45 +345,55 @@ fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorI
         overruns,
         messages_received,
         budget_cycles,
+        stack,
     })
 }
 
 // ---------------------------------------------------------------------------
-// Chunk 3 — tree view (pure derivation over a Chunk-1 snapshot)
+// Tree view: a pure derivation over a snapshot
 // ---------------------------------------------------------------------------
 
-/// One node in the parentage forest. `children` are the actors whose recorded
-/// parent edge points at this node's pid.
+/// One node in the parentage forest returned by [`tree`]. `children` are the
+/// actors whose recorded parent (see [`ActorInfo::supervisor`]) points at
+/// this node's actor.
 #[derive(Debug, Clone)]
 pub struct TreeNode {
     pub info: ActorInfo,
-    /// The actor's recorded parent was absent from the snapshot (already
-    /// Done/Vacant, or itself a tombstone), so it was re-rooted under the forest
-    /// sentinel rather than dropped — the tree stays total (DECISION D8).
+    /// True if this actor's recorded parent was not found in the snapshot
+    /// (it had already exited, or was itself missing), so this node was
+    /// placed at the top of the forest instead of being dropped. This keeps
+    /// every actor in the snapshot visible somewhere in the tree, even one
+    /// whose parent is gone.
     pub orphaned: bool,
     pub children: Vec<TreeNode>,
 }
 
-/// The parentage forest. Roots are actors parented at `ROOT_PID` (genuine
-/// roots) plus re-rooted orphans. The edge is *spawned-by / parent*, not
-/// necessarily supervision (DECISION D9) — see [`ActorInfo::supervisor`].
+/// The parentage forest: every actor from a snapshot, arranged by who spawned
+/// whom. Roots are actors with no parent in the snapshot (including the
+/// run's own root actor) plus any orphaned actors (see [`TreeNode::orphaned`]).
+/// This mirrors spawn parentage, not necessarily a supervision tree; see
+/// [`ActorInfo::supervisor`].
 #[derive(Debug, Clone)]
 pub struct RuntimeTree {
     pub format_version: u16,
     pub roots: Vec<TreeNode>,
 }
 
-/// Take a live [`snapshot`] and fold it into the parentage forest.
+/// Take a fresh [`snapshot`] and fold it into the parentage forest.
 pub fn tree() -> RuntimeTree {
     tree_from(snapshot())
 }
 
-/// Fold an existing snapshot into a forest by grouping each actor under its
-/// parent pid — a single O(n) pass, no new reads. Exposed separately so a
-/// consumer that already holds a snapshot (or a synthetic one, in tests) can
-/// derive the tree without a second scan.
+/// Fold an existing snapshot into a parentage forest by grouping each actor
+/// under its parent, without taking a new snapshot. Useful if you already
+/// have one (for example, one built in a test, or one you took earlier and
+/// want to inspect again) and want the tree view of it without re-reading
+/// the runtime.
 pub fn tree_from(snap: RuntimeSnapshot) -> RuntimeTree {
-    let RuntimeSnapshot { format_version, actors } = snap;
+    let RuntimeSnapshot {
+        format_version,
+        actors,
+    } = snap;
 
     let mut index_of: HashMap<Pid, usize> = HashMap::with_capacity(actors.len());
     for (i, a) in actors.iter().enumerate() {
@@ -254,7 +411,7 @@ pub fn tree_from(snap: RuntimeSnapshot) -> RuntimeTree {
             children_of.entry(parent).or_default().push(i);
         } else {
             // Parent is the forest sentinel (genuine root) or absent from the
-            // snapshot (orphan, D8) — either way a root of the forest.
+            // snapshot (orphan): either way, a root of the forest.
             orphaned[i] = parent != ROOT_PID;
             roots.push(i);
         }
@@ -267,7 +424,10 @@ pub fn tree_from(snap: RuntimeSnapshot) -> RuntimeTree {
         .into_iter()
         .filter_map(|i| build_node(i, &children_of, &orphaned, &mut slots))
         .collect();
-    RuntimeTree { format_version, roots: root_nodes }
+    RuntimeTree {
+        format_version,
+        roots: root_nodes,
+    }
 }
 
 fn build_node(
@@ -285,5 +445,9 @@ fn build_node(
                 .collect()
         })
         .unwrap_or_default();
-    Some(TreeNode { info, orphaned: orphaned[i], children })
+    Some(TreeNode {
+        info,
+        orphaned: orphaned[i],
+        children,
+    })
 }

src/io.rs

@@ -13,44 +13,68 @@
 //! leaves the actor, no copying through an intermediary thread. Built on
 //! these are the conveniences `read(fd, &mut buf)` and `write(fd, &buf)`.
 //!
-//! Architecture
-//! ============
-//! Per `run()`, two OS threads:
-//!   - **epoll thread**: owns the epollfd. Loops in `epoll_wait`. On a
-//!     ready fd, pushes `Completion::FdReady { pid, fd, events }` to the
-//!     shared completion queue and writes the scheduler-wake pipe. On the
-//!     shutdown pipe (also registered in epollfd), exits.
-//!   - **pool thread**: blocks on the request mpsc. Runs the closure
-//!     inside `catch_unwind`, pushes `Completion::Blocking { pid, result }`,
-//!     writes the scheduler-wake pipe.
+//! Architecture (RFC 018: driver-enqueues)
+//! =======================================
+//! Per `run()`, two OS threads, each a *producer* behind the runtime's
+//! two-call contract — make the actor runnable (`unpark_at`, whose enqueue
+//! tail wakes a parked scheduler), nothing else:
 //!
-//! Both threads share a single `completions: Arc<Mutex<VecDeque<Completion>>>`
-//! and the same scheduler-wake pipe.
+//!   - **epoll thread**: owns `epoll_wait` on the epollfd. On a ready fd it
+//!     removes the parked waiter from the shared `waiters` map and DELs the
+//!     fd (both under the waiters lock — see below), then unparks the
+//!     actor directly. On the shutdown pipe (also registered in the
+//!     epollfd), exits.
+//!   - **pool thread**: blocks on the request mpsc. Runs the closure inside
+//!     `catch_unwind`, stashes the result in the actor's slot
+//!     (`pending_io_result`, under the cold lock, generation-checked),
+//!     decrements the runtime's `io_outstanding`, and unparks the actor.
 //!
-//! `epoll_ctl` (register/unregister fd interest) is called by the
-//! scheduler thread *directly* on the epollfd. That's well-defined per
-//! `epoll_ctl(2)`: a thread may be calling `epoll_wait` on the epollfd
-//! while another thread calls `epoll_ctl`. Avoids needing a second mpsc
-//! and a second wake mechanism.
+//! There is no shared completion queue and no wake pipe: each producer
+//! routes its own completion, so the whole byte-vs-completion visibility
+//! discipline of the drain era — and the stranded-completion hazards it
+//! defended against — is unrepresentable. Producers reach the runtime
+//! through a `Weak<RuntimeInner>`: upgraded per completion (the path is
+//! syscall-bound; the refcount op is noise) and avoiding an Arc cycle
+//! through `RuntimeInner::io`.
+//!
+//! `epoll_ctl` (register fd interest) is called by the scheduler thread
+//! directly on the epollfd. That's well-defined per `epoll_ctl(2)`: a
+//! thread may be calling `epoll_wait` on the epollfd while another thread
+//! calls `epoll_ctl`.
 //!
 //! Epoll mode
 //! ==========
 //! Level-triggered with EPOLLONESHOT. After a wakeup the kernel
 //! auto-disarms the fd, so we never get two wakeups for one
-//! `wait_readable` call. The scheduler explicitly `EPOLL_CTL_DEL`s the fd
-//! on completion to free the slot for re-registration. Net effect: each
+//! `wait_readable` call. The epoll thread explicitly `EPOLL_CTL_DEL`s the
+//! fd on readiness to free the slot for re-registration. Net effect: each
 //! `wait_readable(fd)` is one ADD, one wakeup, one DEL — symmetric and
 //! stateless between calls.
 //!
+//! ## The waiters lock is the ADD/DEL serialization
+//!
+//! Registration (scheduler thread: check-vacant, defensive DEL, ADD,
+//! insert) and readiness consumption (epoll thread: remove, DEL) each run
+//! entirely under the `waiters` mutex. This is what makes the
+//! oneshot-rearm race unrepresentable: a woken actor re-registering the
+//! same fd cannot interleave with the epoll thread's DEL for the *previous*
+//! registration — whichever takes the lock second sees a consistent
+//! kernel-side state. Lock order: `io` (the runtime's outer mutex, held by
+//! scheduler-side callers) → `waiters` → slot/queue leaves via `unpark_at`.
+//! The epoll thread takes `waiters` without `io` — it must never take
+//! `io`, both for lock-order hygiene and because teardown holds `io` while
+//! joining it.
+//!
 //! Fd hygiene
 //! ==========
 //! An actor stopped while waiting on an fd unwinds out of `wait_fd`'s park;
 //! a drop guard there (armed after a successful register, forgotten on a
-//! normal wake) removes the `waiters` entry iff it is still that wait's
-//! `(pid, epoch)` and only then `EPOLL_CTL_DEL`s the fd — an entry already
-//! consumed by a racing `FdReady` means the fd may carry someone else's
-//! fresh registration, which must be left alone. `epoll_register` keeps a
-//! defensive bare DEL before ADD as belt-and-braces.
+//! normal wake) calls [`IoThread::cancel_waiter`], which removes the
+//! `waiters` entry iff it is still that wait's `(pid, epoch)` and only then
+//! `EPOLL_CTL_DEL`s the fd — an entry already consumed by the epoll thread
+//! means the fd may carry someone else's fresh registration, which must be
+//! left alone. `epoll_register` keeps a defensive bare DEL before ADD as
+//! belt-and-braces.
 //!
 //! Buffers used with `read`/`write` should be on fds opened with
 //! `O_NONBLOCK`. If they aren't, the syscall may block the scheduler
@@ -68,13 +92,14 @@
 //! they have no equivalent panic-propagation path.
 
 use crate::pid::Pid;
+use crate::runtime::RuntimeInner;
 use std::any::Any;
-use std::collections::{HashMap, VecDeque};
+use std::collections::HashMap;
 use std::io;
 use std::os::fd::RawFd;
 use std::panic;
-use std::sync::mpsc;
-use std::sync::{Arc, Mutex};
+use std::sync::atomic::Ordering;
+use std::sync::{mpsc, Arc, Mutex, Weak};
 use std::thread::JoinHandle as OsJoinHandle;
 
 // ---------------------------------------------------------------------------
@@ -86,45 +111,31 @@ use std::thread::JoinHandle as OsJoinHandle;
 pub type IoResult = Result<Box<dyn Any + Send>, Box<dyn Any + Send>>;
 
 struct Request {
-    /// The submitter's park-epoch — carried through to the `Blocking`
-    /// completion so the wake is epoch-matched.
+    /// The submitter's park-epoch — the eventual wake is epoch-matched.
     epoch: u32,
     pid: Pid,
     /// The work to perform. Returns the wire-form result directly.
     work: Box<dyn FnOnce() -> IoResult + Send>,
 }
 
-/// Completion message from either IO thread back to the scheduler.
-pub enum Completion {
-    /// A `block_on_io` closure has finished (Ok = return value, Err = panic
-    /// payload).
-    Blocking { pid: Pid, epoch: u32, result: IoResult },
-    /// An fd registered via `wait_readable`/`wait_writable` is ready. The
-    /// scheduler looks up the parked pid in `waiters`, unparks it, and
-    /// removes the entry. `pid` isn't in this variant because the epoll
-    /// thread doesn't have access to the `waiters` map; the scheduler
-    /// thread owns that.
-    FdReady { fd: RawFd, events: u32 },
-}
+/// The parked-waiter map, shared between scheduler-side registration and
+/// the epoll thread's readiness consumption. See the module docs on why
+/// this single lock is the ADD/DEL serialization.
+type Waiters = Arc<Mutex<HashMap<RawFd, (Pid, u32)>>>;
 
 // ---------------------------------------------------------------------------
-// IoThread — created per `run()`, owned by `SchedulerState`.
+// IoThread — created per `run()`, owned by `RuntimeInner::io`.
 // ---------------------------------------------------------------------------
 
 pub struct IoThread {
-    // ----- Channels & queues -----
-
     /// Submission queue into the blocking-work pool.
     tx: mpsc::Sender<Request>,
-    /// Shared completion queue, fed by both the pool and the epoll thread.
-    completions: Arc<Mutex<VecDeque<Completion>>>,
-    /// Pipe the scheduler polls in its idle path. Both IO threads write to
-    /// `wake_write` after pushing a completion.
-    wake_read: RawFd,
-    wake_write: RawFd,
+    /// One parked actor per registered fd. Populated by `epoll_register`,
+    /// consumed by the epoll thread on readiness or `cancel_waiter` on an
+    /// unwound wait.
+    waiters: Waiters,
 
     // ----- Epoll machinery -----
-
     /// The epollfd, owned by `IoThread`. Callable cross-thread via
     /// `epoll_ctl` per the man page.
     epollfd: RawFd,
@@ -133,39 +144,24 @@ pub struct IoThread {
     /// shutdown.
     shutdown_read: RawFd,
     shutdown_write: RawFd,
-    /// One parked actor per registered fd. Populated by `wait_readable` /
-    /// `wait_writable` and drained by the scheduler when a `FdReady`
-    /// completion is processed.
-    pub waiters: HashMap<RawFd, (Pid, u32)>,
 
     // ----- Threads -----
-
     pool_thread: Option<OsJoinHandle<()>>,
     epoll_thread: Option<OsJoinHandle<()>>,
-
-    /// Number of `block_on_io` requests in-flight. Used by the scheduler's
-    /// idle path to decide whether to wait on the pipe or exit. Fd waits
-    /// are not counted here; they're counted by `waiters.len()`.
-    pub outstanding: u32,
 }
 
 impl IoThread {
-    pub fn start() -> io::Result<Self> {
-        // Scheduler-facing wake pipe.
-        let (wake_read, wake_write) = make_pipe()?;
-        // Pool submission channel + shared completion queue.
+    /// Start the pool and epoll threads. `rt` is the producers' route back
+    /// into the runtime (slot table + unpark protocol); a `Weak` so the
+    /// `RuntimeInner → IoThread → RuntimeInner` cycle never forms.
+    pub(crate) fn start(rt: Weak<RuntimeInner>) -> io::Result<Self> {
+        // Pool submission channel.
         let (tx, rx) = mpsc::channel::<Request>();
-        let completions: Arc<Mutex<VecDeque<Completion>>> =
-            Arc::new(Mutex::new(VecDeque::new()));
+        let waiters: Waiters = Arc::new(Mutex::new(HashMap::new()));
 
         // Epoll machinery.
         let epollfd = unsafe { libc::epoll_create1(libc::EPOLL_CLOEXEC) };
         if epollfd < 0 {
-            // Best-effort fd cleanup before bailing.
-            unsafe {
-                libc::close(wake_read);
-                libc::close(wake_write);
-            }
             return Err(io::Error::last_os_error());
         }
 
@@ -174,8 +170,6 @@ impl IoThread {
             Err(e) => {
                 unsafe {
                     libc::close(epollfd);
-                    libc::close(wake_read);
-                    libc::close(wake_write);
                 }
                 return Err(e);
             }
@@ -202,42 +196,37 @@ impl IoThread {
                 libc::close(epollfd);
                 libc::close(shutdown_read);
                 libc::close(shutdown_write);
-                libc::close(wake_read);
-                libc::close(wake_write);
             }
             return Err(e);
         }
 
         // Spawn pool thread.
-        let pool_comps = completions.clone();
+        let pool_rt = rt.clone();
         let pool_thread = std::thread::Builder::new()
             .name("smarm-io-pool".into())
-            .spawn(move || pool_loop(rx, pool_comps, wake_write))?;
+            .spawn(move || pool_loop(rx, pool_rt))?;
 
         // Spawn epoll thread.
-        let epoll_comps = completions.clone();
+        let epoll_waiters = waiters.clone();
         let epoll_thread = std::thread::Builder::new()
             .name("smarm-io-epoll".into())
-            .spawn(move || epoll_loop(epollfd, epoll_comps, wake_write))?;
+            .spawn(move || epoll_loop(epollfd, epoll_waiters, rt))?;
 
         Ok(Self {
             tx,
-            completions,
-            wake_read,
-            wake_write,
+            waiters,
             epollfd,
             shutdown_read,
             shutdown_write,
-            waiters: HashMap::new(),
             pool_thread: Some(pool_thread),
             epoll_thread: Some(epoll_thread),
-            outstanding: 0,
         })
     }
 
-    /// Hand a request to the pool. Increments `outstanding`.
+    /// Hand a request to the pool. The caller (scheduler.rs) increments
+    /// `io_outstanding` BEFORE calling — the pool decrements on completion,
+    /// and an increment that trailed the completion would underflow.
     pub fn submit(&mut self, pid: Pid, epoch: u32, work: Box<dyn FnOnce() -> IoResult + Send>) {
-        self.outstanding += 1;
         // Send can only fail if the pool has hung up, which only happens
         // on shutdown. submit during shutdown is a bug.
         if self.tx.send(Request { pid, epoch, work }).is_err() {
@@ -245,39 +234,13 @@ impl IoThread {
         }
     }
 
-    /// Drain every available completion. Caller (the scheduler) routes the
-    /// results and updates `outstanding` / `waiters` accordingly.
-    pub fn drain_completions(&mut self) -> Vec<Completion> {
-        let mut q = match self.completions.lock() {
-            Ok(g) => g,
-            Err(e) => panic!("smarm: io completions lock poisoned (core corrupt): {e}"),
-        };
-        let mut out = Vec::with_capacity(q.len());
-        while let Some(c) = q.pop_front() {
-            out.push(c);
-        }
-        out
-    }
-
-    pub fn wake_fd(&self) -> RawFd {
-        self.wake_read
-    }
-
-    /// Write the wake pipe directly: rouse every scheduler thread blocked in
-    /// its idle `poll_wake`. Used by the terminal (AllDone) path — an idle
-    /// sibling may be blocked on a snapshot that nothing will ever refresh
-    /// (an orphaned timer deadline, or `io_outstanding` from a waiter that
-    /// was stop-cancelled and so never produces a completion).
-    pub fn wake(&self) {
-        wake_scheduler(self.wake_write);
-    }
-
     /// Register interest in `fd` becoming readable/writable; record `pid`
-    /// as the parked waiter. The epoll thread will push a `FdReady`
-    /// completion when the kernel signals.
+    /// as the parked waiter. The epoll thread unparks it on readiness.
+    /// The caller increments `io_fd_waiters` BEFORE calling (mirror of
+    /// `submit`'s contract) and decrements it again if this errors.
     ///
-    /// EPOLLONESHOT: one wakeup per registration. The scheduler must
-    /// `epoll_del` on completion to free the slot for re-registration.
+    /// EPOLLONESHOT: one wakeup per registration; the epoll thread DELs on
+    /// readiness, `cancel_waiter` DELs on an unwound wait.
     pub fn epoll_register(
         &mut self,
         fd: RawFd,
@@ -286,20 +249,24 @@ impl IoThread {
         readable: bool,
         writable: bool,
     ) -> io::Result<()> {
+        let mut waiters = match self.waiters.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: io waiters lock poisoned (core corrupt): {e}"),
+        };
         // Two actors waiting on the same fd would be a misuse: the kernel
         // delivers exactly one EPOLLONESHOT wakeup, so the second waiter
         // would hang. Reject up front.
-        if self.waiters.contains_key(&fd) {
+        if waiters.contains_key(&fd) {
             return Err(io::Error::new(
                 io::ErrorKind::AlreadyExists,
                 "fd already has a parked waiter",
             ));
         }
 
-        // Belt-and-braces: the unwind guard in `wait_fd` is responsible for
-        // cleaning up a stopped waiter's registration, but a bare DEL is
-        // harmless if the fd isn't registered (ENOENT) and removes any leak
-        // a path we haven't thought of might leave behind.
+        // Belt-and-braces: `cancel_waiter` is responsible for cleaning up a
+        // stopped waiter's registration, but a bare DEL is harmless if the
+        // fd isn't registered (ENOENT) and removes any leak a path we
+        // haven't thought of might leave behind.
         unsafe {
             libc::epoll_ctl(self.epollfd, libc::EPOLL_CTL_DEL, fd, std::ptr::null_mut());
         }
@@ -315,26 +282,35 @@ impl IoThread {
             events,
             u64: fd as u64,
         };
-        let r = unsafe {
-            libc::epoll_ctl(self.epollfd, libc::EPOLL_CTL_ADD, fd, &mut ev as *mut _)
-        };
+        let r =
+            unsafe { libc::epoll_ctl(self.epollfd, libc::EPOLL_CTL_ADD, fd, &mut ev as *mut _) };
         if r < 0 {
             return Err(io::Error::last_os_error());
         }
-        self.waiters.insert(fd, (pid, epoch));
+        waiters.insert(fd, (pid, epoch));
         Ok(())
     }
 
-    /// Remove `fd` from the epollfd. Called by the scheduler after a
-    /// `FdReady` completion, so the next `wait_readable(fd)` can ADD again.
-    ///
-    /// Does NOT touch `waiters` — that's the scheduler's bookkeeping; this
-    /// is purely the kernel-side cleanup.
-    pub fn epoll_deregister(&mut self, fd: RawFd) {
+    /// Remove `fd`'s waiter iff it is still `(pid, epoch)`, DELing the fd
+    /// from the epollfd in the same critical section. Returns whether the
+    /// entry was removed (the caller then decrements `io_fd_waiters`).
+    /// `false` means the epoll thread consumed the registration first —
+    /// the fd may already carry someone else's fresh ADD; hands off.
+    pub fn cancel_waiter(&mut self, fd: RawFd, pid: Pid, epoch: u32) -> bool {
+        let mut waiters = match self.waiters.lock() {
+            Ok(g) => g,
+            Err(e) => panic!("smarm: io waiters lock poisoned (core corrupt): {e}"),
+        };
+        if waiters.get(&fd) == Some(&(pid, epoch)) {
+            waiters.remove(&fd);
             // EPOLL_CTL_DEL of an already-removed fd returns ENOENT; ignore.
             unsafe {
                 libc::epoll_ctl(self.epollfd, libc::EPOLL_CTL_DEL, fd, std::ptr::null_mut());
             }
+            true
+        } else {
+            false
+        }
     }
 }
 
@@ -354,7 +330,10 @@ impl Drop for IoThread {
         let real_tx = std::mem::replace(&mut self.tx, dead_tx);
         drop(real_tx);
 
-        // 3. Join both threads.
+        // 3. Join both threads. Safe even while the caller holds the
+        //    runtime's `io` mutex: neither thread ever takes it (they reach
+        //    the runtime through a Weak they upgrade per completion, and
+        //    the epoll thread's only lock is `waiters`).
         if let Some(h) = self.epoll_thread.take() {
             let _ = h.join();
         }
@@ -367,8 +346,6 @@ impl Drop for IoThread {
             libc::close(self.epollfd);
             libc::close(self.shutdown_read);
             libc::close(self.shutdown_write);
-            libc::close(self.wake_read);
-            libc::close(self.wake_write);
         }
     }
 }
@@ -379,36 +356,38 @@ impl Drop for IoThread {
 const SHUTDOWN_EPOLL_TOKEN: u64 = u64::MAX;
 
 // ---------------------------------------------------------------------------
-// Pool loop
+// Pool loop (producer: Blocking completions)
 // ---------------------------------------------------------------------------
 
-fn pool_loop(
-    rx: mpsc::Receiver<Request>,
-    completions: Arc<Mutex<VecDeque<Completion>>>,
-    wake_write: RawFd,
-) {
+fn pool_loop(rx: mpsc::Receiver<Request>, rt: Weak<RuntimeInner>) {
     while let Ok(Request { pid, epoch, work }) = rx.recv() {
         let result: IoResult = match panic::catch_unwind(panic::AssertUnwindSafe(work)) {
             Ok(r) => r,
             Err(payload) => Err(payload),
         };
-        match completions.lock() {
-            Ok(mut g) => g.push_back(Completion::Blocking { pid, epoch, result }),
-            Err(e) => panic!("smarm: io completions lock poisoned (core corrupt): {e}"),
+        let Some(inner) = rt.upgrade() else { return };
+        // Stash the result under the cold lock (generation-checked: an
+        // actor stopped with the op in flight discards it), decrement the
+        // in-flight count, then wake through the epoch-matched unpark. The
+        // unpark's enqueue tail wakes a parked scheduler; the actor stays
+        // `live` until it resumes and finalizes, so the decrement's
+        // ordering against the termination verdict is not load-bearing.
+        if let Some(slot) = inner.slot_at(pid) {
+            let mut cold = slot.cold.lock();
+            if slot.generation() == pid.generation() {
+                cold.pending_io_result = Some(result);
             }
-        wake_scheduler(wake_write);
+        }
+        inner.io_outstanding.fetch_sub(1, Ordering::AcqRel);
+        inner.unpark_at(pid, epoch);
     }
 }
 
 // ---------------------------------------------------------------------------
-// Epoll loop
+// Epoll loop (producer: FdReady completions)
 // ---------------------------------------------------------------------------
 
-fn epoll_loop(
-    epollfd: RawFd,
-    completions: Arc<Mutex<VecDeque<Completion>>>,
-    wake_write: RawFd,
-) {
+fn epoll_loop(epollfd: RawFd, waiters: Waiters, rt: Weak<RuntimeInner>) {
     // Buffer for epoll_wait. 64 is plenty for our scale; if a real load
     // appears that needs more, this is a one-line change.
     const MAX_EVENTS: usize = 64;
@@ -416,12 +395,7 @@ fn epoll_loop(
 
     loop {
         let n = unsafe {
-            libc::epoll_wait(
-                epollfd,
-                events.as_mut_ptr(),
-                MAX_EVENTS as libc::c_int,
-                -1,
-            )
+            libc::epoll_wait(epollfd, events.as_mut_ptr(), MAX_EVENTS as libc::c_int, -1)
         };
 
         if n < 0 {
@@ -436,29 +410,36 @@ fn epoll_loop(
         }
 
         let mut shutdown_requested = false;
-        let mut pushed_any = false;
-        {
-            let mut q = match completions.lock() {
-                Ok(g) => g,
-                Err(e) => panic!("smarm: io completions lock poisoned (core corrupt): {e}"),
-            };
         for ev in events.iter().take(n as usize) {
             if ev.u64 == SHUTDOWN_EPOLL_TOKEN {
                 shutdown_requested = true;
                 continue;
             }
             let fd = ev.u64 as RawFd;
-                let evs = ev.events;
-                q.push_back(Completion::FdReady {
-                    fd,
-                    events: evs,
-                });
-                pushed_any = true;
+            // Consume the registration: remove + DEL under the waiters
+            // lock (the ADD/DEL serialization — see module docs). A
+            // vanished entry means `cancel_waiter` beat us: the wake is
+            // already moot.
+            let entry = {
+                let mut w = match waiters.lock() {
+                    Ok(g) => g,
+                    Err(e) => {
+                        panic!("smarm: io waiters lock poisoned (core corrupt): {e}")
+                    }
+                };
+                let entry = w.remove(&fd);
+                if entry.is_some() {
+                    unsafe {
+                        libc::epoll_ctl(epollfd, libc::EPOLL_CTL_DEL, fd, std::ptr::null_mut());
                     }
                 }
-
-        if pushed_any {
-            wake_scheduler(wake_write);
+                entry
+            };
+            if let Some((pid, epoch)) = entry {
+                let Some(inner) = rt.upgrade() else { return };
+                inner.io_fd_waiters.fetch_sub(1, Ordering::AcqRel);
+                inner.unpark_at(pid, epoch);
+            }
         }
         if shutdown_requested {
             return;
@@ -466,27 +447,8 @@ fn epoll_loop(
     }
 }
 
-/// Write one byte to the scheduler's wake pipe. Retries on EINTR; ignores
-/// EAGAIN (pipe full means there's already an outstanding wake we haven't
-/// consumed yet, which is sufficient).
-fn wake_scheduler(wake_write: RawFd) {
-    let buf: [u8; 1] = [0];
-    unsafe {
-        loop {
-            let n = libc::write(wake_write, buf.as_ptr() as *const _, 1);
-            if n < 0 {
-                let e = *libc::__errno_location();
-                if e == libc::EINTR {
-                    continue;
-                }
-            }
-            break;
-        }
-    }
-}
-
 // ---------------------------------------------------------------------------
-// Pipe helpers (unchanged from v0.2)
+// Pipe helper
 // ---------------------------------------------------------------------------
 
 fn make_pipe() -> io::Result<(RawFd, RawFd)> {
@@ -497,50 +459,3 @@ fn make_pipe() -> io::Result<(RawFd, RawFd)> {
     }
     Ok((fds[0], fds[1]))
 }
-
-/// Drain pending bytes from the wake pipe. Nonblocking (pipe is O_NONBLOCK).
-///
-/// DISCIPLINE: called only by the phase-1 drain-lock winner, immediately
-/// before `drain_completions`. Bytes are the notification channel for
-/// completions; consuming one anywhere else can strand the completion it
-/// announces (see the lost-wakeup note at the call site in `schedule_loop`).
-pub fn drain_wake_pipe(fd: RawFd) {
-    let mut buf = [0u8; 64];
-    loop {
-        let n = unsafe { libc::read(fd, buf.as_mut_ptr() as *mut _, buf.len()) };
-        if n <= 0 {
-            break;
-        }
-    }
-}
-
-/// Block on `fd` for up to `timeout`, returning when either there's data
-/// to read or the timeout elapses. `None` for `timeout` means wait forever.
-pub fn poll_wake(fd: RawFd, timeout: Option<std::time::Duration>) {
-    let timeout_ms: libc::c_int = match timeout {
-        None => -1,
-        Some(d) => {
-            let ms = d.as_millis();
-            if ms > i32::MAX as u128 {
-                i32::MAX
-            } else {
-                ms as i32
-            }
-        }
-    };
-    let mut pfd = libc::pollfd {
-        fd,
-        events: libc::POLLIN,
-        revents: 0,
-    };
-    loop {
-        let r = unsafe { libc::poll(&mut pfd as *mut _, 1, timeout_ms) };
-        if r < 0 {
-            let e = unsafe { *libc::__errno_location() };
-            if e == libc::EINTR {
-                continue;
-            }
-        }
-        break;
-    }
-}

src/lib.rs

@@ -11,34 +11,36 @@
 //!
 //! See `LOOM.md` for the design intent and the deferred-for-later list.
 
-pub mod stack;
-pub mod context;
-pub mod preempt;
-pub mod pid;
 pub mod actor;
+pub mod causal;
 pub mod channel;
-pub mod scheduler;
-pub mod supervisor;
-pub mod timer;
-pub mod io;
-pub mod mutex;
-pub mod monitor;
-pub mod registry;
-pub mod pg;
-pub mod link;
+pub mod context;
 pub mod gen_server;
 pub mod gen_statem;
 pub mod introspect;
+pub mod io;
+pub mod link;
+pub mod monitor;
+pub mod mutex;
 #[cfg(feature = "observer")]
 pub mod observer;
-pub mod runtime;
+pub(crate) mod park;
+pub mod pg;
+pub mod pid;
+pub mod preempt;
 pub(crate) mod raw_mutex;
-pub(crate) mod slot_state;
-pub(crate) mod sync_shim;
+pub mod registry;
 #[doc(hidden)] // pub only so benches/rq_micro.rs can drive the raw structures
 pub mod run_queue;
+pub mod runtime;
+pub mod scheduler;
+pub(crate) mod signal;
+pub(crate) mod slot_state;
+pub mod stack;
+pub mod supervisor;
+pub(crate) mod sync_shim;
+pub mod timer;
 pub mod trace;
-pub mod causal;
 
 // ---------------------------------------------------------------------------
 // Global allocator
@@ -57,33 +59,39 @@ pub use channel::{
 };
 pub use gen_server::{
     call, cast, shutdown, whereis_server, CallError, CallTimeoutError, CastError, GenServer,
-    NamedGenServerBuilder, GenServerBuilder, GenServerCtx, GenServerName, GenServerRef, TimerHandle, Watcher,
+    GenServerBuilder, GenServerCtx, GenServerName, GenServerRef, NamedGenServerBuilder,
+    TimerHandle, Watcher,
 };
 pub use gen_statem::{
-    CallError as GenStatemCallError, Cx, Machine, Reply, Resolution, SendError as GenStatemSendError,
-    GenStatemRef,
+    CallError as GenStatemCallError, Cx, GenStatemRef, Machine, Reply, Resolution,
+    SendError as GenStatemSendError,
 };
 pub use introspect::{
     actor_info, snapshot, tree, tree_from, ActorInfo, ActorState, RuntimeSnapshot, RuntimeTree,
-    TreeNode, SNAPSHOT_FORMAT_VERSION,
+    StackInfo, TreeNode, SNAPSHOT_FORMAT_VERSION,
 };
+pub use link::{link, trap_exit, unlink, ExitSignal};
+pub use monitor::{
+    demonitor, mark_watchable, monitor, terminal_reason, Down, DownReason, Monitor, MonitorId,
+};
+pub use mutex::{LockTimeout, Mutex, MutexGuard};
 #[cfg(feature = "observer")]
 pub use observer::{ObserverReply, ObserverRequest};
-pub use link::{link, trap_exit, unlink, ExitSignal};
-pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
-pub use mutex::{LockTimeout, Mutex, MutexGuard};
+pub use pg::{
+    dispatch, join, leave, members, members_as, pick, pick_as, Incarnation, Member, NodeId,
+};
 pub use pid::{Addressable, Erased, Name, Pid, RawPid};
-pub use pg::{dispatch, join, leave, members, members_as, pick, pick_as, Incarnation, Member, NodeId};
 pub use registry::{
-    install, lookup_as, register, send, send_dyn, send_to, unregister, whereis, RegisterError,
-    SendError,
+    install, lookup_as, register, resolve_name, send, send_dyn, send_to, unregister, whereis,
+    NameResolution, RegisterError, SendError,
 };
 pub use runtime::{init, Config, Runtime};
 pub use scheduler::{
     block_on_io, cancel_timer, request_stop, run, self_pid, send_after, send_after_named,
-    send_after_named_wall, send_after_wall, sleep, sleep_wall,
-    spawn, spawn_addr, spawn_under, wait_readable, wait_readable_timeout, wait_writable,
-    wait_writable_timeout, yield_now, FdArm, JoinError, JoinHandle,
+    send_after_named_wall, send_after_wall, sleep, sleep_wall, spawn, spawn_addr, spawn_addr_with,
+    spawn_under, spawn_under_with, spawn_with, try_spawn, try_spawn_under_with, wait_readable,
+    wait_readable_timeout, wait_writable, wait_writable_timeout, yield_now, FdArm, JoinError,
+    JoinHandle, SpawnError, SpawnOpts,
 };
 pub use supervisor::{ChildSpec, OneForOne, Restart, Signal, Strategy};
 pub use timer::TimerId;

src/link.rs

@@ -157,7 +157,10 @@ pub fn link<A>(target: Pid<A>) {
     });
     match my_trap {
         Some(tx) => {
-            let _ = tx.send(ExitSignal { from: target, reason: DownReason::NoProc });
+            let _ = tx.send(ExitSignal {
+                from: target,
+                reason: DownReason::NoProc,
+            });
         }
         None => request_stop(me),
     }

src/monitor.rs

@@ -1,49 +1,85 @@
-//! Process monitors.
+//! Find out when another actor dies, without it knowing or caring that you're
+//! watching.
 //!
-//! `monitor(target)` asks the runtime to deliver a single [`Down`] when
-//! `target` terminates, and hands back a [`Monitor`] — the [`Receiver`] to read
-//! it from, plus the identity (`id`, `target`) needed to take the registration
-//! back down with [`demonitor`]. A monitor is:
+//! Say one actor manages a pool of workers and needs to know when a worker
+//! exits, so it can replace it. The worker does not need to know it is being
+//! watched, and nothing about the worker's own behavior should change because
+//! someone is watching it. That is what [`monitor`] is for: call
+//! `monitor(target)` to get a [`Monitor`], and read exactly one [`Down`]
+//! message off `monitor.rx` whenever `target` terminates, however it
+//! terminates.
 //!
-//!   - **unidirectional** — the watcher learns of the target's death, but the
-//!     target learns nothing of the watcher, and the watcher is unaffected by
-//!     the death beyond the notification (contrast a *link*, which propagates
-//!     failure);
-//!   - **one-shot** — exactly one `Down` is ever sent for a given monitor.
-//!     The returned channel closes afterwards, so a second `recv()` yields
-//!     `Err(RecvError)`.
+//! ```
+//! use smarm::{monitor, run, spawn, DownReason};
 //!
-//! This generalizes the older single-`supervisor_channel` mechanism: a
-//! supervisor is just a hard-wired monitor that the parent installs at spawn
-//! time. Here any actor may monitor any pid, any number of times.
+//! run(|| {
+//!     let worker = spawn(|| {
+//!         // does some work, then returns
+//!     });
+//!     let pid = worker.pid();
 //!
-//! ## Reasons
+//!     let m = monitor(pid);
+//!     let _ = worker.join();
 //!
-//! [`DownReason`] is deliberately payload-free. A panicking actor's payload
-//! has a single owner and is delivered to whoever `join()`s the actor (as
-//! `JoinError`); a monitor only learns *that* it panicked, not the value.
-//! Monitoring a pid that is already gone (reclaimed, or never alive) yields
-//! [`DownReason::NoProc`] immediately, mirroring Erlang's `noproc`.
+//!     let down = m.rx.recv().expect("monitor channel closed before Down");
+//!     assert_eq!(down.pid, pid);
+//!     assert_eq!(down.reason, DownReason::Exit);
+//! });
+//! ```
 //!
-//! ## Demonitoring
+//! A monitor is one-directional and one-shot:
 //!
-//! Each `monitor()` registration is tagged with a process-unique [`MonitorId`].
-//! [`demonitor`] removes the registration named by a [`Monitor`] from its
-//! target's slot, returning `Some(id)` if a live registration was found or
-//! `None` if it had already fired (or the target is gone). Dropping the
-//! [`Monitor`] afterwards discards any `Down` that the target had *already*
-//! queued — the equivalent of Erlang's `demonitor(Ref, [flush])`.
+//! - **One-directional**: the watcher learns that the target died, but the
+//!   target is completely unaffected. It never learns it was being watched,
+//!   and its own behavior and lifetime do not change because of the monitor.
+//!   This is the opposite of a [`link`](mod@crate::link), which is bidirectional:
+//!   linking two actors means an abnormal death on either side can bring the
+//!   other down too. Reach for a monitor when you just want to *know*; reach
+//!   for a link when a peer's crash should actually stop you.
+//! - **One-shot**: you get exactly one [`Down`] per `monitor()` call, then the
+//!   channel closes. Calling `monitor` again on the same target (or a
+//!   different one) gives you an independent registration with its own
+//!   [`Monitor`] and its own one-shot channel; nothing stops you from
+//!   monitoring the same actor many times over; each call is watched and
+//!   fires on its own.
 //!
-//! ## Races
+//! ## Why a monitor never hands you the panic value
 //!
-//! Registration (below) and `finalize_actor` (in `runtime`) both run under the
-//! shared-state mutex, so a target that is still alive when its monitor is
-//! registered is guaranteed to deliver a real `Down`; there is no window in
-//! which the death slips between the liveness check and the registration.
-//! `demonitor` is protected by the generation half of the pid: if the target
-//! has died and its slot index been recycled, `slot_mut(target)` fails the
-//! generation check and `demonitor` is a clean no-op — it can never strip a
-//! *different* actor's monitor that happens to share the slot index.
+//! If the target panicked, [`Down`] tells you *that* it panicked
+//! ([`DownReason::Panic`]), but not the panic's payload. The payload has a
+//! single owner: it is handed to whichever caller `join()`s the actor's
+//! [`JoinHandle`](crate::JoinHandle), as a `JoinError`. A monitor only needs
+//! to know that something went wrong, not reproduce the exact value that
+//! caused it, so it gets the reason and nothing else.
+//!
+//! Monitoring a target that is already gone (it finished and was cleaned up,
+//! or the pid never pointed at a real actor) is not an error: you get a
+//! [`Down`] with [`DownReason::NoProc`] right away, instead of waiting
+//! forever for something that already happened.
+//!
+//! ## Stopping a monitor early
+//!
+//! [`demonitor`] cancels a monitor before it fires. If the registration was
+//! still live, it removes it and returns `Some` of the monitor's id: no
+//! `Down` will arrive on that channel from here on. If the target had already
+//! died and its `Down` already sent, there is nothing left to cancel and
+//! `demonitor` returns `None`; the `Down` you already have (or that is
+//! already sitting in the channel) is unaffected.
+//!
+//! If you want to cancel *and* make sure a `Down` that already arrived is
+//! discarded without reading it, just drop the [`Monitor`]: dropping it closes
+//! its receiver, and any queued `Down` is dropped along with it.
+//!
+//! ## Correctness notes for implementers
+//!
+//! A target that is still alive at the moment `monitor()` registers is
+//! guaranteed to eventually produce a real `Down`: registration and the
+//! target's own termination bookkeeping run under the same lock, so there is
+//! no window in which the target could die without the just-added
+//! registration seeing it. `demonitor` is similarly race-free against a target
+//! that has since died and had its slot reused by a new, unrelated actor: it
+//! is checked against the exact monitored incarnation, so it can never remove
+//! a different actor's registration by accident, it simply reports `None`.
 
 use crate::channel::{channel, Receiver, Sender};
 use crate::pid::Pid;
@@ -51,8 +87,8 @@ use crate::scheduler::with_runtime;
 
 /// Why a monitored actor went down.
 ///
-/// `Copy` because it carries no payload — see the module docs for why the
-/// panic payload is *not* included here.
+/// Carries no payload: see the module docs for why a monitor never receives
+/// the panic value itself.
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum DownReason {
     /// The target returned normally.
@@ -76,21 +112,22 @@ pub struct Down {
     pub reason: DownReason,
 }
 
-/// A process-unique identifier for one `monitor()` registration.
+/// A unique identifier for one [`monitor`] registration.
 ///
-/// Opaque and `Copy`. Allocated from a monotonic counter in shared state, so
-/// it is never reused for the lifetime of the runtime — distinct `monitor()`
-/// calls on the same target get distinct ids, which is what lets [`demonitor`]
-/// tear down exactly one of several monitors on a target.
+/// Opaque and `Copy`. Never reused for the life of the runtime, so if you
+/// monitor the same target more than once, each call's id is distinct. This
+/// is what lets [`demonitor`] tear down exactly one of several monitors on
+/// the same target without disturbing the others.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct MonitorId(pub(crate) u64);
 
 /// A live monitor: the receiving end of the one-shot [`Down`] channel, plus the
 /// identity needed to [`demonitor`] it.
 ///
-/// Read the notification from [`Monitor::rx`]. Not `Clone` (the receiver is a
-/// single consumer). Dropping it closes the receiving end; if a `Down` was
-/// already queued it is discarded with the channel.
+/// Read the notification from [`Monitor::rx`]. Not `Clone`, since only one
+/// side is meant to consume it. Dropping a `Monitor` closes the receiving
+/// end; if a `Down` had already arrived but was never read, it is discarded
+/// along with it.
 pub struct Monitor {
     /// This registration's process-unique id.
     pub id: MonitorId,
@@ -110,10 +147,11 @@ pub fn monitor<A>(target: Pid<A>) -> Monitor {
     let target = target.erase();
     let (tx, rx) = channel::<Down>();
 
-    // Register under the target's cold lock. `tx.clone()` takes the channel's
-    // own lock — a Channel-class RawMutex, explicitly permitted *under* a Leaf
-    // (cold) lock by the lock order (see raw_mutex.rs). We must still not
-    // *send* under the lock, as `Sender::send` can unpark a parked receiver,
+    // Implementation note: registration happens under the target's cold
+    // lock. `tx.clone()` takes the channel's own lock, a Channel-class
+    // RawMutex, which is explicitly permitted under a Leaf (cold) lock by
+    // the lock order documented in raw_mutex.rs. We must still not *send*
+    // under the lock, since `Sender::send` can unpark a parked receiver,
     // and there's no reason to nest that.
     let (id, registered) = with_runtime(|inner| {
         let id = inner.alloc_monitor_id();
@@ -133,26 +171,89 @@ pub fn monitor<A>(target: Pid<A>) -> Monitor {
     });
 
     if !registered {
-        let _ = tx.send(Down { pid: target, reason: DownReason::NoProc });
+        let _ = tx.send(Down {
+            pid: target,
+            reason: DownReason::NoProc,
+        });
     }
 
     Monitor { id, target, rx }
 }
 
-/// Cancel the monitor `m`. Returns `Some(id)` if a live registration was found
-/// on the target's slot and removed, or `None` if there was nothing to remove
-/// — the target already fired its `Down` (the registration is drained on
-/// finalize), was never alive (`NoProc`), or has been reclaimed.
+/// Flag `target`'s tenancy as watchable: its death will stamp the slot's
+/// terminal record (see [`terminal_reason`]), exactly as registering a name
+/// does. The bridge calls this wherever a smarm pid is *encoded across the
+/// boundary* — a contract reply, an introspection listing — because BEAM can
+/// only watch pids it holds, and can only hold pids that crossed. Keeping the
+/// bit rare is what keeps the record alive: anonymous never-exported churn
+/// (holder threads, egress tasks) stays ineligible and cannot evict a
+/// watchable tenancy's record from a LIFO-recycled slot.
 ///
-/// This stops any *future* `Down`. To also discard a `Down` the target may have
-/// *already* queued (the finalize-races-demonitor case), drop `m` afterwards;
-/// dropping the [`Monitor`] closes its receiver and the queued notice goes with
-/// it — the analogue of Erlang's `demonitor(Ref, [flush])`.
+/// Generation-checked and live-screened: marking a pid whose tenancy already
+/// ended is a no-op — its record either exists (it was flagged before dying)
+/// or is honestly unknowable. Same `Runtime::run()` context contract as
+/// [`monitor`].
+pub fn mark_watchable<A>(target: Pid<A>) {
+    let target = target.erase();
+    with_runtime(|inner| {
+        if let Some(slot) = inner.slot_at(target) {
+            // Cold lock FIRST: finalize publishes Done and checks the
+            // watchable bit under this same lock, so the mark either lands
+            // before finalize reads it (the death stamps) or observes the
+            // tenancy already dead (no-op). No lost-stamp window between an
+            // unlocked liveness read and the flag set.
+            let mut cold = slot.cold.lock();
+            if slot.is_live_for(target) {
+                cold.watchable = true;
+            }
+        }
+    });
+}
+
+/// The terminal [`DownReason`] of the tenancy `target` names, if that tenancy
+/// ever registered a name and is the *most recent named* death of its slot:
+/// finalize stamps the slot with `(generation, reason)` for once-registered
+/// tenancies (anonymous green-thread churn does not stamp — nor evict), and
+/// the record survives reclaim and the next tenant's install, until the next
+/// *named* tenant of the slot itself dies. `None` means the pid never lived,
+/// is still alive, never held a name, or its record was overwritten by a
+/// later named tenancy's death — callers fall back to `NoProc` semantics.
+///
+/// This exists for watch-installers that raced their target's death (bridge
+/// soak signature 4): a `NoProc` observed at install time can be upgraded to
+/// the real reason while the record still matches, which is exactly what an
+/// install that had won the race would have delivered. It does NOT change
+/// [`monitor`]'s own semantics — monitoring a stale pid still queues `NoProc`,
+/// the same shape Erlang gives — the upgrade is the caller's deliberate act.
+/// Same context contract as [`monitor`]: must run inside `Runtime::run()`.
+pub fn terminal_reason<A>(target: Pid<A>) -> Option<DownReason> {
+    let target = target.erase();
+    with_runtime(|inner| {
+        let slot = inner.slot_at(target)?;
+        let cold = slot.cold.lock();
+        match cold.terminal {
+            Some((generation, reason)) if generation == target.generation() => Some(reason),
+            _ => None,
+        }
+    })
+}
+
+/// Cancel the monitor `m`. Returns `Some(id)` if a live registration was found
+/// and removed, so no `Down` will arrive on `m.rx` from here on. Returns
+/// `None` if there was nothing left to remove: the target had already gone
+/// down and its `Down` was already sent (or is already sitting in the
+/// channel, unread).
+///
+/// This only stops a *future* `Down`. If you also want to discard a `Down`
+/// that already arrived (or is about to, in a race with this call), drop `m`
+/// instead of, or in addition to, calling this: dropping the [`Monitor`]
+/// closes its receiver and any queued notice is discarded with it.
 pub fn demonitor(m: &Monitor) -> Option<MonitorId> {
-    // Remove the registration under the target's cold lock, but move the
-    // `Sender` *out* and let it drop only after the lock is released:
-    // dropping the last sender runs `Sender::drop`, which may unpark a parked
-    // receiver — legal under a cold lock, but pointless to nest.
+    // Implementation note: the registration is removed under the target's
+    // cold lock, but the `Sender` is moved *out* and dropped only after the
+    // lock is released. Dropping the last sender runs `Sender::drop`, which
+    // may unpark a parked receiver; legal under a cold lock, but pointless
+    // to nest.
     let removed: Option<(MonitorId, Sender<Down>)> = with_runtime(|inner| {
         let slot = inner.slot_at(m.target)?;
         let mut cold = slot.cold.lock();

src/mutex.rs

@@ -1,12 +1,89 @@
-//! Actor-aware mutex with mandatory timeout.
+//! Shared mutable state across actors, when a channel is overkill.
 //!
-//! `Mutex<T>` parks the calling *green* thread on contention rather than
-//! blocking the OS thread. Every lock attempt is bounded by a timeout.
+//! smarm actors normally coordinate by sending messages, and for a piece of
+//! owned state the right tool is usually a `gen_server`: one actor holds the
+//! data and everyone else talks to it. Sometimes that is more machinery than
+//! you need, and plain shared, lockable state is simpler: [`Mutex<T>`] is
+//! that escape hatch. It behaves like `std::sync::Mutex<T>`, guarding a value
+//! of type `T` behind a guard that gives you `&mut T` while held, but it is
+//! built for smarm's actors rather than OS threads.
 //!
-//! Internals use `Arc<std::sync::Mutex<...>>` so the type is genuinely
-//! `Send + Sync` and can be shared across scheduler threads.
+//! The key difference from `std::sync::Mutex` is what happens on contention.
+//! [`Mutex::lock`] parks the calling actor (a cooperatively scheduled green
+//! thread) rather than blocking the underlying OS thread, so other actors on
+//! the same OS thread keep running while it waits. And every lock attempt is
+//! bounded by a timeout: an actor that hangs on to the lock forever (stuck in
+//! a bug, or just slow) would otherwise wedge every other actor waiting on
+//! it, so smarm makes the wait bounded by default instead of leaving it up
+//! to you to remember.
 //!
-//! Fairness: FIFO. Poisoning: none. Reentrance: deadlock (caller bug).
+//! ## A first lock
+//!
+//! ```
+//! use smarm::{run, spawn, Mutex};
+//!
+//! run(|| {
+//!     let counter = Mutex::new(0u32);
+//!
+//!     // Mutex::clone() is cheap and hands out another handle to the SAME
+//!     // underlying value, much like Arc::clone: every clone shares one lock
+//!     // and one value, so mutations through one are visible through all.
+//!     let a = counter.clone();
+//!     let b = counter.clone();
+//!
+//!     let h1 = spawn(move || {
+//!         let mut guard = a.lock().unwrap();
+//!         *guard += 1;
+//!     });
+//!     let h2 = spawn(move || {
+//!         let mut guard = b.lock().unwrap();
+//!         *guard += 1;
+//!     });
+//!     h1.join().unwrap();
+//!     h2.join().unwrap();
+//!
+//!     assert_eq!(*counter.lock().unwrap(), 2);
+//! });
+//! ```
+//!
+//! ## Choosing a timeout
+//!
+//! [`Mutex::lock`] waits up to [`DEFAULT_TIMEOUT`] (30 seconds) before giving
+//! up with [`LockTimeout`]. To use a different bound for one call, use
+//! [`Mutex::lock_timeout`] instead; to change the default for every future
+//! `lock()` call on this mutex (including through its clones), use
+//! [`Mutex::set_default_timeout`]. If you never want to wait at all, use
+//! [`Mutex::try_lock`], which returns immediately whether or not the lock was
+//! free.
+//!
+//! ## Fairness and panics
+//!
+//! Waiters are granted the lock in the order they started waiting (FIFO), so
+//! no actor can be starved by later arrivals repeatedly cutting in line.
+//!
+//! This mutex never poisons. `std::sync::Mutex` marks itself poisoned if a
+//! thread panics while holding the lock, because a partly mutated value might
+//! be left behind for the next lock holder to see. smarm's actors already
+//! rely on `Drop` running during unwinding to release the lock, so if a
+//! holder panics, [`MutexGuard::drop`] still runs and the next waiter is
+//! granted the lock normally. It is the same tradeoff `std::sync::Mutex`
+//! offers you if you choose to ignore poisoning: you may see a value left
+//! mid-update by the panicking actor, so a panic inside a critical section is
+//! still a bug worth fixing, just not one that also wedges every future lock
+//! attempt.
+//!
+//! Locking a mutex you already hold (on the same actor) does not queue
+//! behind yourself: it deadlocks, the same way relocking a non-reentrant
+//! `std::sync::Mutex` does. Don't call `lock` while already holding a guard
+//! from the same `Mutex`.
+//!
+//! ## Outside the runtime
+//!
+//! `Mutex<T>` also works when called from plain code that is not running as
+//! a smarm actor (for example, in a test's setup code before calling
+//! [`run`](crate::run)). There, an actor's cooperative park has no meaning,
+//! so a lock attempt instead blocks the calling OS thread directly until the
+//! mutex is free; there is no timeout on this path.
 
 use crate::pid::Pid;
 use crate::scheduler;
@@ -15,8 +92,14 @@ use std::collections::VecDeque;
 use std::sync::{Arc, Mutex as StdMutex};
 use std::time::Duration;
 
+/// How long [`Mutex::lock`] waits for the lock before giving up, unless
+/// overridden per-mutex with [`Mutex::set_default_timeout`] or per-call with
+/// [`Mutex::lock_timeout`].
 pub const DEFAULT_TIMEOUT: Duration = Duration::from_secs(30);
 
+/// Returned by [`Mutex::lock`] / [`Mutex::lock_timeout`] when the timeout
+/// elapses before the lock became available. The lock attempt is abandoned;
+/// nothing was acquired, and the mutex's value is unaffected.
 #[derive(Debug, PartialEq, Eq, Clone, Copy)]
 pub struct LockTimeout;
 
@@ -70,12 +153,16 @@ impl TimerTarget for MutexCore {
             };
             // Remove from waiters only if still there with matching epoch.
             // If the lock was already granted (holder == Some(pid)), the
-            // timer fired after the grant — treat as no-op; the actor
+            // timer fired after the grant: treat as no-op; the actor
             // will see `is_holder == true` and return Ok.
             if st.holder == Some(pid) {
                 return;
             }
-            match st.waiters.iter().position(|w| w.pid == pid && w.epoch == epoch) {
+            match st
+                .waiters
+                .iter()
+                .position(|w| w.pid == pid && w.epoch == epoch)
+            {
                 Some(pos) => {
                     st.waiters.remove(pos);
                     true
@@ -100,6 +187,8 @@ pub struct Mutex<T> {
 }
 
 impl<T> Mutex<T> {
+    /// Wrap `value` in a new mutex, initially unlocked, with the default
+    /// lock timeout ([`DEFAULT_TIMEOUT`]).
     pub fn new(value: T) -> Self {
         Self {
             core: Arc::new(MutexCore::new(DEFAULT_TIMEOUT)),
@@ -107,6 +196,11 @@ impl<T> Mutex<T> {
         }
     }
 
+    /// Change how long future [`lock`](Self::lock) calls on this mutex wait
+    /// before giving up. Applies to every clone of this `Mutex` (they share
+    /// one underlying lock), and to `lock` calls already in progress that
+    /// have not yet started waiting. Does not affect [`lock_timeout`](Self::lock_timeout)
+    /// calls, which always use the timeout passed in.
     pub fn set_default_timeout(&self, timeout: Duration) {
         match self.core.state.lock() {
             Ok(mut st) => st.default_timeout = timeout,
@@ -114,6 +208,12 @@ impl<T> Mutex<T> {
         }
     }
 
+    /// Acquire the lock, waiting up to this mutex's default timeout
+    /// ([`DEFAULT_TIMEOUT`], or whatever [`set_default_timeout`](Self::set_default_timeout)
+    /// last set) if it is currently held elsewhere. Returns a [`MutexGuard`]
+    /// that releases the lock when dropped, or [`LockTimeout`] if the
+    /// deadline passes first. To use a one-off timeout instead of the
+    /// mutex's default, call [`lock_timeout`](Self::lock_timeout) directly.
     pub fn lock(&self) -> Result<MutexGuard<'_, T>, LockTimeout> {
         let timeout = match self.core.state.lock() {
             Ok(st) => st.default_timeout,
@@ -122,6 +222,10 @@ impl<T> Mutex<T> {
         self.lock_timeout(timeout)
     }
 
+    /// Acquire the lock, waiting up to `timeout` (ignoring this mutex's
+    /// default) if it is currently held elsewhere. Returns a [`MutexGuard`]
+    /// that releases the lock when dropped, or [`LockTimeout`] if `timeout`
+    /// elapses first with the lock still unavailable.
     pub fn lock_timeout(&self, timeout: Duration) -> Result<MutexGuard<'_, T>, LockTimeout> {
         // Outside the runtime (e.g. in tests, after run() returns) there is no
         // current actor PID.  Fall back to a blocking std::sync::Mutex acquire.
@@ -146,7 +250,10 @@ impl<T> Mutex<T> {
                     Some(v) => v,
                     None => panic!("smarm: Mutex value missing on free fast path (core corrupt)"),
                 };
-                return Ok(MutexGuard { mutex: self, value: Some(value) });
+                return Ok(MutexGuard {
+                    mutex: self,
+                    value: Some(value),
+                });
             }
         }
 
@@ -157,7 +264,7 @@ impl<T> Mutex<T> {
                 Ok(g) => g,
                 Err(e) => panic!("smarm: mutex state lock poisoned (core corrupt): {e}"),
             };
-            // begin_wait is lock-free — legal under the state lock; this
+            // begin_wait is lock-free (legal under the state lock); this
             // makes the epoch atomic with the registration's visibility to
             // grants and timeouts.
             let epoch = scheduler::begin_wait();
@@ -170,7 +277,7 @@ impl<T> Mutex<T> {
         scheduler::insert_wait_timer(deadline, me, target, epoch);
         scheduler::park_current();
 
-        // Resumed — precisely: only our grant or our timer can wake this
+        // Resumed, precisely: only our grant or our timer can wake this
         // wait (both epoch-stamped; a stop wake unwinds out of
         // park_current). The one-shot interpretation below is therefore
         // exhaustive. Are we the holder?
@@ -187,12 +294,18 @@ impl<T> Mutex<T> {
                 Some(v) => v,
                 None => panic!("smarm: Mutex value missing after grant (core corrupt)"),
             };
-            Ok(MutexGuard { mutex: self, value: Some(value) })
+            Ok(MutexGuard {
+                mutex: self,
+                value: Some(value),
+            })
         } else {
             Err(LockTimeout)
         }
     }
 
+    /// Acquire the lock only if it is immediately available: never parks and
+    /// never waits. Returns `Some` with a [`MutexGuard`] if the lock was
+    /// free, `None` if it is currently held elsewhere.
     pub fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
         let me = crate::actor::current_pid()?;
         let mut st = match self.core.state.lock() {
@@ -212,7 +325,10 @@ impl<T> Mutex<T> {
             Some(v) => v,
             None => panic!("smarm: Mutex value missing on try_lock free path (core corrupt)"),
         };
-        Some(MutexGuard { mutex: self, value: Some(value) })
+        Some(MutexGuard {
+            mutex: self,
+            value: Some(value),
+        })
     }
 
     /// Blocking fallback used when called outside the smarm runtime.
@@ -226,16 +342,28 @@ impl<T> Mutex<T> {
                 Ok(mut g) => g.take(),
                 Err(e) => panic!("smarm: mutex value lock poisoned (core corrupt): {e}"),
             };
-            if let Some(v) = v { break v; }
+            if let Some(v) = v {
+                break v;
+            }
             std::thread::yield_now();
         };
-        Ok(MutexGuard { mutex: self, value: Some(value) })
+        Ok(MutexGuard {
+            mutex: self,
+            value: Some(value),
+        })
     }
 }
 
 impl<T> Clone for Mutex<T> {
+    /// Cheap: hands back another handle to the same underlying lock and
+    /// value, the way `Arc::clone` does. All clones of a `Mutex` share one
+    /// lock and one protected value; locking through any clone excludes
+    /// every other clone.
     fn clone(&self) -> Self {
-        Self { core: self.core.clone(), value: self.value.clone() }
+        Self {
+            core: self.core.clone(),
+            value: self.value.clone(),
+        }
     }
 }
 
@@ -247,6 +375,10 @@ unsafe impl<T: Send> Sync for Mutex<T> {}
 // Guard
 // ---------------------------------------------------------------------------
 
+/// Grants access to the value inside a [`Mutex`] while the lock is held.
+/// Dereferences to `&T` and `&mut T`. Dropping the guard releases the lock
+/// and, if another actor is waiting, wakes the next one in arrival order.
+/// Returned by [`Mutex::lock`], [`Mutex::lock_timeout`], and [`Mutex::try_lock`].
 pub struct MutexGuard<'a, T> {
     mutex: &'a Mutex<T>,
     value: Option<T>,
@@ -277,9 +409,7 @@ impl<T: std::fmt::Debug> std::fmt::Debug for MutexGuard<'_, T> {
             Some(v) => v,
             None => panic!("smarm: MutexGuard value missing (core corrupt)"),
         };
-        f.debug_tuple("MutexGuard")
-            .field(value)
-            .finish()
+        f.debug_tuple("MutexGuard").field(value).finish()
     }
 }
 

src/pg.rs

@@ -221,7 +221,9 @@ pub(crate) struct ProcessGroups {
 
 impl ProcessGroups {
     pub(crate) fn new() -> Self {
-        Self { groups: HashMap::new() }
+        Self {
+            groups: HashMap::new(),
+        }
     }
 
     /// Insert `ms` into `group`. Idempotent on the *member*: if the member is
@@ -323,20 +325,33 @@ impl ProcessGroups {
     fn members_where(&self, group: &str, mut is_live: impl FnMut(Pid) -> bool) -> Vec<Pid> {
         self.groups
             .get(group)
-            .map(|v| v.iter().map(|e| e.member.pid).filter(|&p| is_live(p)).collect())
+            .map(|v| {
+                v.iter()
+                    .map(|e| e.member.pid)
+                    .filter(|&p| is_live(p))
+                    .collect()
+            })
             .unwrap_or_default()
     }
 
     /// The first live member of `group` in insertion order — stateless
     /// first-live `pick`, with the same read-path backstop as `members_where`.
     fn first_member_where(&self, group: &str, mut is_live: impl FnMut(Pid) -> bool) -> Option<Pid> {
-        self.groups.get(group)?.iter().map(|e| e.member.pid).find(|&p| is_live(p))
+        self.groups
+            .get(group)?
+            .iter()
+            .map(|e| e.member.pid)
+            .find(|&p| is_live(p))
     }
 }
 
 /// Build the full member identity for `pid` from runtime identity.
 fn member_for(inner: &crate::runtime::RuntimeInner, pid: Pid) -> Member {
-    Member { node: inner.node_id, incarnation: inner.incarnation, pid }
+    Member {
+        node: inner.node_id,
+        incarnation: inner.incarnation,
+        pid,
+    }
 }
 
 /// Is `pid` a live actor right now? Generation-checked atomic slot-word read,
@@ -367,7 +382,10 @@ pub fn join<A>(group: impl Into<String>, pid: Pid<A>) -> bool {
     let mon = monitor(pid);
 
     let (rejected, reaped) = with_runtime(|inner| {
-        let ms = Membership { member: member_for(inner, pid), monitor: mon };
+        let ms = Membership {
+            member: member_for(inner, pid),
+            monitor: mon,
+        };
         let mut pg = inner.process_groups.lock();
         let reaped = pg.reap_group(&group);
         let rejected = pg.join(&group, ms);
@@ -507,7 +525,11 @@ mod tests {
         let (tx, rx) = channel::<Down>();
         let ms = Membership {
             member: member(index, generation),
-            monitor: Monitor { id: MonitorId(0), target: pid, rx },
+            monitor: Monitor {
+                id: MonitorId(0),
+                target: pid,
+                rx,
+            },
         };
         (ms, tx)
     }
@@ -518,7 +540,10 @@ mod tests {
         let (a, _ta) = synth(1, 0);
         let (b, _tb) = synth(1, 0);
         assert!(pg.join("workers", a).is_none(), "first join inserts");
-        assert!(pg.join("workers", b).is_some(), "second identical join is handed back");
+        assert!(
+            pg.join("workers", b).is_some(),
+            "second identical join is handed back"
+        );
         assert_eq!(pg.members_of("workers"), vec![member(1, 0)]);
     }
 
@@ -542,7 +567,10 @@ mod tests {
         let (a, _ta) = synth(1, 0);
         let (b, _tb) = synth(1, 1);
         assert!(pg.join("g", a).is_none());
-        assert!(pg.join("g", b).is_none(), "different generation is a distinct member");
+        assert!(
+            pg.join("g", b).is_none(),
+            "different generation is a distinct member"
+        );
         assert_eq!(pg.members_of("g"), vec![member(1, 0), member(1, 1)]);
     }
 
@@ -555,16 +583,27 @@ mod tests {
         pg.join("g", b);
         assert!(pg.leave("g", member(1, 0)).is_some());
         assert_eq!(pg.members_of("g"), vec![member(2, 0)]);
-        assert!(pg.leave("g", member(1, 0)).is_none(), "second leave finds nothing");
+        assert!(
+            pg.leave("g", member(1, 0)).is_none(),
+            "second leave finds nothing"
+        );
         assert!(pg.leave("g", member(2, 0)).is_some());
         assert!(pg.members_of("g").is_empty(), "group is now empty");
-        assert!(pg.leave("never", member(9, 0)).is_none(), "leaving an unknown group is a no-op");
+        assert!(
+            pg.leave("never", member(9, 0)).is_none(),
+            "leaving an unknown group is a no-op"
+        );
     }
 
     #[test]
     fn remove_where_sweeps_every_group() {
         let mut pg = ProcessGroups::new();
-        for (g, (m, _t)) in [("a", synth(1, 0)), ("a", synth(2, 0)), ("b", synth(1, 0)), ("c", synth(3, 0))] {
+        for (g, (m, _t)) in [
+            ("a", synth(1, 0)),
+            ("a", synth(2, 0)),
+            ("b", synth(1, 0)),
+            ("c", synth(3, 0)),
+        ] {
             pg.join(g, m);
         }
         // Death of pid index 1 (any generation) evicts it everywhere.
@@ -582,8 +621,16 @@ mod tests {
         let pid = Pid::new(1, 0);
         let (tx, rx) = channel::<Down>();
         let dead = Membership {
-            member: Member { node: DEFAULT_NODE_ID, incarnation: Incarnation::new(7), pid },
-            monitor: Monitor { id: MonitorId(0), target: pid, rx },
+            member: Member {
+                node: DEFAULT_NODE_ID,
+                incarnation: Incarnation::new(7),
+                pid,
+            },
+            monitor: Monitor {
+                id: MonitorId(0),
+                target: pid,
+                rx,
+            },
         };
         let _keep = tx;
         let (live, _tl) = synth(2, 0);
@@ -614,9 +661,17 @@ mod tests {
         pg.join("b", b1);
         // pid 1 dies: its group-a monitor receives a Down. Its group-b monitor
         // has not — reap must still sweep pid 1 out of b by the pid predicate.
-        ta1.send(Down { pid: Pid::new(1, 0), reason: DownReason::Exit }).unwrap();
+        ta1.send(Down {
+            pid: Pid::new(1, 0),
+            reason: DownReason::Exit,
+        })
+        .unwrap();
         let evicted = pg.reap_group("a");
-        assert_eq!(evicted.len(), 2, "pid 1's memberships in both a and b are evicted");
+        assert_eq!(
+            evicted.len(),
+            2,
+            "pid 1's memberships in both a and b are evicted"
+        );
         assert_eq!(pg.members_of("a"), vec![member(2, 0)]);
         assert!(pg.members_of("b").is_empty(), "swept from b too; pruned");
     }
@@ -646,8 +701,16 @@ mod tests {
         let dead = Pid::new(1, 0);
         let oracle = |pid: Pid| pid != dead;
 
-        assert_eq!(pg.members_where("g", oracle), vec![Pid::new(2, 0)], "dead pid filtered from read");
-        assert_eq!(pg.first_member_where("g", oracle), Some(Pid::new(2, 0)), "pick skips the dead first member");
+        assert_eq!(
+            pg.members_where("g", oracle),
+            vec![Pid::new(2, 0)],
+            "dead pid filtered from read"
+        );
+        assert_eq!(
+            pg.first_member_where("g", oracle),
+            Some(Pid::new(2, 0)),
+            "pick skips the dead first member"
+        );
 
         // Backstop does not evict — that stays the monitor's job; raw storage
         // still holds both until reap runs.

src/pid.rs

@@ -79,7 +79,10 @@ impl Pid<Erased> {
     /// here; typing happens at typed-actor boundaries via [`Pid::from_raw`].
     #[inline]
     pub const fn new(index: u32, generation: u32) -> Self {
-        Self { raw: RawPid::new(index, generation), _marker: PhantomData }
+        Self {
+            raw: RawPid::new(index, generation),
+            _marker: PhantomData,
+        }
     }
 }
 
@@ -90,7 +93,10 @@ impl<A> Pid<A> {
     /// resolution paths.
     #[inline]
     pub(crate) const fn from_raw(raw: RawPid) -> Self {
-        Self { raw, _marker: PhantomData }
+        Self {
+            raw,
+            _marker: PhantomData,
+        }
     }
 
     /// The raw identity, dropping the actor type — the key for identity-only
@@ -192,7 +198,10 @@ impl<M> Name<M> {
     /// associated constants at call sites.
     #[inline]
     pub const fn new(name: &'static str) -> Self {
-        Self { name, _marker: PhantomData }
+        Self {
+            name,
+            _marker: PhantomData,
+        }
     }
 
     /// The underlying registry key.

src/preempt.rs

@@ -98,10 +98,13 @@ pub(crate) fn clear_current_slot() {
     CURRENT_SLOT.with(|c| c.set(std::ptr::null()));
 }
 
-/// RFC 007 (`smarm-causal`) — raw pointer to the on-CPU actor's slot, null on
-/// the scheduler's own stack. Same lifetime argument as `note_overrun`: the
-/// slot is never reclaimed while its actor is on-CPU.
-#[cfg(feature = "smarm-causal")]
+/// Raw pointer to the on-CPU actor's slot, null on the scheduler's own
+/// stack. Same lifetime argument as `note_overrun`: the slot is never
+/// reclaimed while its actor is on-CPU. Consumers: the `smarm-causal`
+/// profiler (RFC 007) and — unconditionally — the SIGSEGV classifier
+/// (RFC 019 §7), which additionally relies on this being a plain load of a
+/// const-initialized TLS Cell (no lazy init, no allocation, no dtor): safe
+/// from a signal handler.
 #[inline]
 pub(crate) fn current_slot_ptr() -> *const crate::runtime::Slot {
     CURRENT_SLOT.with(|c| c.get())

src/raw_mutex.rs

@@ -166,7 +166,10 @@ impl<T> RawMutex<T> {
         {
             self.lock_slow();
         }
-        RawMutexGuard { m: self, prev_preempt }
+        RawMutexGuard {
+            m: self,
+            prev_preempt,
+        }
     }
 
     #[cold]

src/registry.rs

@@ -1,55 +1,107 @@
-//! Named mailbox registry — resolve a name (or pid) to a *messageable* actor.
+//! Give an actor a name so other actors can find it and message it.
 //!
-//! ## What changed (RFC 014)
+//! Without the registry, the only way to reach an actor is to already be
+//! holding its [`Pid`], usually because you spawned it yourself or someone
+//! passed it to you. That is fine for a worker you just created, but it does
+//! not work for a well-known service that arbitrary parts of your program
+//! need to find independently, like a logger, a config store, or a
+//! connection pool. The registry solves this: an actor claims a name once,
+//! and from then on any other actor can look that name up, or send to it
+//! directly, without ever having been handed a `Pid`.
 //!
-//! The old registry was a `name <-> pid` bimap: `whereis` handed back a `Pid`
-//! you could not send to, because a pid is just `(index, generation)` with no
-//! delivery endpoint. This rework makes resolution yield something messageable.
+//! ```
+//! use smarm::{channel, register, run, send, spawn, unregister, whereis, Name};
 //!
-//! Two facts shape the structure:
+//! const COUNTER: Name<u64> = Name::new("counter");
 //!
-//! 1. **A name resolves to a single actor.** Many actors under one label is
-//!    what *process groups* (`pg`) are for; the registry is one-name-one-actor
-//!    (several names *may* point at the same actor).
-//! 2. **Channels are typed**, so an actor has no single untyped mailbox. An
-//!    actor instead owns a *set* of typed channels — one [`Sender`] per message
-//!    type it accepts. So the registry maps name/pid to a [`Mailbox`]: a small
-//!    structure holding that actor's pid plus all of its typed channels, keyed
-//!    by message [`TypeId`].
+//! run(|| {
+//!     let (ready_tx, ready_rx) = channel::<()>();
+//!     let (tx, rx) = channel::<u64>();
 //!
-//! Resolution is therefore: `name -> pid` (single actor) `-> Mailbox -> the
-//! channel for message type M`. A `Name<Cmd>` and a `Name<Admin>` on the *same*
-//! actor select *different* channels purely by their type parameter, so
-//! capability separation (RFC 014 §4.7) needs no extra machinery.
+//!     let worker = spawn(move || {
+//!         // Claim the name for this actor's inbox. Any actor holding
+//!         // `COUNTER` can now reach this one by name.
+//!         register(COUNTER, tx).unwrap();
+//!         ready_tx.send(()).unwrap();
+//!         assert_eq!(rx.recv().unwrap(), 42);
+//!     });
 //!
-//! ## Type erasure is contained
+//!     ready_rx.recv().unwrap(); // wait for the worker to register
 //!
-//! Each stored channel is a `Box<dyn Any + Send>` that is concretely a
-//! `Sender<M>`, filed under `TypeId::of::<M>()`. A resolve for `M` looks up
-//! that exact `TypeId` and downcasts to `Sender<M>` — keyed by the very type we
-//! downcast to, so the downcast cannot fail on correct data; a failure is a
-//! smarm bug, asserted in debug. The phantom `M` on [`Name`] re-imposes the
-//! type at the call site, so callers never touch the erasure.
+//!     // Look the name up, or just send to it directly.
+//!     assert_eq!(whereis("counter"), Some(worker.pid()));
+//!     send(COUNTER, 42).unwrap();
 //!
-//! ## Cleanup is lazy (prune-on-contact)
+//!     worker.join().unwrap();
 //!
-//! As before, there is no `finalize` hook and no name field on the slot. Every
-//! operation that touches a binding checks the target pid's liveness via the
-//! generation-checked slot word; a binding to a dead actor behaves as absent
-//! and is pruned on contact (its [`Mailbox`] and every name pointing at it are
-//! dropped). The cost is a dead binding lingering until something looks at it;
-//! the payoff is zero coupling to the actor lifecycle.
+//!     // The name dies with the actor: nobody holds it anymore.
+//!     assert_eq!(whereis("counter"), None);
+//! });
+//! ```
 //!
-//! ## Locking
+//! ## Names carry a message type
 //!
-//! One `RawMutex` (Leaf class) in `RuntimeInner`, exactly like the old
-//! registry. The fold (name index *and* handles under the one lock) is what
-//! keeps a name-addressed `send` on a single Leaf — `raw_mutex` panics on a
-//! second Leaf acquired while one is held. The send path clones the `Sender`
-//! **under** the Leaf lock (a `Sender::clone` takes a Channel lock, permitted
-//! under a Leaf), then **releases** the Leaf and only *then* sends — a send can
-//! unpark a receiver, and wakeup-bearing work runs outside the Leaf. Order is
-//! **Leaf -> Channel**, as `pg`/`finalize`.
+//! A [`Name<M>`] is a plain string plus a type parameter `M`: the message
+//! type that name expects to receive. [`Name::new`] is `const`, so the usual
+//! pattern is a module-level constant like `COUNTER` above, shared by every
+//! caller. The type parameter means a name is only ever sent the kind of
+//! message it was declared for. If two different constants share the same
+//! string but have different message types, they still address two
+//! independent channels on the same actor: registering both just gives that
+//! actor two ways to be reached, one per message type. This is how you give
+//! one actor a "public" channel and a separate, differently-typed "admin"
+//! channel under related names, without inventing an enum to merge them.
+//!
+//! ## One actor per name, looked up fresh every time
+//!
+//! A name always points at exactly one actor at a time (contrast a *process
+//! group*, from the [`pg`](crate::pg) module, which is one name mapping to
+//! many actors). Unlike a plain [`Pid`], which names one specific actor
+//! forever and stops working the moment that actor dies, a name is
+//! re-resolved on every [`send`]: if the actor holding it dies and a new one
+//! registers under the same name, the next `send` reaches the new holder
+//! automatically. Use a name for a long-lived service whose exact identity
+//! you do not want to track by hand; use a `Pid` when you already have one
+//! and want to talk to that exact actor.
+//!
+//! ## Registration ends when the actor does
+//!
+//! There is no separate step to clean up a name when its actor exits: dying
+//! is enough. The next operation that touches a dead binding (a [`whereis`],
+//! a [`send`], or another actor's [`register`] of the same name) notices the
+//! actor is gone and clears the stale entry as a side effect, so the name
+//! becomes free again. [`unregister`] is only for a live actor voluntarily
+//! giving up a name it no longer wants; nothing has to call it on the way
+//! out.
+//!
+//! ## Implementation notes
+//!
+//! These details matter if you are working on smarm itself; they are not
+//! part of the public contract.
+//!
+//! Internally, each live actor that has published at least one channel owns
+//! a `Mailbox`: its pid plus a set of typed channels, keyed by the message
+//! type's `TypeId`. A stored channel is a `Box<dyn Any + Send>` that
+//! is concretely a `Sender<M>`; resolving for `M` looks up that exact
+//! `TypeId` and downcasts, so the downcast cannot fail on correct data (a
+//! failure would be a bug in the registry itself, checked in debug builds).
+//! Registering a name therefore means: find or create the actor's mailbox,
+//! insert the channel under its type, and point the name at the actor's pid.
+//!
+//! There is no callback when an actor exits. Every operation that touches a
+//! binding checks the target pid's liveness directly against the scheduler's
+//! slot table (which also tracks a generation counter, so a dead actor's
+//! reused slot index is never mistaken for the same actor). A binding to a
+//! dead actor is treated as absent and dropped right there. This keeps the
+//! registry decoupled from actor teardown, at the cost of a dead binding
+//! lingering until something happens to look at it.
+//!
+//! The whole registry (both the name index and the per-actor mailboxes) sits
+//! behind one lock, which is what lets a name-addressed [`send`] resolve and
+//! clone the target's sender in a single critical section. The sender is
+//! cloned while that lock is held, then the lock is released before the
+//! actual send, since delivering a message can wake a parked receiver and
+//! that wakeup work should not run while the registry is locked.
 
 use crate::channel::Sender;
 use crate::pid::{Addressable, Name, Pid};
@@ -80,28 +132,33 @@ impl std::fmt::Display for RegisterError {
 
 impl std::error::Error for RegisterError {}
 
-/// Why a name-addressed [`send`] did not deliver. Carries the message back so
-/// the caller never loses it (mirrors [`crate::channel::SendError`]).
+/// Why a send did not deliver. Every variant carries the undelivered message
+/// back, mirroring [`crate::channel::SendError`], so a failed send never
+/// silently drops what you tried to send.
 ///
-/// `Debug`/`Display` are hand-written so neither demands `M: Debug` — the
-/// payload is returned, not printed.
+/// `Debug` and `Display` are hand-written so neither requires `M: Debug`,
+/// since the payload is handed back to you, not printed.
 pub enum SendError<M> {
-    /// No live actor is currently registered under this name. Name-addressed
-    /// [`send`] only; the pid-addressed counterpart is [`SendError::Dead`].
+    /// No live actor is currently registered under this name. Returned only
+    /// by name-addressed [`send`]; the pid-addressed counterpart of "nothing
+    /// there" is [`SendError::Dead`].
     Unresolved(M),
-    /// The pid-addressed actor is no longer the live incarnation this pid names
-    /// — it has died, even if its slot now holds a *different* actor (a direct
-    /// `Pid<A>` send never redirects; contrast name-addressed [`send`]). Pid
-    /// paths ([`send_to`] / [`send_dyn`]) only.
+    /// The actor this pid identifies has died, even if its slot has since
+    /// been taken over by a different, live actor. A direct `Pid<A>` send
+    /// never redirects to that new occupant; contrast name-addressed
+    /// [`send`], which would reach it. Returned by the pid-addressed sends,
+    /// [`send_to`] and [`send_dyn`].
     Dead(M),
-    /// The actor is live but exposes no channel for this message type.
+    /// The actor is live but has not published a channel for this message
+    /// type.
     NoChannel(M),
-    /// The actor's channel for this message type is closed (its receiver is gone).
+    /// The actor's channel for this message type is closed (its receiver has
+    /// been dropped).
     Closed(M),
-    /// No live member to deliver to — a [`dispatch`](crate::dispatch) over an
-    /// empty (or all-dead) process group. Group-addressed dispatch only; the
-    /// name-addressed counterpart is [`SendError::Unresolved`]. The message is
-    /// handed back undelivered.
+    /// No live member was available to deliver to: returned by
+    /// [`dispatch`](crate::dispatch) when the target process group is empty
+    /// or every member in it has died. The name-addressed counterpart of
+    /// this case is [`SendError::Unresolved`].
     NoMember(M),
 }
 
@@ -138,7 +195,9 @@ impl<M> std::fmt::Display for SendError<M> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             SendError::Unresolved(_) => write!(f, "no live actor registered under that name"),
-            SendError::Dead(_) => write!(f, "the addressed actor is no longer the live incarnation"),
+            SendError::Dead(_) => {
+                write!(f, "the addressed actor is no longer the live incarnation")
+            }
             SendError::NoChannel(_) => write!(f, "actor has no channel for this message type"),
             SendError::Closed(_) => write!(f, "the actor's channel for this type is closed"),
             SendError::NoMember(_) => write!(f, "no live member in the process group"),
@@ -150,9 +209,9 @@ impl<M> std::error::Error for SendError<M> {}
 
 /// A registry-stored channel, type-erased over its message type. The stored
 /// object must serve two readers: `clone_sender` (downcast back to the concrete
-/// `Sender<M>`) and the RFC 016 snapshot (queued length without knowing `M`).
-/// A bare `Box<dyn Any>` gives the first but not the second, so we erase behind
-/// this small trait instead.
+/// `Sender<M>`) and the runtime introspection snapshot (queued length without
+/// knowing `M`). A bare `Box<dyn Any>` gives the first but not the second, so
+/// we erase behind this small trait instead.
 trait ErasedSender: Send {
     fn as_any(&self) -> &dyn Any;
     fn queued_len(&self) -> usize;
@@ -169,7 +228,7 @@ impl<M: Send + 'static> ErasedSender for Sender<M> {
 
 /// One typed channel of an actor, type-erased. Concretely a `Sender<M>` filed
 /// under `TypeId::of::<M>()`; `msg_type` is `type_name::<M>()`, kept for
-/// observers (RFC 014 §4.5) and as the debug cross-check on the downcast.
+/// observability tooling and as the debug cross-check on the downcast.
 struct Channel {
     sender: Box<dyn ErasedSender>,
     msg_type: &'static str,
@@ -185,7 +244,10 @@ struct Mailbox {
 
 impl Mailbox {
     fn new(pid: Pid) -> Self {
-        Self { pid, channels: HashMap::new() }
+        Self {
+            pid,
+            channels: HashMap::new(),
+        }
     }
 
     /// Clone the `Sender<M>` for this actor, if it has one. Called **under the
@@ -204,7 +266,7 @@ impl Mailbox {
     }
 }
 
-/// Per-actor registry view handed to RFC 016 introspection: registered names
+/// Per-actor registry view handed to runtime introspection: registered names
 /// and summed mailbox depth, tagged with the mailbox's `pid` so a stale
 /// incarnation can be filtered against the slab. Covers only *published*
 /// channels (`register` / `install` / `spawn_addr` / gen_server start); an
@@ -219,47 +281,54 @@ pub(crate) struct MailboxInfo {
 /// The directory. Invariant (held under the registry lock): every value in
 /// `by_name` is the full [`Pid`] (index *and* generation) of an actor that
 /// published a [`Mailbox`] into `by_index` at registration time. Stale entries
-/// (dead holders — including holders whose slot has since been re-tenanted by
-/// a different actor) violate nothing — they are pruned on contact, and the
+/// (dead holders, including holders whose slot has since been re-tenanted by
+/// a different actor) violate nothing: they are pruned on contact, and the
 /// generation makes "dead" decidable even after slot reuse.
 pub(crate) struct Registry {
     /// `pid.index() -> the actor's mailbox`. The handle store.
     by_index: HashMap<u32, Mailbox>,
     /// `name -> holder pid`. Several names may map to one actor. The full pid
     /// (not just the index) is load-bearing: an index alone cannot tell a dead
-    /// holder from the live actor now tenanting its recycled slot, which made
-    /// such a name read as live-held — unresolvable *and* unregisterable — and
-    /// would misdeliver to a same-typed tenant (soak20 signature 2).
+    /// holder from the live actor now tenanting its recycled slot. Comparing
+    /// only the index would make such a name read as live-held (unresolvable
+    /// and unregisterable at once) and could misdeliver to whatever new,
+    /// same-typed actor now sits in that slot.
     by_name: HashMap<&'static str, Pid>,
 }
 
 impl Registry {
     pub(crate) fn new() -> Self {
-        Self { by_index: HashMap::new(), by_name: HashMap::new() }
+        Self {
+            by_index: HashMap::new(),
+            by_name: HashMap::new(),
+        }
     }
 
-    /// Drop a dead holder's artifacts: every name bound to it, and its mailbox
-    /// — but only while the mailbox is still *its own*. A recycled slot's
-    /// mailbox belongs to the live tenant (publish replaces it wholesale on
-    /// pid mismatch) and is left untouched.
+    /// Drop a dead holder's artifacts: every name bound to it, and its
+    /// mailbox, but only while the mailbox is still *its own*. A recycled
+    /// slot's mailbox belongs to the live tenant (publish replaces it
+    /// wholesale on pid mismatch) and is left untouched.
     fn prune_holder(&mut self, holder: Pid) {
         self.by_name.retain(|_, p| *p != holder);
-        if self.by_index.get(&holder.index()).is_some_and(|mb| mb.pid == holder) {
+        if self
+            .by_index
+            .get(&holder.index())
+            .is_some_and(|mb| mb.pid == holder)
+        {
             self.by_index.remove(&holder.index());
         }
     }
 
-    /// RFC 016 snapshot input: per-slot-index registry view — the actor's
-    /// registered names (inverted from `by_name`) and its mailbox depth (queued
-    /// messages summed across every published typed channel). Built in one pass
-    /// under the registry Leaf; the per-channel `queued_len` takes a Channel
-    /// lock, legal under the Leaf (Leaf → Channel). Carries each mailbox's full
-    /// `pid` so the caller can discard a stale incarnation's entry against the
-    /// slab's live generation. Names are matched to mailboxes by *full pid*, so
-    /// a stale name (dead holder) still annotates the corpse's own mailbox if
-    /// that survives, but never a recycled slot's new tenant; names that attach
-    /// to no mailbox are dropped — they violate no invariant and get pruned on
-    /// next contact.
+    /// Runtime introspection input: per-slot-index registry view, giving the
+    /// actor's registered names (inverted from `by_name`) and its mailbox
+    /// depth (queued messages summed across every published typed channel).
+    /// Carries each mailbox's full `pid` so the caller can discard a stale
+    /// incarnation's entry against the slab's live generation. Names are
+    /// matched to mailboxes by *full pid*, so a stale name (dead holder)
+    /// still annotates the corpse's own mailbox if that survives, but never a
+    /// recycled slot's new tenant; names that attach to no mailbox are
+    /// dropped, since that violates no invariant and they get pruned on next
+    /// contact.
     pub(crate) fn introspect_map(&self) -> HashMap<u32, MailboxInfo> {
         let mut names: HashMap<Pid, Vec<&'static str>> = HashMap::new();
         for (&name, &pid) in &self.by_name {
@@ -282,8 +351,9 @@ impl Registry {
 
     /// Single-actor form of [`introspect_map`](Self::introspect_map): the
     /// registry view for one slot index, or `None` if no mailbox is published
-    /// there. Used by `actor_info` so its cost stays proportional to the one
-    /// actor rather than locking every channel in the runtime.
+    /// there. Used by the runtime's per-actor introspection so its cost stays
+    /// proportional to the one actor rather than locking every channel in the
+    /// runtime.
     pub(crate) fn introspect_one(&self, idx: u32) -> Option<MailboxInfo> {
         let mb = self.by_index.get(&idx)?;
         let depth: usize = mb.channels.values().map(|c| c.sender.queued_len()).sum();
@@ -292,7 +362,11 @@ impl Registry {
             .iter()
             .filter_map(|(&n, &p)| (p == mb.pid).then_some(n))
             .collect();
-        Some(MailboxInfo { pid: mb.pid, names, depth: depth.min(u32::MAX as usize) as u32 })
+        Some(MailboxInfo {
+            pid: mb.pid,
+            names,
+            depth: depth.min(u32::MAX as usize) as u32,
+        })
     }
 }
 
@@ -301,14 +375,18 @@ fn live(inner: &crate::runtime::RuntimeInner, pid: Pid) -> bool {
     inner.slot_at(pid).is_some_and(|s| s.is_live_for(pid))
 }
 
-/// Publish the current actor's `Sender<M>` under `name`, capturing the channel
-/// so the name becomes messageable. Idempotent for the same `(name, type)`;
-/// registering a *second* type under the same (or another) name on the same
-/// actor just adds another channel to the actor's mailbox.
+/// Give the current actor's channel a name, so other actors can find and
+/// message it by that name instead of needing its [`Pid`].
 ///
-/// Fails with [`RegisterError::NameTaken`] if the name is held by a *different*
-/// live actor (a binding to a dead actor is pruned and the name treated as
-/// free). Panics if called outside `Runtime::run()`.
+/// Calling this again with the same `(name, type)` from the same actor is
+/// harmless. Registering a *second* message type under the same (or a
+/// different) name from the same actor just adds another typed channel to
+/// that actor's mailbox; it does not replace the first.
+///
+/// Fails with [`RegisterError::NameTaken`] if the name is currently held by a
+/// *different* live actor. A name held by an actor that has since died is not
+/// considered taken: it is quietly reclaimed and handed to you. Panics if
+/// called outside [`run`](crate::run).
 pub fn register<M: Send + 'static>(name: Name<M>, tx: Sender<M>) -> Result<(), RegisterError> {
     register_with(self_pid(), name.as_str(), tx)
 }
@@ -316,8 +394,8 @@ pub fn register<M: Send + 'static>(name: Name<M>, tx: Sender<M>) -> Result<(), R
 /// Bind `name` to `pid`'s mailbox and publish `tx` under `M`'s [`TypeId`], for
 /// an explicit (already-live) actor rather than `self`. The shared core of
 /// [`register`] (which passes `self_pid()`) and the parent-side server-name
-/// bind in `gen_server` (which names a freshly spawned server before its body
-/// has run, so the name resolves the instant `start()` returns). Same collision
+/// bind in `gen_server`, which names a freshly spawned server before its body
+/// has run, so the name resolves the instant `start()` returns. Same collision
 /// rules and lock discipline as `register`.
 pub(crate) fn register_with<M: Send + 'static>(
     me: Pid,
@@ -325,6 +403,16 @@ pub(crate) fn register_with<M: Send + 'static>(
     tx: Sender<M>,
 ) -> Result<(), RegisterError> {
     with_runtime(|inner| {
+        // Stamp-eligibility for the terminal record (soak sig 4): flag the
+        // tenancy BEFORE the binding lands and outside the registry lock (no
+        // nesting), so no successfully-registered actor can die unflagged.
+        // A register that then fails leaves a harmless overshoot; a stale
+        // `me` is screened by the same live() the binding requires below.
+        if live(inner, me) {
+            if let Some(slot) = inner.slot_at(me) {
+                slot.cold.lock().watchable = true;
+            }
+        }
         let mut reg = inner.registry.lock();
         if !live(inner, me) {
             return Err(RegisterError::NoProc);
@@ -337,8 +425,8 @@ pub(crate) fn register_with<M: Send + 'static>(
             } else {
                 // Dead holder: free the name (and its other stale artifacts).
                 // Liveness is judged against the *stored* pid, generation
-                // included — a recycled slot's live tenant no longer makes a
-                // dead name read as taken (soak20 signature 2).
+                // included, so a recycled slot's live tenant no longer makes a
+                // dead name read as taken.
                 reg.prune_holder(holder);
             }
         }
@@ -355,26 +443,32 @@ pub(crate) fn register_with<M: Send + 'static>(
 /// index from a dead prior incarnation (pid mismatch) is replaced wholesale.
 /// Caller holds the registry lock and has established that `me` is live.
 fn publish_channel<M: Send + 'static>(reg: &mut Registry, me: Pid, tx: Sender<M>) {
-    let mb = reg.by_index.entry(me.index()).or_insert_with(|| Mailbox::new(me));
+    let mb = reg
+        .by_index
+        .entry(me.index())
+        .or_insert_with(|| Mailbox::new(me));
     if mb.pid != me {
         *mb = Mailbox::new(me);
     }
     mb.channels.insert(
         TypeId::of::<M>(),
-        Channel { sender: Box::new(tx), msg_type: type_name::<M>() },
+        Channel {
+            sender: Box::new(tx),
+            msg_type: type_name::<M>(),
+        },
     );
 }
 
 /// Publish the current actor's `Sender<A::Msg>` into its mailbox **without**
 /// binding a name, and hand back the typed [`Pid<A>`] that addresses this
-/// actor directly. This is the opt-in, lazy install of RFC 014 §5: an actor
-/// that wants to be reachable by a direct, identity-bound [`Pid<A>`] (rather
-/// than only via a re-resolving [`Name`]) calls this once with its inbox
-/// sender, then hands the returned pid out.
+/// actor directly.
 ///
-/// Unlike [`register`] there is no name to collide on, and `self` is always a
-/// live actor inside `run()`, so this is infallible. Panics if called outside
-/// `Runtime::run()`.
+/// This is for an actor that wants to be reachable directly by its pid,
+/// rather than only through a re-resolving [`Name`]: call this once with your
+/// inbox sender, then hand the returned `Pid<A>` to whoever should be able to
+/// message you. Unlike [`register`] there is no name to collide on, and the
+/// current actor is always live while inside `run()`, so this cannot fail.
+/// Panics if called outside [`run`](crate::run).
 pub fn install<A: Addressable>(tx: Sender<A::Msg>) -> Pid<A> {
     let me = self_pid();
     with_runtime(|inner| {
@@ -390,22 +484,27 @@ pub fn install<A: Addressable>(tx: Sender<A::Msg>) -> Pid<A> {
 /// Publish `tx` into `pid`'s mailbox under `M`'s [`TypeId`], for an explicit
 /// (freshly minted, already-live) actor rather than `self`. The parent-side
 /// half of [`spawn_addr`](crate::spawn_addr): the spawner makes the inbox and
-/// publishes the sender here *before* handing back the `Pid<A>`, so an immediate
-/// `send_to` on the returned pid always resolves — the address is live the
-/// instant the caller holds it, with no dependence on the body having run yet.
+/// publishes the sender here *before* handing back the `Pid<A>`, so an
+/// immediate `send_to` on the returned pid always resolves. The address is
+/// live the instant the caller holds it, with no dependence on the spawned
+/// actor's body having run yet.
 ///
-/// Caller guarantees `pid` is the just-installed actor (Queued, this exact
+/// Caller guarantees `pid` is the just-installed actor (queued, this exact
 /// incarnation); `publish_channel` replaces any stale leftover at the slot.
 pub(crate) fn install_for<M: Send + 'static>(pid: Pid, tx: Sender<M>) {
     with_runtime(|inner| {
         let mut reg = inner.registry.lock();
-        debug_assert!(live(inner, pid), "install_for: pid must be a freshly spawned, live actor");
+        debug_assert!(
+            live(inner, pid),
+            "install_for: pid must be a freshly spawned, live actor"
+        );
         publish_channel::<M>(&mut reg, pid, tx);
     });
 }
 
-/// The single actor currently registered under `name`, or `None` if unbound or
-/// no longer live (the stale binding is pruned on the way out).
+/// Look up which actor currently holds `name`, if any. Returns `None` if the
+/// name is unbound, or if it was bound to an actor that has since died (the
+/// stale binding is cleared as a side effect of this call).
 pub fn whereis(name: &str) -> Option<Pid> {
     with_runtime(|inner| {
         let mut reg = inner.registry.lock();
@@ -421,67 +520,122 @@ pub fn whereis(name: &str) -> Option<Pid> {
     })
 }
 
-/// Resolve `name` to a *typed* [`Pid<A>`] — the identity-bound counterpart of
-/// [`whereis`] (RFC 014 §4.4). Recovers the compile-checked
-/// [`send_to`] path from a durable name: looks the name up,
-/// then re-types the erased pid as `Pid<A>` via the unchecked
-/// [`assert_type`](crate::pid::assert_type) primitive. A wrong `A` is not
-/// unsound — it degrades to [`SendError::NoChannel`] on the next send (routing
-/// is by message `TypeId`), never a misdelivery. `None` if unbound or dead.
+/// What a name is bound to, three-valued (bridge soak signature 4).
 ///
-/// Panics if called outside `Runtime::run()`.
+/// [`Live`](NameResolution::Live) is [`whereis`]'s `Some`.
+/// [`Corpse`](NameResolution::Corpse) carries the *stored* holder pid of a
+/// dead-but-unpruned binding — a state Erlang cannot represent (its name
+/// death unregisters atomically; smarm's prune is lazy), captured here before
+/// the prune that `whereis` performs discards it, so the caller can consult
+/// [`terminal_reason`](crate::monitor::terminal_reason) for the tenancy's
+/// real down reason. [`Unbound`](NameResolution::Unbound) matches Erlang's
+/// unregistered name.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum NameResolution {
+    /// The stored holder is live (generation-checked); the binding stands.
+    Live(Pid),
+    /// The stored holder is dead. The binding was pruned on the way out —
+    /// the name heals exactly as `whereis` heals it; only the evidence is
+    /// returned instead of discarded. A second resolve is `Unbound`.
+    Corpse(Pid),
+    /// No binding stored (never registered, or already pruned by any reader).
+    Unbound,
+}
+
+/// Resolve `name` like [`whereis`], but keep the corpse: the dead-holder arm
+/// returns the stored pid it pruned instead of a bare `None`. Same lock
+/// discipline and pruning behavior as `whereis`; same `Runtime::run()`
+/// context contract.
+pub fn resolve_name(name: &str) -> NameResolution {
+    with_runtime(|inner| {
+        let mut reg = inner.registry.lock();
+        let Some(&pid) = reg.by_name.get(name) else {
+            return NameResolution::Unbound;
+        };
+        if live(inner, pid) {
+            NameResolution::Live(pid)
+        } else {
+            reg.prune_holder(pid);
+            NameResolution::Corpse(pid)
+        }
+    })
+}
+
+/// Like [`whereis`], but returns a *typed* [`Pid<A>`] instead of a bare
+/// [`Pid`], so a follow-up [`send_to`] is compile-checked instead of needing
+/// the untyped [`send_dyn`] escape hatch. `None` if the name is unbound or its
+/// holder has died.
+///
+/// The type `A` is not checked against what the name's holder actually
+/// published: if you pick the wrong `A`, this still succeeds, but the next
+/// send against the returned pid degrades to [`SendError::NoChannel`] rather
+/// than reaching the wrong actor or the wrong channel.
+///
+/// Panics if called outside [`run`](crate::run).
 pub fn lookup_as<A: Addressable>(name: &str) -> Option<Pid<A>> {
     whereis(name).map(crate::pid::assert_type::<A>)
 }
 
-/// Resolve `name` to its actor's pid and a cloned `Sender<M>`, under the Leaf
-/// lock (clone-under-lock, then release). The crate-internal building block for
-/// `gen_server`'s by-name addressing: a named server publishes its inbox as a
-/// `Sender<Envelope<G>>` (via [`register_with`]), and `whereis_server` / `call`
-/// / `cast` recover that exact typed sender here to rebuild a `GenServerRef<G>`.
-/// `None` if unbound, dead (pruned on the way out), or holding no `M` channel.
+/// Resolve `name` to its actor's pid and a cloned `Sender<M>`, all under one
+/// lock acquisition. The crate-internal building block for `gen_server`'s
+/// by-name addressing: a named server publishes its inbox as a
+/// `Sender<Envelope<G>>` (via [`register_with`]), and the server's `call` /
+/// `cast` / `whereis_server` recover that exact typed sender here to rebuild a
+/// `GenServerRef<G>`. `None` if unbound, dead (pruned on the way out), or
+/// holding no `M` channel.
 pub(crate) fn resolve_named_sender<M: Send + 'static>(name: &str) -> Option<(Pid, Sender<M>)> {
     with_runtime(|inner| {
         let mut reg = inner.registry.lock();
         let pid = *reg.by_name.get(name)?;
         if !live(inner, pid) {
             // Stored-pid liveness, generation included: a name whose holder
-            // died is pruned (heals) even if the slot has a new tenant —
-            // previously the tenant's mailbox made the name unresolvable
-            // *without* pruning, wedging it for the tenant's lifetime.
+            // died is pruned (heals) even if the slot has a new tenant.
+            // Otherwise the tenant's mailbox would make the name unresolvable
+            // without pruning, wedging it for the tenant's lifetime.
             reg.prune_holder(pid);
             return None;
         }
         // A live holder's mailbox is its own (publish replaces wholesale on
         // pid mismatch, and one live actor per slot), so index lookup is safe.
-        let tx = reg.by_index.get(&pid.index()).and_then(Mailbox::clone_sender::<M>)?;
+        let tx = reg
+            .by_index
+            .get(&pid.index())
+            .and_then(Mailbox::clone_sender::<M>)?;
         Some((pid, tx))
     })
 }
 
-/// Remove the binding for `name`, returning the actor it pointed at if still
-/// live. Only the *name* is freed; the actor's mailbox (and any other names for
-/// it) remain. A binding to a dead actor is reported as `None`.
+/// Give up a name. Returns the actor it pointed at, if that actor was still
+/// live. Only the *name* is freed; the actor's mailbox (and any other names
+/// bound to it) are unaffected. A binding to an already-dead actor reports
+/// `None`, since there was nothing live to release.
 pub fn unregister(name: &str) -> Option<Pid> {
     with_runtime(|inner| {
         let mut reg = inner.registry.lock();
         let pid = reg.by_name.remove(name)?;
-        if live(inner, pid) { Some(pid) } else { None }
+        if live(inner, pid) {
+            Some(pid)
+        } else {
+            None
+        }
     })
 }
 
-/// Resolve `name` to its actor's `Sender<M>` and deliver `msg`. The whole point
-/// of the rework: a name you can *send* to.
+/// Look `name` up and deliver `msg` to whichever actor currently holds it.
+/// This is the point of naming an actor: a name you can send a message to
+/// directly, without a separate lookup step.
 ///
-/// Errors (message returned in every case): [`SendError::Unresolved`] if no
-/// live actor holds the name, [`SendError::NoChannel`] if that actor has no
-/// channel for `M`, [`SendError::Closed`] if its `M` channel's receiver is
-/// gone. Panics if called outside `Runtime::run()`.
+/// On failure the message comes back to you, wrapped in the [`SendError`]
+/// variant that explains why: [`SendError::Unresolved`] if no live actor
+/// currently holds the name, [`SendError::NoChannel`] if the actor that holds
+/// it never published a channel for `M`, or [`SendError::Closed`] if it
+/// published one but has since dropped the receiving end. Panics if called
+/// outside [`run`](crate::run).
 pub fn send<M: Send + 'static>(name: Name<M>, msg: M) -> Result<(), SendError<M>> {
     let key = name.as_str();
     with_runtime(|inner| {
-        // Resolve + clone the sender under the Leaf lock, then drop the lock
-        // before sending (a send can unpark a receiver).
+        // Resolve + clone the sender under the registry lock, then drop the
+        // lock before sending (a send can unpark a receiver).
         let tx = {
             let mut reg = inner.registry.lock();
             let pid = match reg.by_name.get(key) {
@@ -489,43 +643,51 @@ pub fn send<M: Send + 'static>(name: Name<M>, msg: M) -> Result<(), SendError<M>
                 None => return Err(SendError::Unresolved(msg)),
             };
             if !live(inner, pid) {
-                // Stored-pid liveness (generation included). Previously this
-                // checked the slot's *current* mailbox pid, so a recycled
-                // slot's live tenant passed — and a same-typed tenant would
-                // have received the message (misdelivery), a differently
-                // typed one a misleading NoChannel.
+                // Stored-pid liveness (generation included), so a recycled
+                // slot's new live tenant is never mistaken for the name's
+                // original (now-dead) holder.
                 reg.prune_holder(pid);
                 return Err(SendError::Unresolved(msg));
             }
-            match reg.by_index.get(&pid.index()).and_then(Mailbox::clone_sender::<M>) {
+            match reg
+                .by_index
+                .get(&pid.index())
+                .and_then(Mailbox::clone_sender::<M>)
+            {
                 Some(tx) => tx,
                 None => return Err(SendError::NoChannel(msg)),
             }
         };
-        tx.send(msg).map_err(|crate::channel::SendError(m)| SendError::Closed(m))
+        tx.send(msg)
+            .map_err(|crate::channel::SendError(m)| SendError::Closed(m))
     })
 }
 
 /// Resolve a *raw* pid to its mailbox and deliver `msg` on the channel for `M`,
 /// with **no redirect**. The stored mailbox must be this exact incarnation
-/// (generation included) and still live; otherwise the actor this pid named is
-/// gone and the result is [`SendError::Dead`] — even when the slot now holds a
-/// different, live actor (which we leave untouched). Shared by [`send_to`]
-/// (typed, `M = A::Msg`, channel guaranteed on an installed actor) and
-/// [`send_dyn`] (explicit `M`, where `NoChannel` is a real outcome).
+/// (generation included) and still live; otherwise the actor this pid named
+/// is gone and the result is [`SendError::Dead`], even when the slot now
+/// holds a different, live actor (which is left untouched). Shared by
+/// [`send_to`] (typed, `M = A::Msg`, channel guaranteed on an installed
+/// actor) and [`send_dyn`] (explicit `M`, where `NoChannel` is a real
+/// outcome).
 fn send_to_pid<M: Send + 'static>(
     inner: &crate::runtime::RuntimeInner,
     pid: Pid,
     msg: M,
 ) -> Result<(), SendError<M>> {
-    // Resolve + clone the sender under the Leaf lock, then drop the lock before
-    // sending (a send can unpark a receiver) — Leaf -> Channel, as name `send`.
+    // Resolve + clone the sender under the registry lock, then drop the lock
+    // before sending (a send can unpark a receiver), same order as `send`.
     let tx = {
         let mut reg = inner.registry.lock();
         match reg.by_index.get(&pid.index()).map(|m| m.pid) {
             // Exact incarnation, still alive: its `M` channel, or NoChannel.
             Some(stored) if stored == pid && live(inner, pid) => {
-                match reg.by_index.get(&pid.index()).and_then(Mailbox::clone_sender::<M>) {
+                match reg
+                    .by_index
+                    .get(&pid.index())
+                    .and_then(Mailbox::clone_sender::<M>)
+                {
                     Some(tx) => tx,
                     None => return Err(SendError::NoChannel(msg)),
                 }
@@ -540,35 +702,40 @@ fn send_to_pid<M: Send + 'static>(
             _ => return Err(SendError::Dead(msg)),
         }
     };
-    tx.send(msg).map_err(|crate::channel::SendError(m)| SendError::Closed(m))
+    tx.send(msg)
+        .map_err(|crate::channel::SendError(m)| SendError::Closed(m))
 }
 
-/// Deliver `msg` to the exact actor named by `pid` — RFC 014 §4.2's direct,
-/// identity-bound addressing mode. Unlike name-addressed [`send`] there is **no
-/// redirect**: if that incarnation has died the message comes back as
-/// [`SendError::Dead`], even if its slot now holds a different actor.
+/// Deliver `msg` directly to the exact actor identified by `pid`. Unlike
+/// name-addressed [`send`], there is **no redirect**: if that specific actor
+/// has died, the message comes back as [`SendError::Dead`], even if its slot
+/// has since been taken over by a different, live actor. Use this when you
+/// already hold a `Pid<A>` and want to talk to that one actor specifically;
+/// use [`send`] with a [`Name`] when you want whichever actor currently holds
+/// a name.
 ///
 /// The message type is the actor's `A::Msg`, so on a live actor that has
-/// installed its inbox (via [`install`] or [`register`]) the channel is always
-/// present; [`SendError::NoChannel`] therefore means the actor is live but
-/// never published a `Pid<A>`-reachable inbox. Panics if called outside
-/// `Runtime::run()`.
+/// installed its inbox (via [`install`] or [`register`]) the channel is
+/// always present; [`SendError::NoChannel`] therefore means the actor is live
+/// but never published a `Pid<A>`-reachable inbox. Panics if called outside
+/// [`run`](crate::run).
 pub fn send_to<A: Addressable>(pid: Pid<A>, msg: A::Msg) -> Result<(), SendError<A::Msg>> {
     with_runtime(|inner| send_to_pid::<A::Msg>(inner, pid.erase(), msg))
 }
 
-/// The explicit bare-pid escape hatch (RFC 014 §4.6): deliver `msg` of type `M`
-/// to `pid` when all you hold is an untyped [`Pid`] — a pid off a [`Down`], or
-/// out of a future `members()` — so the typed [`send_to`] is unavailable.
+/// The escape hatch for sending to a bare, untyped [`Pid`] when the typed
+/// [`send_to`] is unavailable, for example a pid recovered from a [`Down`]
+/// notification or a group's `members()` list, where you no longer know the
+/// actor's message type at compile time.
 ///
-/// This is the one send whose message type can genuinely be wrong: the actor
-/// may be live yet expose no channel for `M`, returning [`SendError::NoChannel`]
-/// (on the typed paths that downcast collapses to a `debug_assert`). It is
-/// named and documented as the fallible fallback so the typed `Pid<A>` /
-/// `Name<M>` paths stay the obvious default and an agentic caller reaches for a
-/// present primitive instead of inventing a workaround. Liveness is identical
-/// to [`send_to`]: identity-bound, no redirect, [`SendError::Dead`] once the
-/// addressed incarnation is gone. Panics if called outside `Runtime::run()`.
+/// Because the message type is not checked at compile time here, this is the
+/// one send that can genuinely be live-but-wrong: the actor may be alive yet
+/// expose no channel for `M`, in which case you get [`SendError::NoChannel`]
+/// back instead of a misdelivery. Liveness and redirect behavior are
+/// otherwise identical to [`send_to`]: identity-bound, no redirect,
+/// [`SendError::Dead`] once the addressed incarnation is gone. Prefer
+/// `send_to` with a typed `Pid<A>` whenever you have one; reach for this only
+/// when you don't. Panics if called outside [`run`](crate::run).
 ///
 /// [`Down`]: crate::Down
 pub fn send_dyn<M: Send + 'static>(pid: Pid, msg: M) -> Result<(), SendError<M>> {

src/run_queue.rs

@@ -222,7 +222,10 @@ impl MpmcRing {
             if diff == 0 {
                 // Our turn: claim the position.
                 match self.enqueue_pos.0.compare_exchange_weak(
-                    pos, pos + 1, Ordering::Relaxed, Ordering::Relaxed,
+                    pos,
+                    pos + 1,
+                    Ordering::Relaxed,
+                    Ordering::Relaxed,
                 ) {
                     Ok(_) => {
                         // SAFETY: the claim gives us exclusive write access
@@ -250,7 +253,10 @@ impl MpmcRing {
             let diff = seq as isize - (pos + 1) as isize;
             if diff == 0 {
                 match self.dequeue_pos.0.compare_exchange_weak(
-                    pos, pos + 1, Ordering::Relaxed, Ordering::Relaxed,
+                    pos,
+                    pos + 1,
+                    Ordering::Relaxed,
+                    Ordering::Relaxed,
                 ) {
                     Ok(_) => {
                         // SAFETY: the claim gives us exclusive read access;
@@ -464,19 +470,36 @@ mod tests {
 
         let popped = popped.lock().unwrap();
         assert_eq!(popped.len(), total, "count mismatch");
-        let set: HashSet<u64> = popped.iter().map(|p| ((p.index() as u64) << 32) | p.generation() as u64).collect();
+        let set: HashSet<u64> = popped
+            .iter()
+            .map(|p| ((p.index() as u64) << 32) | p.generation() as u64)
+            .collect();
         assert_eq!(set.len(), total, "duplicate or lost element");
         assert_eq!(pop(&q), None);
     }
 
     #[test]
     fn mpmc_exactly_once_contended() {
-        exactly_once(MpmcRing::new(8, 4096), |q, p| q.push(p), |q| q.pop(), 4, 4, 1000);
+        exactly_once(
+            MpmcRing::new(8, 4096),
+            |q, p| q.push(p),
+            |q| q.pop(),
+            4,
+            4,
+            1000,
+        );
     }
 
     #[test]
     fn striped_exactly_once_contended() {
-        exactly_once(StripedRing::new(8, 4096), |q, p| q.push(p), |q| q.pop(), 4, 4, 1000);
+        exactly_once(
+            StripedRing::new(8, 4096),
+            |q, p| q.push(p),
+            |q| q.pop(),
+            4,
+            4,
+            1000,
+        );
     }
 
     #[test]

src/signal.rs

@@ -0,0 +1,330 @@
+//! RFC 019 §7 — overflow diagnostics.
+//!
+//! One process-global SIGSEGV handler, installed once at [`crate::runtime::init`]
+//! (before any scheduler thread exists, so the PRIOR save is unracing), plus a
+//! per-scheduler-thread `sigaltstack` registered at `schedule_loop` entry — a
+//! guard hit means the faulting stack has no room to run anything, so the
+//! altstack is not optional.
+//!
+//! The handler classifies `si_addr` against the *current* actor only, reached
+//! through `preempt::CURRENT_SLOT` — a const-initialized `Cell<*const Slot>`
+//! whose access is a plain TLS load (no lazy init, no allocation, no dtor
+//! registration), and which every scheduler thread has materialized before an
+//! actor can run on it. The slot's diag atomics (`diag_stack_top` & co) are
+//! written in `install_actor` before the Release publish and are only consulted
+//! here while the actor is on-CPU, so they cannot be stale.
+//!
+//! Two classification tiers:
+//!  - **In-guard**: definitive. Rust frames probe pages in order
+//!    (`__rust_probestack`), so Rust overflow always lands here; so does any C
+//!    built with `-fstack-clash-protection` (distro-packaged libraries), and —
+//!    with the 1 MiB default guard — nearly every unprobed frame too.
+//!  - **Overshoot**: within [`OVERSHOOT_SLOP`] *below* the guard. An unprobed
+//!    frame (cargo-built C via `cc` almost never enables clash protection)
+//!    large enough to step over the guard in one `sub rsp`. Attribution is
+//!    "probable": the address is in unmapped VA that nothing else owns, an
+//!    actor was on-CPU, and the distance fits a frame — the diagnostic says so.
+//!
+//! Classified faults print one line (async-signal-safe: stack buffer +
+//! `write(2)`, no fmt, no alloc, no locks) and re-raise with default
+//! disposition — no unwind, no resume, no fail-soft (jarred; UB-adjacent from
+//! a handler). Unclassified faults reinstate the PRIOR handler and refault, so
+//! std's own "thread ... has overflowed its stack" diagnostics for OS-thread
+//! stacks survive our presence. Reinstating deregisters us for good, which is
+//! fine: the process is dying either way.
+
+use std::cell::Cell;
+use std::mem::MaybeUninit;
+use std::sync::atomic::Ordering;
+use std::sync::Once;
+
+/// Tier-2 window below the guard. Matches the guard default (and the kernel's
+/// `stack_guard_gap`): a frame that out-jumps both the guard and this window
+/// in one displacement is past what a diagnostic can honestly attribute.
+pub(crate) const OVERSHOOT_SLOP: usize = 1024 * 1024;
+
+/// Per-scheduler-thread signal stack. MINSIGSTKSZ is ~11 KiB on AVX-512
+/// hardware; 64 KiB leaves the formatter room without mattering to anyone.
+/// One per OS thread, never freed: scheduler threads live for the process in
+/// practice, and repeated `run()`s on reused threads re-use the registration
+/// (the TLS flag), so the leak is bounded by the OS thread count.
+const ALTSTACK_SIZE: usize = 64 * 1024;
+
+static INSTALL: Once = Once::new();
+/// The handler that was installed before ours (std's, typically). Written
+/// exactly once inside INSTALL — which completes in `runtime::init` before
+/// any scheduler thread (and thus any classifiable fault) can exist — and
+/// only read from the handler afterwards.
+static mut PRIOR: MaybeUninit<libc::sigaction> = MaybeUninit::uninit();
+
+thread_local! {
+    /// Whether this OS thread has registered its altstack.
+    static ALTSTACK_SET: Cell<bool> = const { Cell::new(false) };
+}
+
+/// Where a fault landed relative to the current actor's stack.
+#[derive(Debug, PartialEq, Eq)]
+pub(crate) enum FaultClass {
+    /// Inside `[top − reserve − guard, top − reserve)`: the guard region.
+    Guard,
+    /// Within `OVERSHOOT_SLOP` below the guard: stepped over it. Payload is
+    /// the distance below `guard_lo`.
+    Overshoot(usize),
+    /// Not ours to explain.
+    Foreign,
+}
+
+/// Pure classifier — all edges unit-tested below. `top` is the stack's usable
+/// top, `reserve`/`guard` its shape; both page-rounded by `Stack::new`.
+pub(crate) fn classify(addr: usize, top: usize, reserve: usize, guard: usize) -> FaultClass {
+    let guard_hi = top.wrapping_sub(reserve);
+    let guard_lo = guard_hi.wrapping_sub(guard);
+    if addr >= guard_lo && addr < guard_hi {
+        FaultClass::Guard
+    } else if addr < guard_lo && addr >= guard_lo.saturating_sub(OVERSHOOT_SLOP) {
+        FaultClass::Overshoot(guard_lo - addr)
+    } else {
+        FaultClass::Foreign
+    }
+}
+
+/// Install the process-global handler. Idempotent; called from
+/// `runtime::init`.
+pub(crate) fn install_once() {
+    INSTALL.call_once(|| unsafe {
+        let mut sa: libc::sigaction = std::mem::zeroed();
+        sa.sa_sigaction = handler as *const () as usize;
+        sa.sa_flags = libc::SA_SIGINFO | libc::SA_ONSTACK;
+        libc::sigemptyset(&mut sa.sa_mask);
+        let prior = &mut *std::ptr::addr_of_mut!(PRIOR);
+        libc::sigaction(libc::SIGSEGV, &sa, prior.as_mut_ptr());
+    });
+}
+
+/// Register this OS thread's altstack (idempotent per thread). Called at
+/// `schedule_loop` entry, so every thread that can run an actor has one.
+pub(crate) fn register_altstack() {
+    ALTSTACK_SET.with(|set| {
+        if set.get() {
+            return;
+        }
+        unsafe {
+            let sp = libc::mmap(
+                std::ptr::null_mut(),
+                ALTSTACK_SIZE,
+                libc::PROT_READ | libc::PROT_WRITE,
+                libc::MAP_PRIVATE | libc::MAP_ANONYMOUS,
+                -1,
+                0,
+            );
+            if sp == libc::MAP_FAILED {
+                // Degrade: no altstack means a guard hit dies without the
+                // message (handler can't run) — the pre-RFC behavior, never
+                // incorrectness.
+                return;
+            }
+            let ss = libc::stack_t {
+                ss_sp: sp,
+                ss_flags: 0,
+                ss_size: ALTSTACK_SIZE,
+            };
+            libc::sigaltstack(&ss, std::ptr::null_mut());
+        }
+        set.set(true);
+    });
+}
+
+// ---------------------------------------------------------------------------
+// The handler
+// ---------------------------------------------------------------------------
+
+unsafe extern "C" fn handler(
+    _sig: libc::c_int,
+    info: *mut libc::siginfo_t,
+    _ctx: *mut libc::c_void,
+) {
+    let slot_ptr = crate::preempt::current_slot_ptr();
+    if !slot_ptr.is_null() {
+        let slot = &*slot_ptr;
+        let top = slot.diag_stack_top.load(Ordering::Relaxed);
+        if top != 0 {
+            let reserve = slot.diag_stack_reserve.load(Ordering::Relaxed);
+            let guard = slot.diag_stack_guard.load(Ordering::Relaxed);
+            let pid = slot.diag_pid.load(Ordering::Relaxed);
+            let addr = (*info).si_addr() as usize;
+            match classify(addr, top, reserve, guard) {
+                FaultClass::Guard => {
+                    let mut b = Buf::new();
+                    b.s("smarm: actor ");
+                    b.pid(pid);
+                    b.s(" overflowed its stack: fault in the guard region, depth-at-fault=");
+                    b.u(top - addr);
+                    b.s(" bytes (reserve=");
+                    b.u(reserve);
+                    b.s(", guard=");
+                    b.u(guard);
+                    b.s("). Raise stack_reserve (SpawnOpts or Config).\n");
+                    b.emit();
+                    die_by_default();
+                    return;
+                }
+                FaultClass::Overshoot(below) => {
+                    let mut b = Buf::new();
+                    b.s("smarm: actor ");
+                    b.pid(pid);
+                    b.s(" probably overflowed its stack: fault ");
+                    b.u(below);
+                    b.s(" bytes below the guard - an unprobed (FFI?) frame stepped over it (reserve=");
+                    b.u(reserve);
+                    b.s(", guard=");
+                    b.u(guard);
+                    b.s("). Raise stack_guard or stack_reserve.\n");
+                    b.emit();
+                    die_by_default();
+                    return;
+                }
+                FaultClass::Foreign => {}
+            }
+        }
+    }
+    // Not ours: put back whoever was there before us and refault into them.
+    let prior = &*std::ptr::addr_of!(PRIOR);
+    libc::sigaction(libc::SIGSEGV, prior.as_ptr(), std::ptr::null_mut());
+}
+
+/// Reset SIGSEGV to default disposition; returning from the handler then
+/// refaults at the same instruction and the process dies the normal death
+/// (core-dumpable, correct wait status), exactly as if we were never here —
+/// but with the message already on stderr.
+unsafe fn die_by_default() {
+    let mut dfl: libc::sigaction = std::mem::zeroed();
+    dfl.sa_sigaction = libc::SIG_DFL;
+    libc::sigemptyset(&mut dfl.sa_mask);
+    libc::sigaction(libc::SIGSEGV, &dfl, std::ptr::null_mut());
+}
+
+// ---------------------------------------------------------------------------
+// Async-signal-safe formatting: fixed buffer, decimal itoa, one write(2).
+// ---------------------------------------------------------------------------
+
+struct Buf {
+    b: [u8; 320],
+    len: usize,
+}
+
+impl Buf {
+    fn new() -> Self {
+        Buf {
+            b: [0; 320],
+            len: 0,
+        }
+    }
+    fn s(&mut self, s: &str) {
+        for &c in s.as_bytes() {
+            if self.len < self.b.len() {
+                self.b[self.len] = c;
+                self.len += 1;
+            }
+        }
+    }
+    fn u(&mut self, mut n: usize) {
+        let mut tmp = [0u8; 20];
+        let mut i = tmp.len();
+        loop {
+            i -= 1;
+            tmp[i] = b'0' + (n % 10) as u8;
+            n /= 10;
+            if n == 0 {
+                break;
+            }
+        }
+        for &c in &tmp[i..] {
+            if self.len < self.b.len() {
+                self.b[self.len] = c;
+                self.len += 1;
+            }
+        }
+    }
+    /// `idx.gen`, unpacked from the install-time packing.
+    fn pid(&mut self, packed: u64) {
+        self.u((packed >> 32) as usize);
+        self.s(".");
+        self.u((packed & 0xffff_ffff) as usize);
+    }
+    fn emit(&self) {
+        unsafe {
+            libc::write(2, self.b.as_ptr() as *const libc::c_void, self.len);
+        }
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Classifier units — the arithmetic edges, before anything integrates.
+// ---------------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::{classify, FaultClass, OVERSHOOT_SLOP};
+
+    const PG: usize = 4096;
+    // A synthetic stack far from address-space edges: top at 1 GiB.
+    const TOP: usize = 1 << 30;
+    const RESERVE: usize = 16 * PG;
+    const GUARD: usize = 4 * PG;
+    const GUARD_HI: usize = TOP - RESERVE;
+    const GUARD_LO: usize = GUARD_HI - GUARD;
+
+    #[test]
+    fn inside_guard_both_edges() {
+        assert_eq!(classify(GUARD_LO, TOP, RESERVE, GUARD), FaultClass::Guard);
+        assert_eq!(
+            classify(GUARD_HI - 1, TOP, RESERVE, GUARD),
+            FaultClass::Guard
+        );
+        assert_eq!(
+            classify(GUARD_LO + GUARD / 2, TOP, RESERVE, GUARD),
+            FaultClass::Guard
+        );
+    }
+
+    #[test]
+    fn usable_region_is_foreign() {
+        // A fault inside the RW stack itself isn't a guard hit and must not
+        // be explained as one.
+        assert_eq!(classify(GUARD_HI, TOP, RESERVE, GUARD), FaultClass::Foreign);
+        assert_eq!(classify(TOP - 1, TOP, RESERVE, GUARD), FaultClass::Foreign);
+    }
+
+    #[test]
+    fn above_top_is_foreign() {
+        assert_eq!(classify(TOP, TOP, RESERVE, GUARD), FaultClass::Foreign);
+        assert_eq!(classify(TOP + PG, TOP, RESERVE, GUARD), FaultClass::Foreign);
+    }
+
+    #[test]
+    fn overshoot_window_edges() {
+        assert_eq!(
+            classify(GUARD_LO - 1, TOP, RESERVE, GUARD),
+            FaultClass::Overshoot(1)
+        );
+        assert_eq!(
+            classify(GUARD_LO - OVERSHOOT_SLOP, TOP, RESERVE, GUARD),
+            FaultClass::Overshoot(OVERSHOOT_SLOP)
+        );
+        assert_eq!(
+            classify(GUARD_LO - OVERSHOOT_SLOP - 1, TOP, RESERVE, GUARD),
+            FaultClass::Foreign
+        );
+    }
+
+    #[test]
+    fn low_address_stack_saturates_not_wraps() {
+        // A stack mapped so low that the slop window would underflow: the
+        // window clips to 0 instead of wrapping around the address space.
+        let top = RESERVE + GUARD + PG; // guard_lo == PG
+        assert_eq!(classify(0, top, RESERVE, GUARD), FaultClass::Overshoot(PG));
+        // Null-page fault still classified only because it IS within slop
+        // here; with a normal-height stack it is Foreign (covered above by
+        // the window-edge test at realistic addresses).
+    }
+}

src/slot_state.rs

@@ -188,8 +188,7 @@ impl StateWord {
         loop {
             let w = self.load();
             debug_assert!(
-                matches!(word_state(w), ST_RUNNING | ST_RUNNING_NOTIFIED)
-                    && word_gen(w) == gen,
+                matches!(word_state(w), ST_RUNNING | ST_RUNNING_NOTIFIED) && word_gen(w) == gen,
                 "yield return from invalid word {w:#x}"
             );
             if self
@@ -247,8 +246,7 @@ impl StateWord {
         loop {
             let w = self.load();
             debug_assert!(
-                matches!(word_state(w), ST_RUNNING | ST_RUNNING_NOTIFIED)
-                    && word_gen(w) == gen,
+                matches!(word_state(w), ST_RUNNING | ST_RUNNING_NOTIFIED) && word_gen(w) == gen,
                 "begin_wait from invalid word {w:#x}"
             );
             let next = word_epoch(w).wrapping_add(1) & EPOCH_MASK;
@@ -342,8 +340,7 @@ impl StateWord {
         loop {
             let w = self.load();
             debug_assert!(
-                matches!(word_state(w), ST_RUNNING | ST_RUNNING_NOTIFIED)
-                    && word_gen(w) == gen,
+                matches!(word_state(w), ST_RUNNING | ST_RUNNING_NOTIFIED) && word_gen(w) == gen,
                 "clear_notify from invalid word {w:#x}"
             );
             if word_state(w) != ST_RUNNING_NOTIFIED {
@@ -372,8 +369,7 @@ impl StateWord {
     pub(crate) fn set_done(&self, gen: u32) {
         let prev = self.0.swap(pack(gen, 0, ST_DONE), Ordering::AcqRel);
         debug_assert!(
-            matches!(word_state(prev), ST_RUNNING | ST_RUNNING_NOTIFIED)
-                && word_gen(prev) == gen,
+            matches!(word_state(prev), ST_RUNNING | ST_RUNNING_NOTIFIED) && word_gen(prev) == gen,
             "finalize from invalid word {prev:#x}"
         );
     }
@@ -538,7 +534,11 @@ mod loom_tests {
             // not a pending notification.
             assert!(word.try_claim(0));
             assert_eq!(word.unpark(0, Some(epoch)), Unpark::Noop);
-            assert_eq!(word_state(word.load()), ST_RUNNING, "stale epoch notified a live run");
+            assert_eq!(
+                word_state(word.load()),
+                ST_RUNNING,
+                "stale epoch notified a live run"
+            );
         });
     }
 

src/stack.rs

@@ -1,32 +1,45 @@
-//! mmap-based growable stack with a guard page below.
+//! mmap-based actor stack with a PROT_NONE guard region below (RFC 019).
 //!
 //! Layout (low → high address):
-//!   [ guard page (PROT_NONE) | stack region ]
+//!   [ guard region (PROT_NONE) | stack region ]
 //!                                              ^ top() — initial stack pointer
 //!
-//! Stacks grow downward. Overflow lands in the guard page → SIGSEGV.
+//! Stacks grow downward. Overflow lands in the guard region → SIGSEGV.
+//!
+//! Both the usable reserve and the guard are caller-chosen (page-rounded).
+//! The reserve is a *virtual* reservation: anonymous mmap is demand-paged,
+//! so RSS is touched-pages, not reserve × actors. The guard costs address
+//! space only. A wide guard (the runtime defaults to 64 KiB) exists for
+//! unprobed FFI frames: Rust frames touch pages in order (probestack), so
+//! one page catches Rust overflow, but a C frame with a large local can
+//! step over a single page in one `sub rsp`.
 
 use std::io;
 
 pub struct Stack {
-    /// Bottom of the entire mmap'd region (start of guard page).
+    /// Bottom of the entire mmap'd region (start of the guard).
     base: *mut u8,
     /// Total mmap'd size: guard_size + stack_size.
     total_size: usize,
-    /// Usable stack size (excluding guard page).
+    /// Usable stack size (excluding the guard).
     stack_size: usize,
+    /// PROT_NONE region below the usable stack.
+    guard_size: usize,
 }
 
 // Stack owns its memory; safe to send across threads.
 unsafe impl Send for Stack {}
 
 impl Stack {
-    /// Allocate a new stack. `stack_size` is the usable region; one page is
-    /// added below as a guard page. Both are rounded up to the page size.
-    pub fn new(stack_size: usize) -> io::Result<Self> {
+    /// Allocate a new stack. `stack_size` is the usable region; `guard_size`
+    /// is mapped PROT_NONE below it. Both are rounded up to the page size
+    /// and must be non-zero.
+    pub fn new(stack_size: usize, guard_size: usize) -> io::Result<Self> {
+        assert!(stack_size > 0, "stack_size must be non-zero");
+        assert!(guard_size > 0, "guard_size must be non-zero");
         let page = page_size();
         let stack_size = round_up(stack_size, page);
-        let guard_size = page;
+        let guard_size = round_up(guard_size, page);
         let total_size = guard_size + stack_size;
 
         let base = unsafe {
@@ -44,16 +57,19 @@ impl Stack {
         }
         let base = base as *mut u8;
 
-        let ret = unsafe {
-            libc::mprotect(base as *mut libc::c_void, guard_size, libc::PROT_NONE)
-        };
+        let ret = unsafe { libc::mprotect(base as *mut libc::c_void, guard_size, libc::PROT_NONE) };
         if ret != 0 {
             let err = io::Error::last_os_error();
             unsafe { libc::munmap(base as *mut libc::c_void, total_size) };
             return Err(err);
         }
 
-        Ok(Self { base, total_size, stack_size })
+        Ok(Self {
+            base,
+            total_size,
+            stack_size,
+            guard_size,
+        })
     }
 
     /// 16-byte-aligned top of the usable region.
@@ -62,14 +78,54 @@ impl Stack {
         (raw_top & !15) as *mut u8
     }
 
-    /// Pointer to the bottom of the usable region (just above the guard page).
+    /// Pointer to the bottom of the usable region (just above the guard).
     pub fn usable_base(&self) -> *mut u8 {
-        unsafe { self.base.add(page_size()) }
+        unsafe { self.base.add(self.guard_size) }
     }
 
     pub fn stack_size(&self) -> usize {
         self.stack_size
     }
+
+    pub fn guard_size(&self) -> usize {
+        self.guard_size
+    }
+
+    /// `(stack_size, guard_size)` after page rounding. The pool rule
+    /// (RFC 019 §1) compares this against the runtime defaults: only
+    /// default-shaped stacks are pooled.
+    pub fn shape(&self) -> (usize, usize) {
+        (self.stack_size, self.guard_size)
+    }
+
+    /// Pool-recycle zap (RFC 019 §6): `MADV_DONTNEED` everything below the
+    /// retained entry end `[top − retain, top)` — the span the next actor's
+    /// shallow frames land in stays resident, the dead spike below it is
+    /// released. The stack is unowned at the call site (its actor is dead),
+    /// so a synchronous eager zap races nothing and the RSS drop is
+    /// immediate — a museum of worst-case spikes is exactly what a pool must
+    /// not be; DONTNEED's ~8× per-page cost vs FREE is irrelevant off the
+    /// hot path. Advisory like the park-path shrink: a failure degrades to
+    /// "the pool keeps RSS", never to incorrectness. No-op (no syscall) when
+    /// `retain` covers the whole usable region — i.e. always, at the 64 KiB
+    /// default reserve.
+    pub(crate) fn recycle_zap(&self, retain: usize) {
+        if let Some((off, len)) = retain_range(self.stack_size, retain, page_size()) {
+            unsafe {
+                libc::madvise(
+                    self.usable_base().add(off) as *mut libc::c_void,
+                    len,
+                    libc::MADV_DONTNEED,
+                );
+            }
+        }
+    }
+}
+
+/// Round `n` up to whole pages — the same rounding `Stack::new` applies, so
+/// runtime defaults stored pre-rounded compare exactly against [`Stack::shape`].
+pub(crate) fn round_to_pages(n: usize) -> usize {
+    round_up(n, page_size())
 }
 
 impl Drop for Stack {
@@ -80,10 +136,174 @@ impl Drop for Stack {
     }
 }
 
-fn page_size() -> usize {
+pub(crate) fn page_size() -> usize {
     unsafe { libc::sysconf(libc::_SC_PAGESIZE) as usize }
 }
 
 fn round_up(n: usize, align: usize) -> usize {
     (n + align - 1) & !(align - 1)
 }
+
+/// The whole-page span the park-path shrink may `MADV_FREE` (RFC 019 §3):
+/// `[page_up(hwm), page_down(sp − redzone))`, or `None` if no full page fits.
+///
+/// `hwm` is the sampled high-water (deepest observed `sp`); everything in
+/// `[hwm, sp)` is below the live frame and dead by definition. One page of
+/// redzone stays resident under live `sp` — it covers the SysV 128-byte red
+/// zone plus spill margin with room to spare. Rounding is inward on both
+/// ends so the result can never touch the redzone, cross `sp`, or dip below
+/// `hwm`; all arithmetic is checked so adversarial inputs (`sp < redzone`,
+/// `hwm ≥ sp`, values near the address-space edges) collapse to `None`
+/// rather than a wild or negative-length range.
+pub(crate) fn shrink_range(hwm: usize, sp: usize, page: usize) -> Option<(usize, usize)> {
+    debug_assert!(page.is_power_of_two());
+    if hwm >= sp {
+        return None;
+    }
+    let redzone = page;
+    let end = sp.checked_sub(redzone)? & !(page - 1); // page_down(sp − redzone)
+    let start = hwm.checked_add(page - 1)? & !(page - 1); // page_up(hwm)
+    if end > start {
+        Some((start, end - start))
+    } else {
+        None
+    }
+}
+
+/// The `(offset_from_usable_base, len)` span the pool recycle DONTNEEDs
+/// (RFC 019 §6): everything below the retained entry end. "Bottom RETAIN of
+/// the stack" is read stack-wise (entry frames = highest addresses of a
+/// downward stack): the retained span is `[top − page_up(retain), top)`, the
+/// zapped span is the rest — retaining the low-address deep end instead
+/// would keep the coldest pages and release the ones the next actor faults
+/// first. `retain` rounds *up* to whole pages (retain more, zap less), so
+/// with `stack_size` page-rounded by `Stack::new` the result is always
+/// page-aligned. Checked math: `retain ≥ stack_size` (notably the default
+/// 64 KiB reserve with the 64 KiB RETAIN) and overflow collapse to `None`.
+pub(crate) fn retain_range(
+    stack_size: usize,
+    retain: usize,
+    page: usize,
+) -> Option<(usize, usize)> {
+    debug_assert!(page.is_power_of_two());
+    let retain = retain.checked_add(page - 1)? & !(page - 1); // page_up(retain)
+    let len = stack_size.checked_sub(retain)?;
+    if len == 0 {
+        return None;
+    }
+    Some((0, len))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{retain_range, shrink_range};
+
+    const PG: usize = 4096;
+
+    #[test]
+    fn retain_covers_whole_stack_is_a_noop() {
+        // The default config: reserve == RETAIN == 64 KiB. No zap, no syscall.
+        assert_eq!(retain_range(16 * PG, 16 * PG, PG), None);
+        assert_eq!(retain_range(PG, PG, PG), None);
+    }
+
+    #[test]
+    fn retain_larger_than_stack_is_a_noop() {
+        assert_eq!(retain_range(16 * PG, 17 * PG, PG), None);
+        assert_eq!(retain_range(PG, usize::MAX, PG), None); // page_up overflows
+    }
+
+    #[test]
+    fn retain_zero_zaps_everything() {
+        assert_eq!(retain_range(16 * PG, 0, PG), Some((0, 16 * PG)));
+    }
+
+    #[test]
+    fn retain_rounds_up_zapping_less() {
+        // 1 byte of retain keeps a whole page.
+        assert_eq!(retain_range(16 * PG, 1, PG), Some((0, 15 * PG)));
+        assert_eq!(retain_range(16 * PG, PG + 1, PG), Some((0, 14 * PG)));
+    }
+
+    #[test]
+    fn retain_one_page_short_of_stack() {
+        assert_eq!(retain_range(2 * PG, PG, PG), Some((0, PG)));
+    }
+
+    #[test]
+    fn retain_range_is_page_aligned() {
+        for size_pg in [1usize, 2, 3, 16, 1024] {
+            for retain in [0usize, 1, PG - 1, PG, PG + 1, 4 * PG, size_pg * PG] {
+                if let Some((off, len)) = retain_range(size_pg * PG, retain, PG) {
+                    assert_eq!(off, 0);
+                    assert_eq!(len % PG, 0);
+                    assert!(len <= size_pg * PG);
+                    assert!(len > 0);
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn empty_and_inverted_spans_are_none() {
+        assert_eq!(shrink_range(0x8000_0000, 0x8000_0000, PG), None); // hwm == sp
+        assert_eq!(shrink_range(0x8000_1000, 0x8000_0000, PG), None); // hwm > sp
+    }
+
+    #[test]
+    fn span_smaller_than_redzone_plus_page_is_none() {
+        let sp = 0x8000_0000;
+        // Everything within redzone+1 page of sp: no full page clears both
+        // the redzone and the page_up(hwm) rounding.
+        assert_eq!(shrink_range(sp - PG, sp, PG), None);
+        assert_eq!(shrink_range(sp - 2 * PG + 1, sp, PG), None);
+    }
+
+    #[test]
+    fn exact_two_pages_frees_one() {
+        let sp = 0x8000_0000;
+        let hwm = sp - 2 * PG;
+        // [hwm, hwm+PG) frees; [sp−PG, sp) is redzone.
+        assert_eq!(shrink_range(hwm, sp, PG), Some((hwm, PG)));
+    }
+
+    #[test]
+    fn unaligned_ends_round_inward() {
+        let sp = 0x8000_0123; // live sp mid-page
+        let hwm = 0x7f00_0abc; // high-water mid-page
+        let (start, len) = shrink_range(hwm, sp, PG).unwrap();
+        assert_eq!(start % PG, 0);
+        assert_eq!(len % PG, 0);
+        assert!(start >= hwm); // never below the sampled high-water
+        assert!(start + len <= (sp - PG) & !(PG - 1)); // never into the redzone
+    }
+
+    #[test]
+    fn result_never_crosses_sp() {
+        // Sweep hwm across every offset of the page straddling the boundary.
+        let sp = 0x8000_0000 + 137;
+        for hwm in (sp - 4 * PG)..(sp) {
+            if let Some((start, len)) = shrink_range(hwm, sp, PG) {
+                assert!(start >= hwm);
+                assert!(start + len + PG <= sp + PG); // end ≤ page_down(sp − PG) < sp
+                assert!(len > 0);
+            }
+        }
+    }
+
+    #[test]
+    fn underflow_near_zero_is_none() {
+        assert_eq!(shrink_range(0, PG - 1, PG), None); // sp < redzone
+        assert_eq!(shrink_range(0, 0, PG), None);
+    }
+
+    #[test]
+    fn big_span_frees_interior() {
+        let sp = 0x8000_0000;
+        let spike = 4 * 1024 * 1024;
+        let hwm = sp - spike;
+        let (start, len) = shrink_range(hwm, sp, PG).unwrap();
+        assert_eq!(start, hwm); // aligned input: starts exactly at hwm
+        assert_eq!(len, spike - PG); // everything but the redzone page
+    }
+}

src/supervisor.rs

@@ -169,7 +169,10 @@ pub struct ChildSpec {
 
 impl ChildSpec {
     pub fn new(restart: Restart, start: impl Fn() + Send + Sync + 'static) -> Self {
-        Self { start: Arc::new(start), restart }
+        Self {
+            start: Arc::new(start),
+            restart,
+        }
     }
 }
 
@@ -392,4 +395,3 @@ impl OneForOne {
         }
     }
 }
-

src/sync_shim.rs

@@ -6,10 +6,19 @@
 //! Build the loom models with: `RUSTFLAGS="--cfg loom" cargo test --lib --release`
 
 #[cfg(loom)]
-pub(crate) use loom::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
+pub(crate) use loom::sync::atomic::{fence, AtomicU64, AtomicUsize, Ordering};
 
 #[cfg(not(loom))]
-pub(crate) use std::sync::atomic::{AtomicU64, AtomicUsize, Ordering};
+pub(crate) use std::sync::atomic::{fence, AtomicU64, AtomicUsize, Ordering};
+
+// park.rs condvar-parker (loom + non-Linux builds only; the Linux non-loom
+// build parks on a futex and never touches these — gating them identically
+// keeps the default build free of unused imports).
+#[cfg(loom)]
+pub(crate) use loom::sync::{Condvar, Mutex};
+
+#[cfg(all(not(loom), not(target_os = "linux")))]
+pub(crate) use std::sync::{Condvar, Mutex};
 
 /// `UnsafeCell` with loom's `with`/`with_mut` access API; pass-through cost
 /// is zero in normal builds (`#[inline]`, newtype over std's cell).

src/timer.rs

@@ -129,7 +129,9 @@ impl Ord for Entry {
         // Earlier deadline first; ties broken by insertion order so the
         // ordering is total. `Reason` and `Pid` deliberately don't
         // participate.
-        self.deadline.cmp(&other.deadline).then_with(|| self.seq.cmp(&other.seq))
+        self.deadline
+            .cmp(&other.deadline)
+            .then_with(|| self.seq.cmp(&other.seq))
     }
 }
 
@@ -141,6 +143,14 @@ impl PartialOrd for Entry {
 
 #[derive(Default)]
 pub struct Timers {
+    /// RFC 018: the scheduler coordination layer. Attached once at
+    /// `RuntimeInner::new`; every insert notes its deadline (min-maintained
+    /// snapshot for the busy-path due-check + the timekeeper re-arm wake)
+    /// and every pop/clear re-anchors the snapshot to the heap minimum.
+    /// All calls happen under the timers mutex — the serialization the
+    /// coordinator's timer protocol mandates. `None` only in unit tests
+    /// that construct a bare `Timers`.
+    coord: Option<std::sync::Arc<crate::park::Coordinator>>,
     /// Reverse-wrapped so the smallest deadline is at the top.
     heap: BinaryHeap<Reverse<Entry>>,
     /// Monotonic counter for the tiebreaker `seq` field (and the `TimerId` of a
@@ -157,7 +167,18 @@ pub struct Timers {
 
 impl Timers {
     pub fn new() -> Self {
-        Self { heap: BinaryHeap::new(), next_seq: 0, armed: std::collections::HashSet::new() }
+        Self {
+            coord: None,
+            heap: BinaryHeap::new(),
+            next_seq: 0,
+            armed: std::collections::HashSet::new(),
+        }
+    }
+
+    /// Attach the scheduler coordination layer (RFC 018). Called once, at
+    /// runtime construction, before any scheduler thread exists.
+    pub(crate) fn attach_coordinator(&mut self, c: std::sync::Arc<crate::park::Coordinator>) {
+        self.coord = Some(c);
     }
 
     /// Insert a `Sleep` timer. Convenience for the common case.
@@ -242,6 +263,13 @@ impl Timers {
             #[cfg(feature = "smarm-causal")]
             wall,
         }));
+        // RFC 018: publish the (possibly new-minimum) deadline to the
+        // busy-path snapshot and wake the timekeeper if it is parked
+        // toward a later one. We hold the timers mutex — the mandated
+        // serialization for both.
+        if let Some(c) = &self.coord {
+            c.note_deadline(deadline);
+        }
         seq
     }
 
@@ -255,6 +283,9 @@ impl Timers {
     pub fn clear(&mut self) {
         self.heap.clear();
         self.armed.clear();
+        if let Some(c) = &self.coord {
+            c.refresh_deadline(None);
+        }
     }
 
     /// Soonest pending deadline, or `None` if the heap is empty.
@@ -324,6 +355,13 @@ impl Timers {
             }
             out.push(entry);
         }
+        // RFC 018: re-anchor the busy-path snapshot to the new heap minimum
+        // (still under the timers mutex). A causal-shift re-queue above went
+        // through `heap.push` directly, so this peek is the one place the
+        // snapshot is guaranteed to catch up.
+        if let Some(c) = &self.coord {
+            c.refresh_deadline(self.peek_deadline());
+        }
         out
     }
 }

src/trace.rs

@@ -16,13 +16,17 @@
 #[cfg(feature = "smarm-trace")]
 #[macro_export]
 macro_rules! te {
-    ($kind:expr) => { $crate::trace::record($kind) };
+    ($kind:expr) => {
+        $crate::trace::record($kind)
+    };
 }
 
 #[cfg(not(feature = "smarm-trace"))]
 #[macro_export]
 macro_rules! te {
-    ($kind:expr) => { () };
+    ($kind:expr) => {
+        ()
+    };
 }
 
 #[cfg(feature = "smarm-trace")]
@@ -109,8 +113,8 @@ mod inner {
     // -----------------------------------------------------------------------
 
     pub fn open() {
-        let path = std::env::var("SMARM_TRACE_FILE")
-            .unwrap_or_else(|_| "smarm_trace.json".to_owned());
+        let path =
+            std::env::var("SMARM_TRACE_FILE").unwrap_or_else(|_| "smarm_trace.json".to_owned());
 
         let (tx, rx) = mpsc::channel::<Msg>();
         let start = Instant::now();
@@ -164,8 +168,11 @@ mod inner {
         // which would try to re-acquire inner.shared (already held at many
         // te!() call sites) -> deadlock. Guard at the very top, before any
         // allocation-capable call.
-        let was_enabled = crate::preempt::PREEMPTION_ENABLED
-            .with(|e| { let v = e.get(); e.set(false); v });
+        let was_enabled = crate::preempt::PREEMPTION_ENABLED.with(|e| {
+            let v = e.get();
+            e.set(false);
+            v
+        });
 
         LOCAL_STATE.with(|cell| {
             let mut opt = cell.borrow_mut();
@@ -197,7 +204,10 @@ mod inner {
     fn drain_thread(rx: mpsc::Receiver<Msg>, path: &str) {
         let f = match std::fs::File::create(path) {
             Ok(f) => f,
-            Err(e) => { eprintln!("[smarm-trace] create failed: {}", e); return; }
+            Err(e) => {
+                eprintln!("[smarm-trace] create failed: {}", e);
+                return;
+            }
         };
         let mut w = std::io::BufWriter::new(f);
         let _ = writeln!(w, "{{\"traceEvents\":[");
@@ -210,7 +220,9 @@ mod inner {
                 Ok(Msg::Event(r)) => {
                     let (name, actor_idx) = chrome_fields(&r.event);
                     let ts_us = r.nanos as f64 / 1000.0;
-                    if !first { let _ = w.write_all(b",\n"); }
+                    if !first {
+                        let _ = w.write_all(b",\n");
+                    }
                     first = false;
                     let _ = write!(w,
                         "{{\"ph\":\"i\",\"ts\":{:.3},\"pid\":{},\"tid\":{},\"name\":{:?},\"s\":\"g\"}}",
@@ -234,8 +246,9 @@ mod inner {
 
     fn chrome_fields(ev: &Event) -> (String, u32) {
         match ev {
-            Event::Spawn { parent, child } =>
-                (format!("spawn c={}", child.index()), parent.index()),
+            Event::Spawn { parent, child } => {
+                (format!("spawn c={}", child.index()), parent.index())
+            }
             Event::Resume(p) => ("resume".into(), p.index()),
             Event::Yield(p) => ("yield".into(), p.index()),
             Event::Park(p) => ("park".into(), p.index()),
@@ -244,9 +257,12 @@ mod inner {
             Event::UnparkDeferred(p) => ("unpark_deferred".into(), p.index()),
             Event::UnparkFlagConsumed(p) => ("unpark_flag_consumed".into(), p.index()),
             Event::Send { sender, receiver } => (
-                format!("send rx={}", receiver
+                format!(
+                    "send rx={}",
+                    receiver
                         .map(|p| p.index().to_string())
-                    .unwrap_or_else(|| "none".into())),
+                        .unwrap_or_else(|| "none".into())
+                ),
                 sender.index(),
             ),
             Event::RecvPark(p) => ("recv_park".into(), p.index()),

tests/cancel.rs

@@ -49,8 +49,14 @@ fn looping_actor_on_check_is_stopped() {
         }
         let _ = h.join();
     });
-    assert!(saw_stopped.load(Ordering::SeqCst), "expected DownReason::Stopped");
-    assert!(dropped.load(Ordering::SeqCst), "Drop guard must run during the cancellation unwind");
+    assert!(
+        saw_stopped.load(Ordering::SeqCst),
+        "expected DownReason::Stopped"
+    );
+    assert!(
+        dropped.load(Ordering::SeqCst),
+        "Drop guard must run during the cancellation unwind"
+    );
 }
 
 #[test]
@@ -79,8 +85,14 @@ fn parked_on_recv_actor_is_stopped() {
         }
         let _ = h.join();
     });
-    assert!(saw_stopped.load(Ordering::SeqCst), "expected DownReason::Stopped");
-    assert!(dropped.load(Ordering::SeqCst), "Drop guard must run on cancellation of a parked actor");
+    assert!(
+        saw_stopped.load(Ordering::SeqCst),
+        "expected DownReason::Stopped"
+    );
+    assert!(
+        dropped.load(Ordering::SeqCst),
+        "Drop guard must run on cancellation of a parked actor"
+    );
 }
 
 #[test]
@@ -185,6 +197,12 @@ fn stop_flagged_while_queued_lands_at_first_park() {
         .recv_timeout(Duration::from_secs(10))
         .expect("runtime deadlocked: stop against a QUEUED actor was lost at its first park");
 
-    assert!(saw_stopped.load(Ordering::SeqCst), "expected DownReason::Stopped");
-    assert!(dropped.load(Ordering::SeqCst), "Drop guard must run during the cancellation unwind");
+    assert!(
+        saw_stopped.load(Ordering::SeqCst),
+        "expected DownReason::Stopped"
+    );
+    assert!(
+        dropped.load(Ordering::SeqCst),
+        "Drop guard must run during the cancellation unwind"
+    );
 }

tests/causal.rs

@@ -24,7 +24,11 @@ fn progress_point_counts() {
         h.join().unwrap();
         let after = smarm::causal::progress_snapshot();
         let delta = |name: &str| {
-            after.iter().find(|(n, _)| n == name).map(|(_, c)| *c).unwrap()
+            after
+                .iter()
+                .find(|(n, _)| n == name)
+                .map(|(_, c)| *c)
+                .unwrap()
                 - before
                     .iter()
                     .find(|(n, _)| n == name)
@@ -45,21 +49,12 @@ fn site_guard_nesting_restores() {
             assert_eq!(smarm::causal::current_site_name(), None);
             {
                 let _outer = smarm::causal_site!("outer");
-                assert_eq!(
-                    smarm::causal::current_site_name().as_deref(),
-                    Some("outer")
-                );
+                assert_eq!(smarm::causal::current_site_name().as_deref(), Some("outer"));
                 {
                     let _inner = smarm::causal_site!("inner");
-                    assert_eq!(
-                        smarm::causal::current_site_name().as_deref(),
-                        Some("inner")
-                    );
+                    assert_eq!(smarm::causal::current_site_name().as_deref(), Some("inner"));
                 }
-                assert_eq!(
-                    smarm::causal::current_site_name().as_deref(),
-                    Some("outer")
-                );
+                assert_eq!(smarm::causal::current_site_name().as_deref(), Some("outer"));
             }
             assert_eq!(smarm::causal::current_site_name(), None);
         });
@@ -88,10 +83,7 @@ fn virtual_speedup_ledger() {
         let bystander = smarm::spawn(move || {
             while !stop2.load(Ordering::Relaxed) {
                 smarm::check!();
-                out2.store(
-                    smarm::causal::my_absorbed_delay_cycles(),
-                    Ordering::Relaxed,
-                );
+                out2.store(smarm::causal::my_absorbed_delay_cycles(), Ordering::Relaxed);
             }
         });
 
@@ -166,10 +158,7 @@ fn runnable_bystander_pays_delay() {
             while !stop_b.load(Ordering::Relaxed) {
                 iters2.fetch_add(1, Ordering::Relaxed);
                 smarm::check!();
-                absorbed2.store(
-                    smarm::causal::my_absorbed_delay_cycles(),
-                    Ordering::Relaxed,
-                );
+                absorbed2.store(smarm::causal::my_absorbed_delay_cycles(), Ordering::Relaxed);
             }
         });
 
@@ -177,8 +166,7 @@ fn runnable_bystander_pays_delay() {
             let i0 = iters.load(Ordering::Relaxed);
             let t = std::time::Instant::now();
             smarm::sleep(Duration::from_millis(150));
-            let rate =
-                (iters.load(Ordering::Relaxed) - i0) as f64 / t.elapsed().as_secs_f64();
+            let rate = (iters.load(Ordering::Relaxed) - i0) as f64 / t.elapsed().as_secs_f64();
             out.store(rate as u64, Ordering::Relaxed);
         };
 
@@ -448,7 +436,10 @@ fn timer_deadline_shifts_with_injected_delay() {
 
     // Raw deadline passed, effective deadline not: nothing fires, entry kept.
     assert!(t.pop_due(now + Duration::from_millis(60)).is_empty());
-    assert!(!t.is_empty(), "shifted entry must be re-queued, not dropped");
+    assert!(
+        !t.is_empty(),
+        "shifted entry must be re-queued, not dropped"
+    );
 
     // Past raw + injected (with margin) it must fire. Chase in case a
     // parallel test injected more debt meanwhile.
@@ -503,7 +494,11 @@ fn wall_timer_ignores_injected_delay() {
     // Just past the raw deadline: the wall entry fires, the virtual one is
     // re-queued at its shifted deadline.
     let due = t.pop_due(now + Duration::from_millis(60));
-    assert_eq!(due.len(), 1, "exactly the wall entry must fire at raw deadline");
+    assert_eq!(
+        due.len(),
+        1,
+        "exactly the wall entry must fire at raw deadline"
+    );
     assert_eq!(due[0].pid, Pid::new(0, 0));
     assert!(!t.is_empty(), "virtual sibling must remain queued, shifted");
 }
@@ -623,7 +618,11 @@ fn wall_send_after_ignores_injected_delay() {
 
     // Just past the raw deadline: only the wall send pops; run its thunk.
     let due = t.pop_due(now + Duration::from_millis(60));
-    assert_eq!(due.len(), 1, "exactly the wall send must fire at raw deadline");
+    assert_eq!(
+        due.len(),
+        1,
+        "exactly the wall send must fire at raw deadline"
+    );
     for e in due {
         if let smarm::timer::Reason::Send { fire } = e.reason {
             fire();

tests/channel.rs

@@ -154,7 +154,10 @@ fn channel_ops_interleaved_with_monitor_churn_multi_thread() {
         }
         consumer.join().unwrap();
     });
-    assert_eq!(total.load(std::sync::atomic::Ordering::Relaxed), (0..32).sum::<i64>());
+    assert_eq!(
+        total.load(std::sync::atomic::Ordering::Relaxed),
+        (0..32).sum::<i64>()
+    );
 }
 
 // ---------------------------------------------------------------------------
@@ -220,7 +223,10 @@ fn recv_timeout_reports_disconnected_on_close() {
 fn recv_timeout_zero_duration_is_a_bounded_poll() {
     run(|| {
         let (_tx, rx) = channel::<i64>();
-        assert_eq!(rx.recv_timeout(Duration::ZERO), Err(RecvTimeoutError::Timeout));
+        assert_eq!(
+            rx.recv_timeout(Duration::ZERO),
+            Err(RecvTimeoutError::Timeout)
+        );
     });
 }
 
@@ -262,7 +268,8 @@ fn recv_timeout_many_waiters_multi_thread() {
             let (tx, rx) = channel::<i64>();
             let got = got2.clone();
             let timed_out = timed_out2.clone();
-            handles.push(spawn(move || match rx.recv_timeout(Duration::from_millis(100)) {
+            handles.push(spawn(move || {
+                match rx.recv_timeout(Duration::from_millis(100)) {
                     Ok(v) => {
                         assert_eq!(v, i);
                         got.fetch_add(1, Ordering::Relaxed);
@@ -271,6 +278,7 @@ fn recv_timeout_many_waiters_multi_thread() {
                         timed_out.fetch_add(1, Ordering::Relaxed);
                     }
                     Err(e) => panic!("unexpected: {e}"),
+                }
             }));
             if i % 2 == 0 {
                 handles.push(spawn(move || {

tests/context.rs

@@ -11,9 +11,15 @@ thread_local! {
     static LOG: Cell<u64> = const { Cell::new(0) };
 }
 
-fn log(v: u64) { LOG.with(|c| c.set(c.get() | v)); }
-fn get_log() -> u64 { LOG.with(|c| c.get()) }
-fn reset_log() { LOG.with(|c| c.set(0)); }
+fn log(v: u64) {
+    LOG.with(|c| c.set(c.get() | v));
+}
+fn get_log() -> u64 {
+    LOG.with(|c| c.get())
+}
+fn reset_log() {
+    LOG.with(|c| c.set(0));
+}
 
 extern "C-unwind" fn actor_simple() {
     log(0x1);
@@ -23,7 +29,7 @@ extern "C-unwind" fn actor_simple() {
 #[test]
 fn actor_runs_and_returns_to_scheduler() {
     reset_log();
-    let stack = Stack::new(64 * 1024).unwrap();
+    let stack = Stack::new(64 * 1024, 4096).unwrap();
     let sp = init_actor_stack(stack.top(), actor_simple);
     set_actor_sp(sp);
     unsafe { switch_to_actor() };
@@ -40,7 +46,7 @@ extern "C-unwind" fn actor_two_steps() {
 #[test]
 fn actor_yields_and_resumes() {
     reset_log();
-    let stack = Stack::new(64 * 1024).unwrap();
+    let stack = Stack::new(64 * 1024, 4096).unwrap();
     let sp = init_actor_stack(stack.top(), actor_two_steps);
     set_actor_sp(sp);
 
@@ -73,7 +79,10 @@ extern "C-unwind" fn actor_reg_check() {
         REG_BEFORE.set([s0, s1, s2, s3]).ok();
         switch_to_scheduler();
 
-        let a0: u64; let a1: u64; let a2: u64; let a3: u64;
+        let a0: u64;
+        let a1: u64;
+        let a2: u64;
+        let a3: u64;
         core::arch::asm!(
             "mov {a0}, r12", "mov {a1}, r13", "mov {a2}, r14", "mov {a3}, r15",
             a0 = out(reg) a0, a1 = out(reg) a1, a2 = out(reg) a2, a3 = out(reg) a3,
@@ -85,11 +94,17 @@ extern "C-unwind" fn actor_reg_check() {
 
 #[test]
 fn callee_saved_registers_survive_yield() {
-    let stack = Stack::new(64 * 1024).unwrap();
+    let stack = Stack::new(64 * 1024, 4096).unwrap();
     let sp = init_actor_stack(stack.top(), actor_reg_check);
     set_actor_sp(sp);
-    unsafe { switch_to_actor(); switch_to_actor(); }
-    assert_eq!(REG_BEFORE.get().copied().unwrap(), REG_AFTER.get().copied().unwrap());
+    unsafe {
+        switch_to_actor();
+        switch_to_actor();
+    }
+    assert_eq!(
+        REG_BEFORE.get().copied().unwrap(),
+        REG_AFTER.get().copied().unwrap()
+    );
 }
 
 // Two actors, independent stacks.
@@ -117,20 +132,24 @@ extern "C-unwind" fn actor_b() {
 
 #[test]
 fn two_actors_dont_corrupt_each_other() {
-    let stack_a = Stack::new(64 * 1024).unwrap();
-    let stack_b = Stack::new(64 * 1024).unwrap();
+    let stack_a = Stack::new(64 * 1024, 4096).unwrap();
+    let stack_b = Stack::new(64 * 1024, 4096).unwrap();
 
     let sp_a = init_actor_stack(stack_a.top(), actor_a);
     let sp_b = init_actor_stack(stack_b.top(), actor_b);
 
-    set_actor_sp(sp_a); unsafe { switch_to_actor() };
+    set_actor_sp(sp_a);
+    unsafe { switch_to_actor() };
     let sp_a = get_actor_sp();
 
-    set_actor_sp(sp_b); unsafe { switch_to_actor() };
+    set_actor_sp(sp_b);
+    unsafe { switch_to_actor() };
     let sp_b = get_actor_sp();
 
-    set_actor_sp(sp_a); unsafe { switch_to_actor() };
-    set_actor_sp(sp_b); unsafe { switch_to_actor() };
+    set_actor_sp(sp_a);
+    unsafe { switch_to_actor() };
+    set_actor_sp(sp_b);
+    unsafe { switch_to_actor() };
 
     assert_eq!(A_VAL.with(|c| c.get()), 0xA00D);
     assert_eq!(B_VAL.with(|c| c.get()), 0xB00D);

tests/fd_select.rs

@@ -11,8 +11,8 @@
 //! OUTSIDE `run` — an in-actor assertion alone passes vacuously.
 
 use smarm::{
-    channel, run, select, select_timeout, spawn, try_select, wait_readable,
-    wait_readable_timeout, wait_writable_timeout, yield_now, FdArm,
+    channel, run, select, select_timeout, spawn, try_select, wait_readable, wait_readable_timeout,
+    wait_writable_timeout, yield_now, FdArm,
 };
 use std::os::fd::RawFd;
 use std::sync::atomic::{AtomicBool, AtomicU32, Ordering};
@@ -33,7 +33,10 @@ impl Pipe {
         let mut fds: [libc::c_int; 2] = [0; 2];
         let r = unsafe { libc::pipe2(fds.as_mut_ptr(), libc::O_CLOEXEC | libc::O_NONBLOCK) };
         assert_eq!(r, 0, "pipe2 failed");
-        Pipe { read: fds[0], write: fds[1] }
+        Pipe {
+            read: fds[0],
+            write: fds[1],
+        }
     }
 }
 
@@ -253,12 +256,18 @@ fn wait_readable_timeout_times_out_then_succeeds_with_data() {
         let (rfd, wfd) = (p.read, p.write);
 
         let start = Instant::now();
-        assert_eq!(wait_readable_timeout(rfd, Duration::from_millis(30)).unwrap(), false);
+        assert_eq!(
+            wait_readable_timeout(rfd, Duration::from_millis(30)).unwrap(),
+            false
+        );
         assert!(start.elapsed() >= Duration::from_millis(30));
 
         // Timed-out wait must leave the fd clean; ready path returns true.
         assert_eq!(raw_write(wfd, b"d"), 1);
-        assert_eq!(wait_readable_timeout(rfd, Duration::from_secs(5)).unwrap(), true);
+        assert_eq!(
+            wait_readable_timeout(rfd, Duration::from_secs(5)).unwrap(),
+            true
+        );
         let mut buf = [0u8; 1];
         assert_eq!(raw_read(rfd, &mut buf), 1);
         ok2.store(true, Ordering::SeqCst);
@@ -274,7 +283,10 @@ fn wait_readable_timeout_wakes_on_late_data() {
         let p = Pipe::new();
         let (rfd, wfd) = (p.read, p.write);
         let h = spawn(move || {
-            assert_eq!(wait_readable_timeout(rfd, Duration::from_secs(5)).unwrap(), true);
+            assert_eq!(
+                wait_readable_timeout(rfd, Duration::from_secs(5)).unwrap(),
+                true
+            );
             let mut buf = [0u8; 1];
             assert_eq!(raw_read(rfd, &mut buf), 1);
             got2.store(buf[0] as u32, Ordering::SeqCst);
@@ -292,7 +304,10 @@ fn wait_writable_timeout_ready_now_on_empty_pipe() {
     run(move || {
         let p = Pipe::new();
         // An empty pipe's write end is writable: ready-now path, no park.
-        assert_eq!(wait_writable_timeout(p.write, Duration::from_secs(5)).unwrap(), true);
+        assert_eq!(
+            wait_writable_timeout(p.write, Duration::from_secs(5)).unwrap(),
+            true
+        );
         ok2.store(true, Ordering::SeqCst);
     });
     assert!(ok.load(Ordering::SeqCst));

tests/gen_server.rs

@@ -403,7 +403,10 @@ fn worker_pool_down_reaches_handle_down() {
     let got = Arc::new(Mutex::new(Vec::new()));
     let got2 = got.clone();
     run(move || {
-        let server = start(Pool { watcher: None, log: Vec::new() });
+        let server = start(Pool {
+            watcher: None,
+            log: Vec::new(),
+        });
         server.cast(PoolCast::SpawnDoomedWorker).unwrap();
         let _ = server.call(()).unwrap(); // sync point: cast handled, worker live
         *got2.lock().unwrap() = server.call(()).unwrap();
@@ -421,7 +424,10 @@ fn watch_dead_pid_is_noproc_down() {
         let h = spawn(|| {});
         let dead = h.pid();
         h.join().unwrap();
-        let server = start(Pool { watcher: None, log: Vec::new() });
+        let server = start(Pool {
+            watcher: None,
+            log: Vec::new(),
+        });
         server.cast(PoolCast::Watch(dead)).unwrap();
         *got2.lock().unwrap() = server.call(()).unwrap();
     });
@@ -497,7 +503,12 @@ impl GenServer for Timed {
 }
 
 fn timed(fired: Arc<Mutex<Vec<u32>>>, cancel_won: Arc<Mutex<Option<bool>>>) -> Timed {
-    Timed { timer: None, fired, cancel_won, last: None }
+    Timed {
+        timer: None,
+        fired,
+        cancel_won,
+        last: None,
+    }
 }
 
 // A one-shot armed from a handler fires into handle_timer with its payload.
@@ -534,7 +545,11 @@ fn cancel_before_fire_suppresses_it() {
         let count = server.call(()).unwrap();
         assert_eq!(count, 0, "cancelled timer must not fire");
     });
-    assert_eq!(*cancel_won.lock().unwrap(), Some(true), "cancel beat the fire");
+    assert_eq!(
+        *cancel_won.lock().unwrap(),
+        Some(true),
+        "cancel beat the fire"
+    );
     assert!(fired.lock().unwrap().is_empty());
 }
 
@@ -549,11 +564,16 @@ fn tick_every_rearms_repeatedly() {
     run(move || {
         let cw = Arc::new(Mutex::new(None));
         let server = start(timed(f2, cw));
-        server.cast(TkCast::Tick(Duration::from_millis(20))).unwrap();
+        server
+            .cast(TkCast::Tick(Duration::from_millis(20)))
+            .unwrap();
         let _ = server.call(()).unwrap(); // sync: periodic armed
         smarm::sleep(Duration::from_millis(130)); // ~6 periods
         let count = server.call(()).unwrap();
-        assert!(count >= 3, "periodic should have re-armed several times, got {count}");
+        assert!(
+            count >= 3,
+            "periodic should have re-armed several times, got {count}"
+        );
     });
     // Every tick delivered the same payload.
     assert!(fired.lock().unwrap().iter().all(|&v| v == 9));
@@ -568,7 +588,9 @@ fn cancel_stops_a_periodic() {
     let c2 = cancel_won.clone();
     run(move || {
         let server = start(timed(f2, c2));
-        server.cast(TkCast::Tick(Duration::from_millis(20))).unwrap();
+        server
+            .cast(TkCast::Tick(Duration::from_millis(20)))
+            .unwrap();
         let _ = server.call(()).unwrap();
         smarm::sleep(Duration::from_millis(70)); // a few ticks
         server.cast(TkCast::CancelLast).unwrap();
@@ -616,11 +638,17 @@ fn idle_fires_repeatedly_on_quiet() {
     let idles = Arc::new(Mutex::new(0));
     let i2 = idles.clone();
     run(move || {
-        let server = start(Idler { window: Duration::from_millis(25), idles: i2 });
+        let server = start(Idler {
+            window: Duration::from_millis(25),
+            idles: i2,
+        });
         smarm::sleep(Duration::from_millis(130)); // quiet ⇒ ~5 windows
         drop(server); // keep the server alive across the quiet span
     });
-    assert!(*idles.lock().unwrap() >= 2, "idle should re-arm and fire several times");
+    assert!(
+        *idles.lock().unwrap() >= 2,
+        "idle should re-arm and fire several times"
+    );
 }
 
 // Traffic within the window keeps idle from firing; only once the inbox goes
@@ -632,7 +660,10 @@ fn traffic_resets_the_idle_window() {
     let before_quiet = Arc::new(Mutex::new(u32::MAX));
     let bq = before_quiet.clone();
     run(move || {
-        let server = start(Idler { window: Duration::from_millis(60), idles: i2 });
+        let server = start(Idler {
+            window: Duration::from_millis(60),
+            idles: i2,
+        });
         // Poke every 25ms (< 60ms window) for ~100ms: each cast resets the
         // window before it can elapse.
         for _ in 0..4 {
@@ -643,8 +674,15 @@ fn traffic_resets_the_idle_window() {
         smarm::sleep(Duration::from_millis(140)); // now genuinely quiet
         drop(server);
     });
-    assert_eq!(*before_quiet.lock().unwrap(), 0, "steady traffic must suppress idle");
-    assert!(*idles.lock().unwrap() >= 1, "idle fires once the inbox falls quiet");
+    assert_eq!(
+        *before_quiet.lock().unwrap(),
+        0,
+        "steady traffic must suppress idle"
+    );
+    assert!(
+        *idles.lock().unwrap() >= 1,
+        "idle fires once the inbox falls quiet"
+    );
 }
 
 // RFC 015 §4.7 — no armed timer survives loop exit. A server with a live
@@ -658,7 +696,9 @@ fn no_timer_survives_exit() {
     let f_read = fired.clone();
     run(move || {
         let server = start(timed(f_server, Arc::new(Mutex::new(None))));
-        server.cast(TkCast::Tick(Duration::from_millis(15))).unwrap();
+        server
+            .cast(TkCast::Tick(Duration::from_millis(15)))
+            .unwrap();
         let _ = server.call(()).unwrap(); // sync: periodic armed
         smarm::sleep(Duration::from_millis(45)); // a couple of ticks
         let mon = smarm::monitor(server.pid());
@@ -667,6 +707,10 @@ fn no_timer_survives_exit() {
         assert!(mon.rx.recv().is_ok());
         let at_exit = f_read.lock().unwrap().len();
         smarm::sleep(Duration::from_millis(90)); // would be several more ticks
-        assert_eq!(f_read.lock().unwrap().len(), at_exit, "no tick may fire after exit");
+        assert_eq!(
+            f_read.lock().unwrap().len(),
+            at_exit,
+            "no tick may fire after exit"
+        );
     });
 }

tests/gen_statem.rs

@@ -100,7 +100,15 @@ fn state_timeout_fires() {
     let got = Arc::new(Mutex::new(0u32));
     let got2 = got.clone();
     run(move || {
-        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 5 });
+        let m = TimerSm::start(
+            T::Idle,
+            TData {
+                enters: 0,
+                st_fires: 0,
+                named_fires: 0,
+                st_window: 5,
+            },
+        );
         m.send(Ev2::Cast(TCast::Arm)).unwrap(); // -> Armed, arms 5ms state-timeout
         smarm::sleep(Duration::from_millis(40)); // let it fire
         *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::StFires(r))).unwrap();
@@ -116,13 +124,25 @@ fn state_timeout_auto_resets_on_transition() {
     let got2 = got.clone();
     run(move || {
         // Long window so the explicit Disarm beats it comfortably.
-        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 50 });
+        let m = TimerSm::start(
+            T::Idle,
+            TData {
+                enters: 0,
+                st_fires: 0,
+                named_fires: 0,
+                st_window: 50,
+            },
+        );
         m.send(Ev2::Cast(TCast::Arm)).unwrap(); // -> Armed, arms 50ms state-timeout
         m.send(Ev2::Cast(TCast::Disarm)).unwrap(); // -> Idle, auto-resets it
         smarm::sleep(Duration::from_millis(80)); // past the original window
         *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::StFires(r))).unwrap();
     });
-    assert_eq!(*got.lock().unwrap(), 0, "auto-reset cancelled the pending state-timeout");
+    assert_eq!(
+        *got.lock().unwrap(),
+        0,
+        "auto-reset cancelled the pending state-timeout"
+    );
 }
 
 // A named timeout survives a state change: armed in Idle, it still fires after
@@ -133,14 +153,26 @@ fn named_timeout_survives_transition() {
     let got2 = got.clone();
     run(move || {
         // Armed's own state-timeout is long so it doesn't interfere.
-        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 200 });
+        let m = TimerSm::start(
+            T::Idle,
+            TData {
+                enters: 0,
+                st_fires: 0,
+                named_fires: 0,
+                st_window: 200,
+            },
+        );
         m.send(Ev2::Cast(TCast::Ping(20))).unwrap(); // arm "ping" for 20ms (in Idle)
         m.send(Ev2::Cast(TCast::Arm)).unwrap(); // -> Armed (ping must survive this)
         m.send(Ev2::Cast(TCast::Disarm)).unwrap(); // -> Idle (and this)
         smarm::sleep(Duration::from_millis(60)); // let "ping" fire
         *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::NamedFires(r))).unwrap();
     });
-    assert_eq!(*got.lock().unwrap(), 1, "named timeout fired across the transitions");
+    assert_eq!(
+        *got.lock().unwrap(),
+        1,
+        "named timeout fired across the transitions"
+    );
 }
 
 // Cancelling a named timeout before its window prevents the fire.
@@ -149,13 +181,25 @@ fn named_timeout_cancel() {
     let got = Arc::new(Mutex::new(99u32));
     let got2 = got.clone();
     run(move || {
-        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 200 });
+        let m = TimerSm::start(
+            T::Idle,
+            TData {
+                enters: 0,
+                st_fires: 0,
+                named_fires: 0,
+                st_window: 200,
+            },
+        );
         m.send(Ev2::Cast(TCast::Ping(30))).unwrap(); // arm "ping" for 30ms
         m.send(Ev2::Cast(TCast::CancelPing)).unwrap(); // cancel before it fires
         smarm::sleep(Duration::from_millis(60)); // past the original window
         *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::NamedFires(r))).unwrap();
     });
-    assert_eq!(*got.lock().unwrap(), 0, "cancel prevented the named-timeout fire");
+    assert_eq!(
+        *got.lock().unwrap(),
+        0,
+        "cancel prevented the named-timeout fire"
+    );
 }
 
 // ===========================================================================
@@ -170,7 +214,15 @@ fn cast_then_call_roundtrip() {
     let got2 = got.clone();
     run(move || {
         // Long state-timeout window so it never fires during the test.
-        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 10_000 });
+        let m = TimerSm::start(
+            T::Idle,
+            TData {
+                enters: 0,
+                st_fires: 0,
+                named_fires: 0,
+                st_window: 10_000,
+            },
+        );
         m.send(Ev2::Cast(TCast::Arm)).unwrap(); // Idle  -> Armed (enter)
         m.send(Ev2::Cast(TCast::Disarm)).unwrap(); // Armed -> Idle  (enter)
         m.send(Ev2::Cast(TCast::Arm)).unwrap(); // Idle  -> Armed (enter)
@@ -178,7 +230,11 @@ fn cast_then_call_roundtrip() {
                                                    // enters = 1 (start) + 4 transitions = 5.
         *got2.lock().unwrap() = m.call(|r| Ev2::Call(TCall::Enters(r))).unwrap();
     });
-    assert_eq!(*got.lock().unwrap(), 5, "one enter on start, one per real transition");
+    assert_eq!(
+        *got.lock().unwrap(),
+        5,
+        "one enter on start, one per real transition"
+    );
 }
 
 // `enter` fires once on start and once per *real* transition; a stay (a call
@@ -188,7 +244,15 @@ fn enter_on_start_and_each_transition_but_not_stay() {
     let got = Arc::new(Mutex::new((0u32, 0u32, 0u32)));
     let got2 = got.clone();
     run(move || {
-        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 10_000 }); // enter -> 1
+        let m = TimerSm::start(
+            T::Idle,
+            TData {
+                enters: 0,
+                st_fires: 0,
+                named_fires: 0,
+                st_window: 10_000,
+            },
+        ); // enter -> 1
         let after_start = m.call(|r| Ev2::Call(TCall::Enters(r))).unwrap();
         // A stay (a counter read returns `prev`) must not bump enters.
         let _ = m.call(|r| Ev2::Call(TCall::StFires(r))).unwrap();
@@ -208,7 +272,15 @@ fn call_to_panicking_handler_is_down() {
     let got = Arc::new(Mutex::new(None::<Result<u32, CallError>>));
     let got2 = got.clone();
     run(move || {
-        let m = TimerSm::start(T::Idle, TData { enters: 0, st_fires: 0, named_fires: 0, st_window: 10_000 });
+        let m = TimerSm::start(
+            T::Idle,
+            TData {
+                enters: 0,
+                st_fires: 0,
+                named_fires: 0,
+                st_window: 10_000,
+            },
+        );
         let r = m.call(|rep| Ev2::Call(TCall::Boom(rep)));
         *got2.lock().unwrap() = Some(r);
     });
@@ -299,7 +371,11 @@ fn postponed_call_answered_after_transition() {
         smarm::sleep(Duration::from_millis(20)); // let the child wake with its reply
         *g2.lock().unwrap() = *taken.lock().unwrap();
     });
-    assert_eq!(*got.lock().unwrap(), Some(42), "postponed call answered by the Filled state");
+    assert_eq!(
+        *got.lock().unwrap(),
+        Some(42),
+        "postponed call answered by the Filled state"
+    );
 }
 
 #[derive(Clone, Copy, PartialEq, Eq, Debug)]

tests/introspect.rs

@@ -56,7 +56,11 @@ fn snapshot_lists_actors_with_parent_edge() {
 
         // The root itself is on-CPU (it's running this code) and rooted under
         // the forest sentinel.
-        let root = snap.actors.iter().find(|a| a.pid == me).expect("root present");
+        let root = snap
+            .actors
+            .iter()
+            .find(|a| a.pid == me)
+            .expect("root present");
         assert_eq!(root.state, ActorState::Running);
         assert_eq!(root.supervisor, smarm::Pid::new(u32::MAX, u32::MAX));
 
@@ -200,7 +204,11 @@ fn tree_places_child_under_its_spawner() {
 
         // The root is parented at the forest sentinel, so it's a genuine root,
         // and the worker it spawned hangs beneath it.
-        let root = t.roots.iter().find(|n| n.info.pid == me).expect("root in forest");
+        let root = t
+            .roots
+            .iter()
+            .find(|n| n.info.pid == me)
+            .expect("root in forest");
         assert!(!root.orphaned);
         assert!(
             root.children.iter().any(|c| c.info.pid == h.pid()),
@@ -237,6 +245,13 @@ fn tree_from_nests_children_and_reroots_orphans() {
         overruns: 0,
         messages_received: 0,
         budget_cycles: 0,
+        stack: smarm::StackInfo {
+            reserve: 0,
+            guard: 0,
+            depth_high_water: 0,
+            parks_since_shrink: 0,
+            shrinks: 0,
+        },
     };
 
     let snap = RuntimeSnapshot {
@@ -251,14 +266,25 @@ fn tree_from_nests_children_and_reroots_orphans() {
     let t = tree_from(snap);
     assert_eq!(t.roots.len(), 2);
 
-    let root = t.roots.iter().find(|n| n.info.pid == root_pid).expect("root present");
+    let root = t
+        .roots
+        .iter()
+        .find(|n| n.info.pid == root_pid)
+        .expect("root present");
     assert!(!root.orphaned);
     assert_eq!(root.children.len(), 1);
     assert_eq!(root.children[0].info.pid, child);
     assert!(!root.children[0].orphaned);
 
-    let o = t.roots.iter().find(|n| n.info.pid == orphan).expect("orphan re-rooted");
-    assert!(o.orphaned, "an actor whose parent is absent must be flagged orphaned");
+    let o = t
+        .roots
+        .iter()
+        .find(|n| n.info.pid == orphan)
+        .expect("orphan re-rooted");
+    assert!(
+        o.orphaned,
+        "an actor whose parent is absent must be flagged orphaned"
+    );
     assert!(o.children.is_empty());
 }
 
@@ -352,3 +378,134 @@ fn budget_cycles_accumulate_when_enabled() {
         h.join().unwrap();
     });
 }
+
+// ---------------------------------------------------------------------------
+// RFC 019 §8 — the stack introspection surface.
+// ---------------------------------------------------------------------------
+
+/// Burn ~`frames` × 4 KiB of stack with a yield at max depth, so the context
+/// save samples the high-water there (RFC 019 §2: hwm is SAMPLED at
+/// deschedule, not tracked continuously).
+#[inline(never)]
+fn burn_stack_yielding(frames: usize) -> u64 {
+    let mut local = [0u8; 4096];
+    local[0] = frames as u8;
+    let below = if frames == 0 {
+        smarm::yield_now();
+        0
+    } else {
+        burn_stack_yielding(frames - 1)
+    };
+    std::hint::black_box(&mut local);
+    below.wrapping_add(local[0] as u64)
+}
+
+#[test]
+fn stack_info_reports_defaults_and_sampled_depth() {
+    run(|| {
+        let (ready_tx, ready_rx) = channel::<()>();
+        let (gate_tx, gate_rx) = channel::<()>();
+
+        let h = spawn(move || {
+            // ~32 KiB deep with a yield at the bottom: the sample point.
+            std::hint::black_box(burn_stack_yielding(8));
+            ready_tx.send(()).unwrap();
+            gate_rx.recv().unwrap();
+        });
+        ready_rx.recv().unwrap();
+
+        let info = spin_until(h.pid(), |a| a.state == ActorState::Parked);
+        let s = info.stack;
+        assert_eq!(s.reserve, 64 * 1024, "default reserve");
+        assert_eq!(
+            s.guard,
+            1024 * 1024,
+            "default guard (kernel stack_guard_gap convention)"
+        );
+        assert!(
+            s.depth_high_water >= 8 * 4096,
+            "hwm sampled at the deep yield: expected ≥ 32 KiB, got {}",
+            s.depth_high_water
+        );
+        assert!(
+            s.depth_high_water < s.reserve,
+            "depth {} cannot exceed the reserve {}",
+            s.depth_high_water,
+            s.reserve
+        );
+        // Parked at the gate right now, never shrunk (64 KiB reserve cannot
+        // cross the shrink threshold).
+        assert!(s.parks_since_shrink >= 1, "the gate park must be counted");
+        assert_eq!(s.shrinks, 0);
+
+        gate_tx.send(()).unwrap();
+        h.join().unwrap();
+    });
+}
+
+#[test]
+fn stack_info_shrink_counters_are_live() {
+    use smarm::runtime::{Config, SHRINK_COOLDOWN, SHRINK_THRESHOLD};
+    use smarm::{spawn_with, SpawnOpts};
+
+    let rt = smarm::runtime::init(Config::exact(1));
+    rt.run(|| {
+        let (park_tx, park_rx) = channel::<()>();
+
+        let spike = 768 * 4096;
+        assert!(spike > SHRINK_THRESHOLD);
+        let worker = spawn_with(
+            SpawnOpts {
+                stack_reserve: Some(8 * 1024 * 1024),
+                ..SpawnOpts::default()
+            },
+            move || {
+                std::hint::black_box(burn_stack_yielding(768));
+                for _ in 0..(SHRINK_COOLDOWN + 8) {
+                    park_rx.recv().unwrap();
+                }
+            },
+        );
+
+        let wpid = worker.pid();
+        // Before any parks complete: the spike depth is visible.
+        let info = spin_until(wpid, |a| a.state == ActorState::Parked);
+        assert!(
+            info.stack.depth_high_water >= spike,
+            "spike should be sampled: {} < {spike}",
+            info.stack.depth_high_water
+        );
+
+        // Cross the cooldown, then read the counters live while the worker
+        // is parked waiting for the remaining rounds (post-join the slot is
+        // reclaimed and the generation check correctly hides it).
+        for _ in 0..(SHRINK_COOLDOWN + 2) {
+            spin_until(wpid, |a| a.state == ActorState::Parked);
+            park_tx.send(()).unwrap();
+        }
+        let info = spin_until(wpid, |a| {
+            a.state == ActorState::Parked && a.stack.shrinks >= 1
+        });
+        let s = info.stack;
+        assert!(
+            s.shrinks >= 1,
+            "cooldown was crossed with a spike above threshold"
+        );
+        assert!(
+            s.parks_since_shrink < SHRINK_COOLDOWN,
+            "counter must reset at shrink: {}",
+            s.parks_since_shrink
+        );
+        assert!(
+            s.depth_high_water < spike,
+            "hwm resets to the shallow park sp at shrink; got {}",
+            s.depth_high_water
+        );
+
+        for _ in 0..6 {
+            spin_until(wpid, |a| a.state == ActorState::Parked);
+            park_tx.send(()).unwrap();
+        }
+        worker.join().unwrap();
+    });
+}

tests/io.rs

@@ -56,8 +56,16 @@ fn other_actors_run_while_block_on_io_is_in_flight() {
     let pos_2 = v.iter().position(|&x| x == 2).unwrap();
     let pos_3 = v.iter().position(|&x| x == 3).unwrap();
     let pos_4 = v.iter().position(|&x| x == 4).unwrap();
-    assert!(pos_2 < pos_4, "B's first step ran after A resumed: {:?}", *v);
-    assert!(pos_3 < pos_4, "B's second step ran after A resumed: {:?}", *v);
+    assert!(
+        pos_2 < pos_4,
+        "B's first step ran after A resumed: {:?}",
+        *v
+    );
+    assert!(
+        pos_3 < pos_4,
+        "B's second step ran after A resumed: {:?}",
+        *v
+    );
 }
 
 #[test]
@@ -76,7 +84,9 @@ fn many_concurrent_block_on_io_calls_all_complete() {
                 cc.fetch_add(n, Ordering::SeqCst);
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
     });
     assert_eq!(counter.load(Ordering::SeqCst), 10);
 }

tests/io_epoll.rs

@@ -144,8 +144,7 @@ fn write_sugar_sends_bytes_to_pipe() {
             // Pipe is empty + has buffer space, so this returns immediately
             // after wait_writable wakes (which happens fast because the
             // kernel marks an empty pipe as immediately writable).
-            let n = smarm::scheduler::write(p_writer.write, b"smarm")
-                .expect("write failed");
+            let n = smarm::scheduler::write(p_writer.write, b"smarm").expect("write failed");
             assert_eq!(n, 5);
             c.fetch_add(1, Ordering::SeqCst);
         });
@@ -209,10 +208,18 @@ fn other_actors_run_while_one_is_parked_on_wait_readable() {
     let pos_lit_a = v.iter().position(|&c| c == b'a').unwrap();
     let big_b_count = v.iter().filter(|&&c| c == b'B').count();
     assert_eq!(big_b_count, 3, "B should have made 3 steps: {:?}", *v);
-    assert!(pos_big_a < pos_lit_a, "A pre-park before A post-park: {:?}", *v);
+    assert!(
+        pos_big_a < pos_lit_a,
+        "A pre-park before A post-park: {:?}",
+        *v
+    );
     // At least the last B step should be before A resumes.
     let last_big_b = v.iter().rposition(|&c| c == b'B').unwrap();
-    assert!(last_big_b < pos_lit_a, "B should finish before A resumes: {:?}", *v);
+    assert!(
+        last_big_b < pos_lit_a,
+        "B should finish before A resumes: {:?}",
+        *v
+    );
 }
 
 // ---------------------------------------------------------------------------

tests/link.rs

@@ -57,7 +57,10 @@ fn linked_pair_one_panics_other_is_stopped() {
             panic!("boom");
         });
 
-        let dn = down_b.rx.recv().expect("monitor channel closed before Down");
+        let dn = down_b
+            .rx
+            .recv()
+            .expect("monitor channel closed before Down");
         assert_eq!(dn.pid, b, "Down reported the wrong pid");
         if matches!(dn.reason, DownReason::Stopped) {
             s.store(true, Ordering::SeqCst);
@@ -152,7 +155,10 @@ fn link_to_dead_pid_stops_a_nontrapping_caller() {
         });
         let b = hb.pid();
         let down_b = monitor(b);
-        let dn = down_b.rx.recv().expect("monitor channel closed before Down");
+        let dn = down_b
+            .rx
+            .recv()
+            .expect("monitor channel closed before Down");
         if matches!(dn.reason, DownReason::Stopped) {
             s.store(true, Ordering::SeqCst);
         }

tests/monitor.rs

@@ -67,7 +67,10 @@ fn monitor_already_dead_target_is_noproc() {
         // and its generation bumped, so `pid` is now stale.
         h.join().unwrap();
         let down = monitor(pid);
-        let d = down.rx.recv().expect("NoProc Down should be delivered immediately");
+        let d = down
+            .rx
+            .recv()
+            .expect("NoProc Down should be delivered immediately");
         assert_eq!(d.pid, pid);
         if matches!(d.reason, DownReason::NoProc) {
             o.store(true, Ordering::SeqCst);
@@ -91,7 +94,11 @@ fn multiple_monitors_all_notified() {
             }
         }
     });
-    assert_eq!(count.load(Ordering::SeqCst), 3, "every monitor should see the Down");
+    assert_eq!(
+        count.load(Ordering::SeqCst),
+        3,
+        "every monitor should see the Down"
+    );
 }
 
 #[test]
@@ -103,8 +110,15 @@ fn demonitor_stops_delivery() {
         let h = spawn(|| {});
         let pid = h.pid();
         let m = monitor(pid);
-        assert_eq!(demonitor(&m), Some(m.id), "live registration should be removed");
-        assert!(m.rx.recv().is_err(), "no Down should arrive after demonitor");
+        assert_eq!(
+            demonitor(&m),
+            Some(m.id),
+            "live registration should be removed"
+        );
+        assert!(
+            m.rx.recv().is_err(),
+            "no Down should arrive after demonitor"
+        );
         let _ = h.join();
     });
 }
@@ -122,7 +136,10 @@ fn demonitor_one_of_many() {
         let _ = h.join();
         assert!(matches!(ms[0].rx.recv().unwrap().reason, DownReason::Exit));
         assert!(matches!(ms[2].rx.recv().unwrap().reason, DownReason::Exit));
-        assert!(ms[1].rx.recv().is_err(), "demonitored channel should be closed");
+        assert!(
+            ms[1].rx.recv().is_err(),
+            "demonitored channel should be closed"
+        );
     });
 }
 
@@ -136,7 +153,11 @@ fn demonitor_after_fire_is_none() {
         let m = monitor(pid);
         let d = m.rx.recv().expect("Down before close");
         assert!(matches!(d.reason, DownReason::Exit));
-        assert_eq!(demonitor(&m), None, "already-fired monitor has nothing to remove");
+        assert_eq!(
+            demonitor(&m),
+            None,
+            "already-fired monitor has nothing to remove"
+        );
         let _ = h.join();
     });
 }

tests/mutex.rs

@@ -3,9 +3,9 @@
 //! needs to be able to park.
 
 use smarm::{run, spawn, yield_now, LockTimeout, Mutex};
+use std::sync::atomic::{AtomicU32, Ordering};
 use std::sync::Arc;
 use std::sync::Mutex as StdMutex;
-use std::sync::atomic::{AtomicU32, Ordering};
 use std::time::{Duration, Instant};
 
 // ---------------------------------------------------------------------------
@@ -111,8 +111,16 @@ fn contended_lock_parks_until_holder_releases() {
     let pos_b_locked = v.iter().position(|s| *s == "B_locked").unwrap();
 
     assert!(pos_a_locked < pos_b_try, "log: {:?}", *v);
-    assert!(pos_b_try < pos_a_dropped, "B should attempt before A drops: {:?}", *v);
-    assert!(pos_a_dropped < pos_b_locked, "B should lock only after A drops: {:?}", *v);
+    assert!(
+        pos_b_try < pos_a_dropped,
+        "B should attempt before A drops: {:?}",
+        *v
+    );
+    assert!(
+        pos_a_dropped < pos_b_locked,
+        "B should lock only after A drops: {:?}",
+        *v
+    );
 }
 
 // ---------------------------------------------------------------------------
@@ -209,7 +217,11 @@ fn waiters_are_granted_the_lock_in_fifo_order() {
     });
 
     let v = order.lock().unwrap().clone();
-    assert_eq!(v, vec![1, 2, 3, 4], "waiters should acquire in arrival order");
+    assert_eq!(
+        v,
+        vec![1, 2, 3, 4],
+        "waiters should acquire in arrival order"
+    );
 }
 
 // ---------------------------------------------------------------------------

tests/observer.rs

@@ -77,8 +77,7 @@ fn observer_reports_none_for_a_forged_pid() {
         // An index that is not in the slab at all — the verb relays the
         // primitive's `None` faithfully.
         let forged = smarm::Pid::new(u32::MAX - 1, 0);
-        let ObserverReply::ActorInfo(none) =
-            obs.call(ObserverRequest::ActorInfo(forged)).unwrap()
+        let ObserverReply::ActorInfo(none) = obs.call(ObserverRequest::ActorInfo(forged)).unwrap()
         else {
             panic!("ActorInfo verb must reply ActorInfo");
         };
@@ -113,7 +112,10 @@ fn observer_sees_a_parked_actor_as_parked() {
             }
             smarm::yield_now();
         }
-        assert!(parked, "observer should eventually report the worker as Parked");
+        assert!(
+            parked,
+            "observer should eventually report the worker as Parked"
+        );
 
         gate_tx.send(()).unwrap();
         worker.join().unwrap();

tests/park_wake.rs

@@ -0,0 +1,69 @@
+//! RFC 018 scheduler park/wake — observable-behavior guards.
+//!
+//! These pin the two timer-latency properties the park/wake swap must
+//! preserve or introduce:
+//!
+//! - `sleep_fires_under_saturation`: due timers fire even when every
+//!   scheduler is busy (nobody parked ⇒ no timekeeper) — the busy-path
+//!   due-check, ratified design point (a). The old drain phase gave this
+//!   for free (timers drained every loop iteration); the new design must
+//!   not lose it.
+//! - `submillisecond_sleep_is_prompt`: a sub-ms sleep completes promptly.
+//!   Under the old wake pipe, `poll_wake`'s `as_millis` truncation turned
+//!   sub-ms deadlines into 0ms busy-polls (correct wall time, pathological
+//!   CPU); under park/wake the futex timespec carries full nanosecond
+//!   precision.
+
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+use std::time::{Duration, Instant};
+
+#[test]
+fn sleep_fires_under_saturation() {
+    let rt = smarm::runtime::init(smarm::runtime::Config::exact(4));
+    rt.run(|| {
+        let stop = Arc::new(AtomicBool::new(false));
+        let mut spinners = Vec::new();
+        // 8 spinners over 4 schedulers: the run queue never empties, so no
+        // scheduler ever parks and no timekeeper exists. Only the busy-path
+        // due-check can fire the sleeper's timer before the spinners quit.
+        for _ in 0..8 {
+            let stop = stop.clone();
+            spinners.push(smarm::spawn(move || {
+                let t0 = Instant::now();
+                while !stop.load(Ordering::Relaxed) && t0.elapsed() < Duration::from_secs(5) {
+                    smarm::yield_now();
+                }
+            }));
+        }
+        let t0 = Instant::now();
+        smarm::sleep(Duration::from_millis(10));
+        let dt = t0.elapsed();
+        stop.store(true, Ordering::Relaxed);
+        for s in spinners {
+            let _ = s.join();
+        }
+        assert!(
+            dt < Duration::from_millis(500),
+            "10ms sleep took {dt:?} under scheduler saturation — busy-path \
+             timer firing is broken (timekeeper-only firing stalls under load)"
+        );
+    });
+}
+
+#[test]
+fn submillisecond_sleep_is_prompt() {
+    let rt = smarm::runtime::init(smarm::runtime::Config::exact(2));
+    rt.run(|| {
+        // Warm one iteration, then measure.
+        smarm::sleep(Duration::from_micros(500));
+        let t0 = Instant::now();
+        smarm::sleep(Duration::from_micros(500));
+        let dt = t0.elapsed();
+        assert!(dt >= Duration::from_micros(400), "woke early: {dt:?}");
+        assert!(
+            dt < Duration::from_millis(100),
+            "500µs sleep took {dt:?} — sub-ms deadline handling is broken"
+        );
+    });
+}

tests/pg.rs

@@ -44,7 +44,10 @@ fn a_dead_actor_vanishes_from_every_group_it_joined() {
         // Drain-on-contact: touching g1 detects the death and sweeps the pid
         // out of every group (g2 included), not just g1.
         assert!(members("g1").is_empty(), "evicted from the touched group");
-        assert!(members("g2").is_empty(), "and swept from the untouched group");
+        assert!(
+            members("g2").is_empty(),
+            "and swept from the untouched group"
+        );
         assert_eq!(pick("g1"), None);
     });
 }
@@ -83,7 +86,11 @@ fn live_members_survive_a_peers_death() {
         tx_a.send(()).unwrap();
         a.join().unwrap();
 
-        assert_eq!(members("svc"), vec![b.pid()], "only the dead peer is reaped");
+        assert_eq!(
+            members("svc"),
+            vec![b.pid()],
+            "only the dead peer is reaped"
+        );
         assert_eq!(pick("svc"), Some(b.pid()));
 
         tx_b.send(()).unwrap();
@@ -125,7 +132,10 @@ fn joining_an_already_dead_pid_is_evicted_on_next_contact() {
         // monitor() on a gone pid queues a NoProc Down immediately, so the
         // membership is reaped the next time the group is touched.
         join("late", pid);
-        assert!(members("late").is_empty(), "dead-at-join member is reaped on read");
+        assert!(
+            members("late").is_empty(),
+            "dead-at-join member is reaped on read"
+        );
         assert_eq!(pick("late"), None);
     });
 }

tests/poison_stop.rs

@@ -41,7 +41,11 @@ fn stop_storm_does_not_poison_runtime() {
         }
         c.fetch_add(1, Ordering::SeqCst);
     });
-    assert_eq!(completed.load(Ordering::SeqCst), 1, "root completed cleanly");
+    assert_eq!(
+        completed.load(Ordering::SeqCst),
+        1,
+        "root completed cleanly"
+    );
 }
 
 /// The sharper repro: a stop-flagged actor whose *next allocation* is the
@@ -85,5 +89,9 @@ fn self_stop_during_spawn_does_not_poison_shared_mutex() {
         }
         c.fetch_add(1, Ordering::SeqCst);
     });
-    assert_eq!(completed.load(Ordering::SeqCst), 1, "root completed cleanly");
+    assert_eq!(
+        completed.load(Ordering::SeqCst),
+        1,
+        "root completed cleanly"
+    );
 }

tests/preempt.rs

@@ -43,10 +43,21 @@ fn check_yields_when_timeslice_expired() {
     let pos_big_b = v.iter().position(|&c| c == b'B').unwrap();
     let pos_lit_a = v.iter().position(|&c| c == b'a').unwrap();
     let pos_lit_b = v.iter().position(|&c| c == b'b').unwrap();
-    assert!(pos_big_a < pos_lit_a, "A's tail ran before B's head: {:?}", *v);
-    assert!(pos_big_b < pos_lit_b, "B's tail ran before A's head: {:?}", *v);
-    assert!(pos_big_a.max(pos_big_b) < pos_lit_a.min(pos_lit_b),
-        "preemption didn't interleave: {:?}", *v);
+    assert!(
+        pos_big_a < pos_lit_a,
+        "A's tail ran before B's head: {:?}",
+        *v
+    );
+    assert!(
+        pos_big_b < pos_lit_b,
+        "B's tail ran before A's head: {:?}",
+        *v
+    );
+    assert!(
+        pos_big_a.max(pos_big_b) < pos_lit_a.min(pos_lit_b),
+        "preemption didn't interleave: {:?}",
+        *v
+    );
 }
 
 #[test]

tests/registry.rs

@@ -65,7 +65,10 @@ fn name_held_by_live_actor_is_taken() {
         ready_rx.recv().unwrap();
         // Root tries to claim a live actor's name for itself -> NameTaken.
         let (tx_b, _rx_b) = channel::<u64>();
-        assert_eq!(register(SVC, tx_b), Err(RegisterError::NameTaken { holder: a.pid() }));
+        assert_eq!(
+            register(SVC, tx_b),
+            Err(RegisterError::NameTaken { holder: a.pid() })
+        );
         send(SVC, 0).unwrap(); // release a (delivers to the holder, a)
         a.join().unwrap();
     });
@@ -105,7 +108,10 @@ fn dead_holder_is_pruned_and_name_taken_over() {
 fn send_errors_unresolved_and_no_channel() {
     run(|| {
         // No actor at all.
-        assert!(matches!(send(Name::<u64>::new("ghost"), 1u64), Err(SendError::Unresolved(_))));
+        assert!(matches!(
+            send(Name::<u64>::new("ghost"), 1u64),
+            Err(SendError::Unresolved(_))
+        ));
 
         let (ready_tx, ready_rx) = channel::<()>();
         let (tx, rx) = channel::<u64>();
@@ -228,7 +234,10 @@ fn send_dyn_delivers_and_reports_wrong_type() {
         let p = h.pid(); // a bare Pid<Erased>, as if recovered off a Down
         send_dyn::<u64>(p, 3u64).unwrap(); // right type: delivered
                                            // Live actor, but it has no channel for &str — the genuinely-fallible case.
-        assert!(matches!(send_dyn::<&'static str>(p, "nope"), Err(SendError::NoChannel(_))));
+        assert!(matches!(
+            send_dyn::<&'static str>(p, "nope"),
+            Err(SendError::NoChannel(_))
+        ));
         done_tx.send(()).unwrap();
         h.join().unwrap();
     });

tests/runtime.rs

@@ -14,10 +14,17 @@
 //!   - No slot leaks under high spawn/join churn
 //!   - Panic on one scheduler thread doesn't kill others
 
-use smarm::{channel, runtime::{Config, Runtime}, spawn, yield_now, JoinHandle};
-use std::sync::{atomic::{AtomicBool, AtomicU64, Ordering}, Arc};
-use std::time::Duration;
+use smarm::{
+    channel,
+    runtime::{Config, Runtime},
+    spawn, yield_now, JoinHandle,
+};
 use std::collections::HashSet;
+use std::sync::{
+    atomic::{AtomicBool, AtomicU64, Ordering},
+    Arc,
+};
+use std::time::Duration;
 
 // ---------------------------------------------------------------------------
 // Helpers
@@ -29,7 +36,9 @@ fn rt(n: usize) -> Runtime {
 }
 
 /// Convenient single-threaded runtime (regression guard).
-fn rt1() -> Runtime { rt(1) }
+fn rt1() -> Runtime {
+    rt(1)
+}
 
 /// Multi-threaded runtime using all available parallelism.
 fn rt_par() -> Runtime {
@@ -79,7 +88,9 @@ fn config_min_1_max_1_is_single_threaded() {
 fn runtime_run_executes_closure() {
     let flag = Arc::new(AtomicBool::new(false));
     let f = flag.clone();
-    rt(1).run(move || { f.store(true, Ordering::SeqCst); });
+    rt(1).run(move || {
+        f.store(true, Ordering::SeqCst);
+    });
     assert!(flag.load(Ordering::SeqCst));
 }
 
@@ -111,8 +122,12 @@ fn runtime_can_be_used_multiple_times_sequentially() {
     let b = Arc::new(AtomicU64::new(0));
     let ac = a.clone();
     let bc = b.clone();
-    r.run(move || { ac.fetch_add(1, Ordering::SeqCst); });
-    r.run(move || { bc.fetch_add(1, Ordering::SeqCst); });
+    r.run(move || {
+        ac.fetch_add(1, Ordering::SeqCst);
+    });
+    r.run(move || {
+        bc.fetch_add(1, Ordering::SeqCst);
+    });
     assert_eq!(a.load(Ordering::SeqCst), 1);
     assert_eq!(b.load(Ordering::SeqCst), 1);
 }
@@ -126,7 +141,9 @@ fn exact_1_spawn_join_works() {
     let v = Arc::new(AtomicU64::new(0));
     let vc = v.clone();
     rt1().run(move || {
-        let h = spawn(move || { vc.store(42, Ordering::SeqCst); });
+        let h = spawn(move || {
+            vc.store(42, Ordering::SeqCst);
+        });
         h.join().unwrap();
     });
     assert_eq!(v.load(Ordering::SeqCst), 42);
@@ -155,7 +172,9 @@ fn exact_1_panic_captured() {
     let s = saw_err.clone();
     rt1().run(move || {
         let h = spawn(|| panic!("oops"));
-        if h.join().is_err() { s.store(true, Ordering::SeqCst); }
+        if h.join().is_err() {
+            s.store(true, Ordering::SeqCst);
+        }
     });
     assert!(saw_err.load(Ordering::SeqCst));
 }
@@ -176,7 +195,9 @@ fn multi_thread_all_actors_complete() {
                 cc.fetch_add(1, Ordering::SeqCst);
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
     });
     assert_eq!(counter.load(Ordering::SeqCst), 100);
 }
@@ -221,7 +242,9 @@ fn multi_thread_many_channels_no_lost_wakeups() {
                 tx.send(1).unwrap();
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
     });
     assert_eq!(count.load(Ordering::SeqCst), PAIRS as u64);
 }
@@ -247,7 +270,9 @@ fn multi_thread_mutex_contention_no_deadlock() {
                 }
             }));
         }
-        for h in handles { h.join().unwrap(); }
+        for h in handles {
+            h.join().unwrap();
+        }
         let g = m.lock_timeout(Duration::from_secs(1)).unwrap();
         t.store(*g, Ordering::SeqCst);
     });
@@ -262,7 +287,9 @@ fn multi_thread_join_across_threads() {
     rt_par().run(move || {
         let h = spawn(move || {
             // Do some work to make scheduling interesting.
-            for _ in 0..10 { yield_now(); }
+            for _ in 0..10 {
+                yield_now();
+            }
             vc.store(1, Ordering::SeqCst);
         });
         h.join().unwrap();
@@ -279,8 +306,7 @@ fn multi_thread_join_across_threads() {
 
 #[test]
 fn actors_run_on_multiple_os_threads() {
-    let thread_ids: Arc<smarm::Mutex<HashSet<u64>>> =
-        Arc::new(smarm::Mutex::new(HashSet::new()));
+    let thread_ids: Arc<smarm::Mutex<HashSet<u64>>> = Arc::new(smarm::Mutex::new(HashSet::new()));
 
     rt_par().run({
         let ids = thread_ids.clone();
@@ -294,11 +320,15 @@ fn actors_run_on_multiple_os_threads() {
                     g.insert(tid);
                 }));
             }
-            for h in handles { h.join().unwrap(); }
+            for h in handles {
+                h.join().unwrap();
+            }
         }
     });
 
-    let n = std::thread::available_parallelism().map(|n| n.get()).unwrap_or(1);
+    let n = std::thread::available_parallelism()
+        .map(|n| n.get())
+        .unwrap_or(1);
 
     let ids = thread_ids.lock_timeout(Duration::from_secs(1)).unwrap();
     // If we have >1 scheduler threads, we expect >1 OS thread IDs.
@@ -326,11 +356,17 @@ fn scheduler_stats_run_queue_len_is_observable() {
     // run() completes (queue len == 0 at quiescence).
     let r = rt_par();
     r.run(|| {
-        for _ in 0..10 { spawn(|| {}); }
+        for _ in 0..10 {
+            spawn(|| {});
+        }
         // Don't join — let them drain naturally.
     });
     let stats = r.stats();
-    assert_eq!(stats.total_run_queue_len(), 0, "queue should be empty after run()");
+    assert_eq!(
+        stats.total_run_queue_len(),
+        0,
+        "queue should be empty after run()"
+    );
 }
 
 #[test]
@@ -359,7 +395,9 @@ fn panic_in_actor_does_not_kill_runtime() {
             }));
         }
         let _ = bad.join(); // expect Err
-        for h in good_handles { h.join().unwrap(); }
+        for h in good_handles {
+            h.join().unwrap();
+        }
     });
     assert_eq!(completed.load(Ordering::SeqCst), 10);
 }
@@ -379,7 +417,9 @@ fn no_slot_leak_under_churn() {
     rt_par().run(move || {
         for _ in 0..500 {
             let cc = c.clone();
-            spawn(move || { cc.fetch_add(1, Ordering::SeqCst); })
+            spawn(move || {
+                cc.fetch_add(1, Ordering::SeqCst);
+            })
             .join()
             .unwrap();
         }
@@ -474,7 +514,11 @@ fn multi_thread_timer_only_no_pipe_contention() {
         }
     });
 
-    assert_eq!(count.load(Ordering::SeqCst), ACTORS as u64, "not all actors completed");
+    assert_eq!(
+        count.load(Ordering::SeqCst),
+        ACTORS as u64,
+        "not all actors completed"
+    );
 
     let elapsed = start.elapsed();
     assert!(
@@ -515,5 +559,46 @@ fn runtime_reusable_after_root_panic() {
     let ran = Arc::new(AtomicBool::new(false));
     let ran_t = ran.clone();
     r.run(move || ran_t.store(true, Ordering::Relaxed));
-    assert!(ran.load(Ordering::Relaxed), "runtime unusable after root panic");
+    assert!(
+        ran.load(Ordering::Relaxed),
+        "runtime unusable after root panic"
+    );
+}
+
+// ---------------------------------------------------------------------------
+// RFC 019 — Config stack knobs
+// ---------------------------------------------------------------------------
+
+/// Burn ~`frames` × 4 KiB of stack; probestack touches pages in order so
+/// exceeding the reserve would hit the guard and SIGSEGV the process.
+#[inline(never)]
+fn burn_stack(frames: usize) -> u64 {
+    let mut local = [0u8; 4096];
+    local[0] = frames as u8;
+    let below = if frames == 0 {
+        0
+    } else {
+        burn_stack(frames - 1)
+    };
+    std::hint::black_box(&mut local);
+    below.wrapping_add(local[0] as u64)
+}
+
+#[test]
+fn config_stack_reserve_permits_deep_recursion() {
+    // ~256 KiB of frames: four times the old fixed 64 KiB reserve. With
+    // Config::stack_reserve raised this must complete; before RFC 019 it
+    // could only segfault.
+    let rt = smarm::runtime::init(Config::exact(1).stack_reserve(1024 * 1024));
+    let done = Arc::new(AtomicBool::new(false));
+    let done2 = done.clone();
+    rt.run(move || {
+        spawn(move || {
+            std::hint::black_box(burn_stack(64));
+            done2.store(true, Ordering::SeqCst);
+        })
+        .join()
+        .unwrap();
+    });
+    assert!(done.load(Ordering::SeqCst));
 }

tests/scheduler.rs

@@ -14,7 +14,9 @@ use std::sync::Arc;
 fn root_actor_runs() {
     let captured = Arc::new(AtomicI64::new(0));
     let c = captured.clone();
-    run(move || { c.store(99, Ordering::SeqCst); });
+    run(move || {
+        c.store(99, Ordering::SeqCst);
+    });
     assert_eq!(captured.load(Ordering::SeqCst), 99);
 }
 
@@ -27,7 +29,9 @@ fn spawn_and_join_returns_exit() {
     let captured = Arc::new(AtomicI64::new(0));
     let c = captured.clone();
     run(move || {
-        let h = spawn(move || { c.store(7, Ordering::SeqCst); });
+        let h = spawn(move || {
+            c.store(7, Ordering::SeqCst);
+        });
         let res = h.join();
         assert!(res.is_ok(), "join returned {:?}", res);
     });
@@ -68,8 +72,7 @@ fn yield_now_interleaves_actors() {
 
 #[test]
 fn self_pid_is_stable_within_an_actor() {
-    let pid_cell: Arc<std::sync::Mutex<Option<smarm::Pid>>> =
-        Arc::new(std::sync::Mutex::new(None));
+    let pid_cell: Arc<std::sync::Mutex<Option<smarm::Pid>>> = Arc::new(std::sync::Mutex::new(None));
     let p2 = pid_cell.clone();
     run(move || {
         let h = spawn(move || {

tests/select.rs

@@ -19,7 +19,12 @@ fn ready_arm_returns_immediately_without_parking() {
         txa.send(42).unwrap();
         let i = select(&[&rxb, &rxa]);
         assert_eq!(i, 1);
-        out2.store(rxa.try_recv().unwrap().expect("ready arm must hold a message"), Ordering::SeqCst);
+        out2.store(
+            rxa.try_recv()
+                .unwrap()
+                .expect("ready arm must hold a message"),
+            Ordering::SeqCst,
+        );
     });
     assert_eq!(out.load(Ordering::SeqCst), 42);
 }
@@ -276,7 +281,10 @@ fn select_timeout_ready_arm_wins_without_arming_a_timer() {
         let (txa, rxa) = channel::<i64>();
         let (_keep_b, rxb) = channel::<i64>();
         txa.send(5).unwrap();
-        assert_eq!(select_timeout(&[&rxb, &rxa], Duration::from_millis(500)), Some(1));
+        assert_eq!(
+            select_timeout(&[&rxb, &rxa], Duration::from_millis(500)),
+            Some(1)
+        );
         assert_eq!(rxa.try_recv().unwrap(), Some(5));
     });
 }
@@ -342,7 +350,10 @@ fn select_timeout_closed_arm_is_ready_not_a_timeout() {
         let (_keep_a, rxa) = channel::<i64>();
         let (txb, rxb) = channel::<i64>();
         drop(txb);
-        assert_eq!(select_timeout(&[&rxa, &rxb], Duration::from_millis(200)), Some(1));
+        assert_eq!(
+            select_timeout(&[&rxa, &rxb], Duration::from_millis(200)),
+            Some(1)
+        );
         assert!(rxb.try_recv().is_err());
     });
 }

tests/spawn_opts.rs

@@ -0,0 +1,235 @@
+//! RFC 019 commit 2 — the `SpawnOpts` surface.
+//!
+//! Covers: per-spawn stack shape overrides on every spawn surface, the
+//! `None ⇒ Config default` resolution, the pool rule from the outside
+//! (obligation 4: a custom-shaped stack never enters the pool), and that a
+//! big reserve behaviorally takes effect (deep recursion completes).
+
+use smarm::runtime::{Config, DEFAULT_STACK_GUARD, DEFAULT_STACK_RESERVE};
+use smarm::{self_pid, spawn, spawn_under_with, spawn_with, GenServerBuilder, SpawnOpts};
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+
+fn rt1() -> smarm::runtime::Runtime {
+    smarm::runtime::init(Config::exact(1))
+}
+
+#[test]
+fn default_spawn_has_default_shape() {
+    rt1().run(|| {
+        let h = spawn(|| {
+            let shape = smarm::introspect::stack_shape(self_pid()).unwrap();
+            assert_eq!(shape, (DEFAULT_STACK_RESERVE, DEFAULT_STACK_GUARD));
+        });
+        h.join().unwrap();
+    });
+}
+
+#[test]
+fn spawn_with_overrides_reserve_and_guard() {
+    rt1().run(|| {
+        let opts = SpawnOpts {
+            stack_reserve: Some(1024 * 1024),
+            guard_size: Some(256 * 1024),
+        };
+        let h = spawn_with(opts, || {
+            let shape = smarm::introspect::stack_shape(self_pid()).unwrap();
+            assert_eq!(shape, (1024 * 1024, 256 * 1024));
+        });
+        h.join().unwrap();
+    });
+}
+
+#[test]
+fn spawn_with_partial_override_keeps_config_default_for_the_rest() {
+    rt1().run(|| {
+        let opts = SpawnOpts {
+            stack_reserve: Some(1024 * 1024),
+            ..SpawnOpts::default()
+        };
+        let h = spawn_with(opts, || {
+            let shape = smarm::introspect::stack_shape(self_pid()).unwrap();
+            assert_eq!(shape, (1024 * 1024, DEFAULT_STACK_GUARD));
+        });
+        h.join().unwrap();
+    });
+}
+
+#[test]
+fn spawn_with_rounds_to_pages() {
+    rt1().run(|| {
+        let opts = SpawnOpts {
+            stack_reserve: Some(64 * 1024 + 1),
+            guard_size: Some(4097),
+        };
+        let h = spawn_with(opts, || {
+            let (reserve, guard) = smarm::introspect::stack_shape(self_pid()).unwrap();
+            assert_eq!(reserve % 4096, 0);
+            assert_eq!(guard % 4096, 0);
+            assert!(reserve >= 64 * 1024 + 1);
+            assert!(guard >= 4097);
+        });
+        h.join().unwrap();
+    });
+}
+
+#[test]
+fn spawn_under_with_takes_opts() {
+    rt1().run(|| {
+        let me = self_pid();
+        let opts = SpawnOpts {
+            stack_reserve: Some(128 * 1024),
+            ..SpawnOpts::default()
+        };
+        let h = spawn_under_with(me, opts, || {
+            let (reserve, _) = smarm::introspect::stack_shape(self_pid()).unwrap();
+            assert_eq!(reserve, 128 * 1024);
+        });
+        h.join().unwrap();
+    });
+}
+
+/// Obligation 4, from the outside: a dead custom stack must not be handed to
+/// the next default spawn. The pool is LIFO, so if the custom stack had been
+/// (wrongly) pushed at death, the very next default-shaped spawn on this
+/// single-threaded runtime would pop it and report a custom shape.
+#[test]
+fn custom_stack_never_enters_the_pool() {
+    rt1().run(|| {
+        spawn_with(
+            SpawnOpts {
+                stack_reserve: Some(512 * 1024),
+                guard_size: Some(128 * 1024),
+            },
+            || {},
+        )
+        .join()
+        .unwrap();
+        let h = spawn(|| {
+            let shape = smarm::introspect::stack_shape(self_pid()).unwrap();
+            assert_eq!(shape, (DEFAULT_STACK_RESERVE, DEFAULT_STACK_GUARD));
+        });
+        h.join().unwrap();
+    });
+}
+
+/// The reverse direction of the pool rule: a default-shaped stack IS pooled
+/// and reused (cap = threads × 4 ≥ 1 here, pool empty at start).
+#[test]
+fn default_stack_is_recycled() {
+    rt1().run(|| {
+        spawn(|| {}).join().unwrap();
+        let h = spawn(|| {
+            let shape = smarm::introspect::stack_shape(self_pid()).unwrap();
+            assert_eq!(shape, (DEFAULT_STACK_RESERVE, DEFAULT_STACK_GUARD));
+        });
+        h.join().unwrap();
+    });
+}
+
+/// Burn ~`frames` × 4 KiB of stack (see tests/runtime.rs twin).
+#[inline(never)]
+fn burn_stack(frames: usize) -> u64 {
+    let mut local = [0u8; 4096];
+    local[0] = frames as u8;
+    let below = if frames == 0 {
+        0
+    } else {
+        burn_stack(frames - 1)
+    };
+    std::hint::black_box(&mut local);
+    below.wrapping_add(local[0] as u64)
+}
+
+#[test]
+fn big_reserve_behaviorally_takes_effect() {
+    // ~1 MiB deep on an 8 MiB per-spawn reserve, runtime default untouched.
+    rt1().run(|| {
+        let done = Arc::new(AtomicBool::new(false));
+        let done2 = done.clone();
+        spawn_with(
+            SpawnOpts {
+                stack_reserve: Some(8 * 1024 * 1024),
+                ..SpawnOpts::default()
+            },
+            move || {
+                std::hint::black_box(burn_stack(256));
+                done2.store(true, Ordering::SeqCst);
+            },
+        )
+        .join()
+        .unwrap();
+        assert!(done.load(Ordering::SeqCst));
+    });
+}
+
+// ---------------------------------------------------------------------------
+// Builder surfaces
+// ---------------------------------------------------------------------------
+
+struct Echo;
+impl smarm::GenServer for Echo {
+    type Call = ();
+    type Reply = (usize, usize);
+    type Cast = ();
+    type Info = ();
+    type Timer = ();
+    fn handle_call(&mut self, _c: ()) -> (usize, usize) {
+        smarm::introspect::stack_shape(self_pid()).unwrap()
+    }
+    fn handle_cast(&mut self, _c: ()) {}
+}
+
+#[test]
+fn gen_server_builder_stack_opts() {
+    rt1().run(|| {
+        let server = GenServerBuilder::new(Echo)
+            .stack_opts(SpawnOpts {
+                stack_reserve: Some(256 * 1024),
+                ..SpawnOpts::default()
+            })
+            .start();
+        let (reserve, guard) = server.call(()).unwrap();
+        assert_eq!(reserve, 256 * 1024);
+        assert_eq!(guard, DEFAULT_STACK_GUARD);
+        server.shutdown();
+    });
+}
+
+struct Probe;
+impl smarm::Machine for Probe {
+    type Ev = smarm::channel::Sender<(usize, usize)>;
+    fn state_timeout_ev() -> Self::Ev {
+        unreachable!("no timers in this test")
+    }
+    fn timeout_ev(_name: &'static str) -> Self::Ev {
+        unreachable!("no timers in this test")
+    }
+    fn on_start(&mut self, _cx: &mut smarm::Cx<Self::Ev>) {}
+    fn handle(
+        &mut self,
+        ev: Self::Ev,
+        _cx: &mut smarm::Cx<Self::Ev>,
+    ) -> smarm::gen_statem::Step<Self::Ev> {
+        let _ = ev.send(smarm::introspect::stack_shape(self_pid()).unwrap());
+        smarm::gen_statem::Step::Stayed
+    }
+}
+
+#[test]
+fn gen_statem_spawn_with_stack_opts() {
+    rt1().run(|| {
+        let m = smarm::gen_statem::spawn_with(
+            SpawnOpts {
+                stack_reserve: Some(256 * 1024),
+                ..SpawnOpts::default()
+            },
+            Probe,
+        );
+        let (tx, rx) = smarm::channel::channel();
+        m.send(tx).unwrap();
+        let (reserve, guard) = rx.recv().unwrap();
+        assert_eq!(reserve, 256 * 1024);
+        assert_eq!(guard, DEFAULT_STACK_GUARD);
+    });
+}

tests/stack.rs

@@ -7,13 +7,13 @@ use smarm::stack::Stack;
 
 #[test]
 fn top_is_16_byte_aligned() {
-    let s = Stack::new(64 * 1024).unwrap();
+    let s = Stack::new(64 * 1024, 4096).unwrap();
     assert_eq!(s.top() as usize % 16, 0);
 }
 
 #[test]
 fn top_is_within_allocation() {
-    let s = Stack::new(64 * 1024).unwrap();
+    let s = Stack::new(64 * 1024, 4096).unwrap();
     let top = s.top() as usize;
     let base = s.usable_base() as usize;
     assert!(top > base);
@@ -22,7 +22,7 @@ fn top_is_within_allocation() {
 
 #[test]
 fn write_and_read_top_of_stack() {
-    let s = Stack::new(64 * 1024).unwrap();
+    let s = Stack::new(64 * 1024, 4096).unwrap();
     let sentinel: u64 = 0xDEAD_BEEF_CAFE_1234;
     unsafe {
         let ptr = s.top().sub(8) as *mut u64;
@@ -33,7 +33,7 @@ fn write_and_read_top_of_stack() {
 
 #[test]
 fn write_and_read_bottom_of_usable_region() {
-    let s = Stack::new(64 * 1024).unwrap();
+    let s = Stack::new(64 * 1024, 4096).unwrap();
     let sentinel: u64 = 0x0102_0304_0506_0708;
     unsafe {
         let ptr = s.usable_base() as *mut u64;
@@ -44,17 +44,17 @@ fn write_and_read_bottom_of_usable_region() {
 
 #[test]
 fn small_stack_allocates() {
-    assert!(Stack::new(4096).is_ok());
+    assert!(Stack::new(4096, 4096).is_ok());
 }
 
 #[test]
 fn large_stack_allocates() {
-    assert!(Stack::new(8 * 1024 * 1024).is_ok());
+    assert!(Stack::new(8 * 1024 * 1024, 4096).is_ok());
 }
 
 #[test]
 fn stack_size_at_least_requested() {
-    let s = Stack::new(64 * 1024).unwrap();
+    let s = Stack::new(64 * 1024, 4096).unwrap();
     assert!(s.stack_size() >= 64 * 1024);
 }
 
@@ -68,15 +68,32 @@ use std::process::Command;
 fn run_as_child_if_requested() {
     match env::var("SMARM_SUBTEST").as_deref() {
         Ok("guard_page_direct") => {
-            let s = Stack::new(64 * 1024).unwrap();
+            let s = Stack::new(64 * 1024, 4096).unwrap();
             unsafe {
                 let guard_ptr = s.usable_base().sub(1);
                 guard_ptr.write_volatile(0xAB);
             }
             std::process::exit(0);
         }
+        Ok("wide_guard_top") => {
+            // One byte below the usable region, 64 KiB guard: must fault.
+            let s = Stack::new(64 * 1024, 64 * 1024).unwrap();
+            unsafe {
+                s.usable_base().sub(1).write_volatile(0xAB);
+            }
+            std::process::exit(0);
+        }
+        Ok("wide_guard_bottom") => {
+            // The very bottom page of a 64 KiB guard: an unprobed C-style
+            // leap over a small guard lands here — must still fault.
+            let s = Stack::new(64 * 1024, 64 * 1024).unwrap();
+            unsafe {
+                s.usable_base().sub(64 * 1024).write_volatile(0xAB);
+            }
+            std::process::exit(0);
+        }
         Ok("stack_overflow") => {
-            let s = Stack::new(64 * 1024).unwrap();
+            let s = Stack::new(64 * 1024, 4096).unwrap();
             unsafe {
                 let mut ptr = s.top().sub(1);
                 let stop = s.usable_base().sub(1);
@@ -107,7 +124,12 @@ fn guard_page_causes_sigsegv() {
     #[cfg(unix)]
     {
         use std::os::unix::process::ExitStatusExt;
-        assert_eq!(status.signal(), Some(11), "expected SIGSEGV, got: {:?}", status);
+        assert_eq!(
+            status.signal(),
+            Some(11),
+            "expected SIGSEGV, got: {:?}",
+            status
+        );
     }
 }
 
@@ -118,6 +140,76 @@ fn stack_overflow_causes_sigsegv() {
     #[cfg(unix)]
     {
         use std::os::unix::process::ExitStatusExt;
-        assert_eq!(status.signal(), Some(11), "expected SIGSEGV, got: {:?}", status);
+        assert_eq!(
+            status.signal(),
+            Some(11),
+            "expected SIGSEGV, got: {:?}",
+            status
+        );
+    }
+}
+
+// ---------------------------------------------------------------------------
+// RFC 019 — explicit shape: rounding, guard accessor, wide-guard coverage.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn sizes_round_up_to_page() {
+    let s = Stack::new(64 * 1024 + 1, 4096 + 1).unwrap();
+    assert_eq!(s.stack_size() % 4096, 0);
+    assert_eq!(s.guard_size() % 4096, 0);
+    assert!(s.stack_size() >= 64 * 1024 + 1);
+    assert!(s.guard_size() >= 4096 + 1);
+}
+
+#[test]
+fn shape_reports_rounded_sizes() {
+    let s = Stack::new(64 * 1024, 64 * 1024).unwrap();
+    assert_eq!(s.shape(), (64 * 1024, 64 * 1024));
+}
+
+#[test]
+fn usable_base_sits_above_guard() {
+    let s = Stack::new(64 * 1024, 64 * 1024).unwrap();
+    // The usable region must start exactly guard_size above the mapping
+    // base: a write at usable_base is legal, one byte below is not (the
+    // subprocess tests below prove the "not").
+    let sentinel: u64 = 0x1111_2222_3333_4444;
+    unsafe {
+        let ptr = s.usable_base() as *mut u64;
+        ptr.write_volatile(sentinel);
+        assert_eq!(ptr.read_volatile(), sentinel);
+    }
+}
+
+#[test]
+fn wide_guard_faults_at_top() {
+    run_as_child_if_requested();
+    let status = spawn_subtest("wide_guard_top");
+    #[cfg(unix)]
+    {
+        use std::os::unix::process::ExitStatusExt;
+        assert_eq!(
+            status.signal(),
+            Some(11),
+            "expected SIGSEGV, got: {:?}",
+            status
+        );
+    }
+}
+
+#[test]
+fn wide_guard_faults_at_bottom() {
+    run_as_child_if_requested();
+    let status = spawn_subtest("wide_guard_bottom");
+    #[cfg(unix)]
+    {
+        use std::os::unix::process::ExitStatusExt;
+        assert_eq!(
+            status.signal(),
+            Some(11),
+            "expected SIGSEGV, got: {:?}",
+            status
+        );
     }
 }

tests/stack_diag.rs

@@ -0,0 +1,153 @@
+//! RFC 019 §7 — overflow diagnostics, observed from outside via subprocess
+//! (mirrors tests/stack.rs's harness, plus stderr capture).
+//!
+//! Four cases:
+//!  - Rust recursion at defaults: probed frames walk into the guard →
+//!    tier-1 definitive message, death by SIGSEGV.
+//!  - FFI canary (96 KiB unprobed C local) at defaults: first touch lands
+//!    inside the 1 MiB guard → tier-1 message.
+//!  - FFI canary with the guard shrunk to 4 KiB: the frame steps over it
+//!    into unmapped VA below → tier-2 "stepped over" message. This is the
+//!    RFC's motivating incident (cargo-vendored gz build) reproduced.
+//!  - FFI canary with reserve raised to 256 KiB: fits, runs clean, exits 0 —
+//!    the §1 knob is the fix, proven by the same frame.
+
+use std::env;
+use std::process::Command;
+
+unsafe extern "C" {
+    fn smarm_canary_burn();
+}
+
+/// Unbounded probed recursion; each frame dirties 4 KiB. black_box defeats
+/// tail-call elision so the walk is real.
+#[inline(never)]
+#[allow(unconditional_recursion)]
+fn recurse_forever(depth: u64) -> u64 {
+    let mut local = [0u8; 4096];
+    local[0] = depth as u8;
+    std::hint::black_box(&mut local);
+    recurse_forever(depth + 1).wrapping_add(local[0] as u64)
+}
+
+fn run_as_child_if_requested() {
+    let mode = match env::var("SMARM_DIAG_SUBTEST") {
+        Ok(m) => m,
+        Err(_) => return,
+    };
+    use smarm::runtime::Config;
+    use smarm::{spawn_with, SpawnOpts};
+    let rt = smarm::runtime::init(Config::exact(1));
+    rt.run(move || {
+        let opts = match mode.as_str() {
+            "rust_overflow" | "ffi_tier1" => SpawnOpts::default(),
+            // Small guard: the canary's 96 KiB displacement clears it.
+            "ffi_tier2" => SpawnOpts {
+                guard_size: Some(4096),
+                ..SpawnOpts::default()
+            },
+            // Enough reserve: the same frame simply fits.
+            "ffi_clean" => SpawnOpts {
+                stack_reserve: Some(256 * 1024),
+                ..SpawnOpts::default()
+            },
+            other => panic!("unknown subtest {other}"),
+        };
+        let is_rust = mode == "rust_overflow";
+        spawn_with(opts, move || {
+            if is_rust {
+                std::hint::black_box(recurse_forever(0));
+            } else {
+                unsafe { smarm_canary_burn() };
+            }
+        })
+        .join()
+        .unwrap();
+    });
+    std::process::exit(0);
+}
+
+fn spawn_subtest(name: &str) -> std::process::Output {
+    let exe = env::current_exe().unwrap();
+    Command::new(exe)
+        .env("SMARM_DIAG_SUBTEST", name)
+        .args(["--test-threads=1", "--quiet"])
+        .output()
+        .expect("failed to spawn subprocess")
+}
+
+#[cfg(unix)]
+fn assert_died_sigsegv(out: &std::process::Output) {
+    use std::os::unix::process::ExitStatusExt;
+    assert_eq!(
+        out.status.signal(),
+        Some(11),
+        "expected death by SIGSEGV, got {:?}; stderr:\n{}",
+        out.status,
+        String::from_utf8_lossy(&out.stderr)
+    );
+}
+
+#[test]
+fn rust_overflow_dies_with_tier1_message() {
+    run_as_child_if_requested();
+    let out = spawn_subtest("rust_overflow");
+    assert_died_sigsegv(&out);
+    let err = String::from_utf8_lossy(&out.stderr);
+    assert!(
+        err.contains("overflowed its stack") && err.contains("in the guard region"),
+        "missing tier-1 diagnostic; stderr:\n{err}"
+    );
+    assert!(
+        err.contains("reserve=65536"),
+        "wrong reserve in message:\n{err}"
+    );
+    assert!(
+        err.contains("guard=1048576"),
+        "wrong guard in message:\n{err}"
+    );
+}
+
+#[test]
+fn ffi_canary_at_defaults_dies_with_tier1_message() {
+    run_as_child_if_requested();
+    let out = spawn_subtest("ffi_tier1");
+    assert_died_sigsegv(&out);
+    let err = String::from_utf8_lossy(&out.stderr);
+    // 96 KiB displacement from a 64 KiB reserve lands ~32 KiB into the
+    // 1 MiB guard: definitively classified.
+    assert!(
+        err.contains("in the guard region"),
+        "wide guard should catch the unprobed frame in tier 1; stderr:\n{err}"
+    );
+}
+
+#[test]
+fn ffi_canary_over_small_guard_dies_with_tier2_message() {
+    run_as_child_if_requested();
+    let out = spawn_subtest("ffi_tier2");
+    assert_died_sigsegv(&out);
+    let err = String::from_utf8_lossy(&out.stderr);
+    assert!(
+        err.contains("stepped over it") && err.contains("below the guard"),
+        "expected tier-2 overshoot attribution; stderr:\n{err}"
+    );
+    assert!(err.contains("guard=4096"), "wrong guard in message:\n{err}");
+}
+
+#[test]
+fn ffi_canary_with_enough_reserve_runs_clean() {
+    run_as_child_if_requested();
+    let out = spawn_subtest("ffi_clean");
+    assert!(
+        out.status.success(),
+        "canary should fit in 256 KiB reserve, got {:?}; stderr:\n{}",
+        out.status,
+        String::from_utf8_lossy(&out.stderr)
+    );
+    let err = String::from_utf8_lossy(&out.stderr);
+    assert!(
+        !err.contains("smarm: actor"),
+        "no diagnostic expected on the clean path; stderr:\n{err}"
+    );
+}

tests/stack_recycle.rs

@@ -0,0 +1,133 @@
+//! RFC 019 commit 5 — pool recycle zaps a dead stack down to its retained
+//! entry end, observed from the outside.
+//!
+//! A default-shaped stack that spiked deep and then died must not carry its
+//! spike into the pool as resident RSS: `recycle_stack` DONTNEEDs everything
+//! below the top `RECYCLE_RETAIN` bytes before pushing. The zap is
+//! synchronous on the death path, so the drop is immediate — but the death
+//! path itself races the observer's `join` return, hence the brief poll.
+//!
+//! Residency is measured with `mincore`, not smaps: a neighboring rw anon
+//! mapping can land flush against the stack top and the kernel merges the
+//! VMAs (observed under the full test run), so per-mapping smaps fields
+//! over-count. The PROT_NONE guard below can never merge, so the usable
+//! base is exactly the anchor VMA's start, and `mincore` counts pages
+//! within [usable_base, usable_base + reserve) regardless of merging.
+
+use smarm::runtime::{Config, RECYCLE_RETAIN};
+use smarm::{channel, spawn, yield_now};
+
+const RESERVE: usize = 4 * 1024 * 1024;
+
+/// Burn ~`frames` × 4 KiB of stack, dirtying every frame.
+#[inline(never)]
+fn burn_stack(frames: usize) -> u64 {
+    let mut local = [0u8; 4096];
+    local[0] = frames as u8;
+    let below = if frames == 0 {
+        0
+    } else {
+        burn_stack(frames - 1)
+    };
+    std::hint::black_box(&mut local);
+    below.wrapping_add(local[0] as u64)
+}
+
+/// Resident-page count over [lo, lo + len) via mincore (len page-aligned).
+fn resident_pages(lo: usize, len: usize) -> usize {
+    let page = 4096;
+    let mut vec = vec![0u8; len / page];
+    let ret = unsafe { libc::mincore(lo as *mut libc::c_void, len, vec.as_mut_ptr()) };
+    assert_eq!(
+        ret,
+        0,
+        "mincore failed: {}",
+        std::io::Error::last_os_error()
+    );
+    vec.iter().filter(|&&b| b & 1 != 0).count()
+}
+
+/// The [start, end) of the VMA containing `addr`.
+fn vma_containing(addr: usize) -> (usize, usize) {
+    let maps = std::fs::read_to_string("/proc/self/maps").unwrap();
+    for line in maps.lines() {
+        if let Some((range, _)) = line.split_once(' ') {
+            if let Some((a, b)) = range.split_once('-') {
+                if let (Ok(start), Ok(end)) =
+                    (usize::from_str_radix(a, 16), usize::from_str_radix(b, 16))
+                {
+                    if start <= addr && addr < end {
+                        return (start, end);
+                    }
+                }
+            }
+        }
+    }
+    panic!("no VMA contains {addr:#x}");
+}
+
+fn vma_exists(addr: usize) -> bool {
+    let maps = std::fs::read_to_string("/proc/self/maps").unwrap();
+    for line in maps.lines() {
+        if let Some((range, _)) = line.split_once(' ') {
+            if let Some((a, b)) = range.split_once('-') {
+                if let (Ok(start), Ok(end)) =
+                    (usize::from_str_radix(a, 16), usize::from_str_radix(b, 16))
+                {
+                    if start <= addr && addr < end {
+                        return true;
+                    }
+                }
+            }
+        }
+    }
+    false
+}
+
+#[test]
+fn recycle_zaps_dead_stack_down_to_retain() {
+    // Default reserve raised so the pool holds big stacks (default-shaped ⇒
+    // pooled) and the zap has something to bite; single scheduler.
+    let rt = smarm::runtime::init(Config::exact(1).stack_reserve(RESERVE));
+    rt.run(|| {
+        let (tx, rx) = channel::<usize>();
+
+        let h = spawn(move || {
+            let probe = 0u8;
+            let anchor = &probe as *const u8 as usize;
+            // The guard below is PROT_NONE and can never merge with the
+            // usable region, so the anchor VMA's start IS the usable base.
+            let (vlo, _) = vma_containing(anchor);
+            // Dirty ~3 MiB of the 4 MiB reserve, then die.
+            std::hint::black_box(burn_stack(768));
+            tx.send(vlo).unwrap();
+        });
+
+        let usable_base = rx.recv().unwrap();
+        h.join().unwrap();
+
+        // The zap span is everything below the retained entry end. DONTNEED
+        // on private anon discards synchronously and unconditionally, so
+        // this must go to exactly zero resident pages; the poll only covers
+        // the death path racing join's return.
+        let zap_len = RESERVE - RECYCLE_RETAIN;
+        let mut resident = usize::MAX;
+        for _ in 0..10_000 {
+            resident = resident_pages(usable_base, zap_len);
+            if resident == 0 {
+                break;
+            }
+            yield_now();
+        }
+        assert_eq!(
+            resident, 0,
+            "recycled stack's zap span still resident: {resident} pages in \
+             [{usable_base:#x}, +{zap_len:#x})"
+        );
+        // Pooled, not munmapped: the mapping must still be there.
+        assert!(
+            vma_exists(usable_base),
+            "default-shaped stack was unmapped instead of pooled"
+        );
+    });
+}

tests/stack_shrink.rs

@@ -0,0 +1,161 @@
+//! RFC 019 commit 3 — park-path stack shrink, observed from the outside.
+//!
+//! The one integration-level claim of the shrink machinery: an actor that
+//! spikes deep, returns shallow, and then parks past the cooldown gets its
+//! dead span MADV_FREE'd — visible as `LazyFree` in `/proc/self/smaps`
+//! within the stack's address range — while everything live survives.
+//!
+//! The high-water mark is *sampled* at context-save, so the spike yields
+//! once at max depth to guarantee a sample there (in production, preemption
+//! provides the quasi-random samples; a test must not rely on luck).
+
+use smarm::runtime::{Config, SHRINK_COOLDOWN, SHRINK_THRESHOLD};
+use smarm::{actor_info, channel, spawn, spawn_with, yield_now, ActorState, SpawnOpts};
+
+/// Burn ~`frames` × 4 KiB of stack, yielding once at the bottom so the
+/// context-save samples `sp` at max depth.
+#[inline(never)]
+fn burn_stack_yielding(frames: usize) -> u64 {
+    let mut local = [0u8; 4096];
+    local[0] = frames as u8;
+    let below = if frames == 0 {
+        yield_now();
+        0
+    } else {
+        burn_stack_yielding(frames - 1)
+    };
+    std::hint::black_box(&mut local);
+    below.wrapping_add(local[0] as u64)
+}
+
+/// Sum the `LazyFree:` kB of every smaps mapping intersecting [lo, hi).
+fn lazy_free_bytes_in(lo: usize, hi: usize) -> usize {
+    let smaps = std::fs::read_to_string("/proc/self/smaps").unwrap();
+    let mut total_kb = 0usize;
+    let mut in_range = false;
+    for line in smaps.lines() {
+        if let Some((range, _)) = line.split_once(' ') {
+            if let Some((a, b)) = range.split_once('-') {
+                if let (Ok(start), Ok(end)) =
+                    (usize::from_str_radix(a, 16), usize::from_str_radix(b, 16))
+                {
+                    in_range = start < hi && end > lo;
+                    continue;
+                }
+            }
+        }
+        if in_range {
+            if let Some(rest) = line.strip_prefix("LazyFree:") {
+                let kb: usize = rest.trim().trim_end_matches(" kB").trim().parse().unwrap();
+                total_kb += kb;
+            }
+        }
+    }
+    total_kb * 1024
+}
+
+#[test]
+fn spike_then_parks_marks_lazyfree_and_keeps_live_data() {
+    // Single scheduler: the controller can gate on the worker being Parked.
+    let rt = smarm::runtime::init(Config::exact(1));
+    rt.run(|| {
+        let (park_tx, park_rx) = channel::<()>();
+        let (done_tx, done_rx) = channel::<(usize, u64)>();
+
+        let spike = 768 * 4096; // ~3 MiB, well past SHRINK_THRESHOLD
+        assert!(spike > SHRINK_THRESHOLD);
+
+        let worker = spawn_with(
+            SpawnOpts {
+                stack_reserve: Some(8 * 1024 * 1024),
+                ..SpawnOpts::default()
+            },
+            move || {
+                // Live data that must survive the shrink, and an anchor
+                // address inside the stack for the smaps scan.
+                let live = [0xA5u8; 64];
+                let anchor = live.as_ptr() as usize;
+
+                // Spike: ~3 MiB deep, sampled at the bottom, unwound.
+                std::hint::black_box(burn_stack_yielding(768));
+
+                // Park past the cooldown. Each recv on the drained inbox is
+                // one park; the controller sends only when it sees us Parked.
+                for _ in 0..(SHRINK_COOLDOWN + 8) {
+                    park_rx.recv().unwrap();
+                }
+
+                // Measure from inside: the stack spans ≤ 8 MiB below anchor.
+                let lazy = lazy_free_bytes_in(anchor - 8 * 1024 * 1024, anchor + 4096);
+                let checksum = live.iter().map(|&b| b as u64).sum();
+                done_tx.send((lazy, checksum)).unwrap();
+            },
+        );
+
+        let wpid = worker.pid();
+        for _ in 0..(SHRINK_COOLDOWN + 8) {
+            // Gate: send only once the worker is genuinely parked so every
+            // round is a real park-on-empty-mailbox.
+            loop {
+                match actor_info(wpid) {
+                    Some(info) if info.state == ActorState::Parked => break,
+                    Some(_) => yield_now(),
+                    None => panic!("worker died early"),
+                }
+            }
+            park_tx.send(()).unwrap();
+        }
+
+        let (lazy, checksum) = done_rx.recv().unwrap();
+        // The spike was ~3 MiB; demand at least 2 MiB marked to leave slack
+        // for the redzone, rounding, and pages the unwind re-dirtied.
+        assert!(
+            lazy >= 2 * 1024 * 1024,
+            "expected ≥ 2 MiB LazyFree in the stack range, got {} bytes",
+            lazy
+        );
+        assert_eq!(
+            checksum,
+            64 * 0xA5u64,
+            "live stack data corrupted by shrink"
+        );
+        worker.join().unwrap();
+    });
+}
+
+/// Steady-state actors must never pay the syscall: an actor that parks a lot
+/// but never spikes past the threshold ends with zero LazyFree in its stack.
+#[test]
+fn shallow_actor_never_shrinks() {
+    let rt = smarm::runtime::init(Config::exact(1));
+    rt.run(|| {
+        let (park_tx, park_rx) = channel::<()>();
+        let (done_tx, done_rx) = channel::<usize>();
+
+        let worker = spawn(move || {
+            let probe = 0u8;
+            let anchor = &probe as *const u8 as usize;
+            for _ in 0..(SHRINK_COOLDOWN + 8) {
+                park_rx.recv().unwrap();
+            }
+            done_tx
+                .send(lazy_free_bytes_in(anchor - 64 * 1024, anchor + 4096))
+                .unwrap();
+        });
+
+        let wpid = worker.pid();
+        for _ in 0..(SHRINK_COOLDOWN + 8) {
+            loop {
+                match actor_info(wpid) {
+                    Some(info) if info.state == ActorState::Parked => break,
+                    Some(_) => yield_now(),
+                    None => panic!("worker died early"),
+                }
+            }
+            park_tx.send(()).unwrap();
+        }
+
+        assert_eq!(done_rx.recv().unwrap(), 0, "steady-state actor was shrunk");
+        worker.join().unwrap();
+    });
+}

tests/stop_with_queued_call.rs

@@ -18,8 +18,8 @@
 //! registry entry guarantees for every named server.
 
 use smarm::{
-    call, channel, init, request_stop, spawn, Config, GenServer, GenServerBuilder, GenServerName,
-    CallError, Receiver, RecvTimeoutError,
+    call, channel, init, request_stop, spawn, CallError, Config, GenServer, GenServerBuilder,
+    GenServerName, Receiver, RecvTimeoutError,
 };
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
@@ -73,7 +73,9 @@ fn named_server_request_stop_releases_queued_caller_with_server_down() {
         let (res_tx, res_rx) = channel::<Result<(), CallError>>();
 
         // 1. Start the named server and keep its ref alive.
-        let server = GenServerBuilder::new(Blocker { gate: Some(gate_rx) })
+        let server = GenServerBuilder::new(Blocker {
+            gate: Some(gate_rx),
+        })
         .named(BLOCKER)
         .start()
         .expect("name should be free");

tests/stress.rs

@@ -10,7 +10,11 @@
 //! out rather than produce a false pass — run with `cargo test -- --timeout`
 //! or under a CI timeout.
 
-use smarm::{channel, runtime::{Config, Runtime}, spawn, yield_now, JoinHandle};
+use smarm::{
+    channel,
+    runtime::{Config, Runtime},
+    spawn, yield_now, JoinHandle,
+};
 use std::sync::{
     atomic::{AtomicU64, AtomicUsize, Ordering},
     Arc,
@@ -199,7 +203,9 @@ fn thundering_herd_all_wake() {
         }
 
         // Let all receivers park before we send.
-        for _ in 0..4 { yield_now(); }
+        for _ in 0..4 {
+            yield_now();
+        }
 
         // Coordinator blasts all channels.
         handles.push(spawn(move || {
@@ -240,8 +246,7 @@ fn concurrent_spawn_join_churn() {
         for _ in 0..PARENTS {
             let tc = t.clone();
             parent_handles.push(spawn(move || {
-                let mut child_handles: Vec<JoinHandle> =
-                    Vec::with_capacity(CHILDREN_PER_PARENT);
+                let mut child_handles: Vec<JoinHandle> = Vec::with_capacity(CHILDREN_PER_PARENT);
 
                 for _ in 0..CHILDREN_PER_PARENT {
                     let tcc = tc.clone();
@@ -292,7 +297,9 @@ fn join_race_child_finishes_first() {
         }
 
         // Yield enough to let children run to completion before we join.
-        for _ in 0..8 { yield_now(); }
+        for _ in 0..8 {
+            yield_now();
+        }
 
         for h in handles {
             // If child already finished, join must return immediately with Ok.
@@ -374,8 +381,7 @@ fn panic_storm_does_not_corrupt_scheduler() {
 fn pid_generation_increments_on_reuse() {
     use smarm::self_pid;
 
-    let pids: Arc<smarm::Mutex<Vec<smarm::Pid>>> =
-        Arc::new(smarm::Mutex::new(Vec::new()));
+    let pids: Arc<smarm::Mutex<Vec<smarm::Pid>>> = Arc::new(smarm::Mutex::new(Vec::new()));
 
     let p = pids.clone();
     rt(1).run(move || {
@@ -392,7 +398,9 @@ fn pid_generation_increments_on_reuse() {
         }
     });
 
-    let g = pids.lock_timeout(std::time::Duration::from_secs(1)).unwrap();
+    let g = pids
+        .lock_timeout(std::time::Duration::from_secs(1))
+        .unwrap();
     // Any two PIDs that share an index must have different generations.
     for i in 0..g.len() {
         for j in (i + 1)..g.len() {

tests/supervisor.rs

@@ -51,7 +51,11 @@ fn transient_child_is_restarted_on_panic_then_settles() {
         });
         sup.join().unwrap();
     });
-    assert_eq!(runs.load(Ordering::SeqCst), 3, "two restarts then a clean exit");
+    assert_eq!(
+        runs.load(Ordering::SeqCst),
+        3,
+        "two restarts then a clean exit"
+    );
 }
 
 #[test]
@@ -167,7 +171,11 @@ fn one_for_all_restarts_a_normally_exited_sibling() {
         sup.join().unwrap();
     });
     assert_eq!(a.load(Ordering::SeqCst), 2, "A: crash then clean run");
-    assert_eq!(b.load(Ordering::SeqCst), 2, "B cycled with the group despite a clean exit");
+    assert_eq!(
+        b.load(Ordering::SeqCst),
+        2,
+        "B cycled with the group despite a clean exit"
+    );
 }
 
 #[test]

tests/terminal_outcome_after_death.rs

@@ -0,0 +1,276 @@
+//! The terminal-record contract (bridge soak signature 4): a watch installed
+//! *after* its target's death — the async-install race the bridge's proxies
+//! live with — must be able to recover the real down reason instead of a
+//! blanket `NoProc`. Two primitives carry it:
+//!
+//!   - `finalize_actor` stamps the slot with `(generation, DownReason)`; the
+//!     record survives reclaim, registry pruning, and the next tenant's
+//!     install, and is overwritten only by the slot's next death.
+//!     [`terminal_reason`] reads it generation-matched.
+//!   - [`resolve_name`] is `whereis` with the corpse kept: the dead-holder arm
+//!     returns the stored pid it prunes ([`NameResolution::Corpse`]) instead
+//!     of discarding the only evidence of *who* died. `Unbound` stays the
+//!     Erlang-shaped `noproc` for names that were never (or are no longer)
+//!     bound.
+//!
+//! `monitor()` of a stale pid still queues plain `NoProc` — the upgrade is a
+//! caller's deliberate act, not a semantics change.
+
+use smarm::{
+    init, mark_watchable, request_stop, resolve_name, terminal_reason, CallError, Config,
+    DownReason, GenServer, GenServerBuilder, GenServerName, NameResolution,
+};
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
+
+const TARGET: GenServerName<Target> = GenServerName::new("terminal_target");
+
+/// Named server that panics on cast — the sig-4 death.
+struct Target;
+
+impl GenServer for Target {
+    type Call = ();
+    type Reply = ();
+    type Cast = ();
+    type Info = ();
+    type Timer = ();
+
+    fn handle_call(&mut self, _req: ()) {}
+    fn handle_cast(&mut self, _op: ()) {
+        panic!("terminal_target: induced panic");
+    }
+}
+
+/// Slot filler for the re-tenancy phase (distinct type, held alive).
+struct Filler;
+
+impl GenServer for Filler {
+    type Call = ();
+    type Reply = ();
+    type Cast = ();
+    type Info = ();
+    type Timer = ();
+
+    fn handle_call(&mut self, _req: ()) {}
+    fn handle_cast(&mut self, _op: ()) {}
+}
+
+#[derive(Debug)]
+struct Observed {
+    exit_reason: Option<DownReason>,
+    anon_reason: Option<DownReason>,
+    /// Anonymous but export-marked while alive — must stamp (sig 5).
+    marked_reason: Option<DownReason>,
+    /// Marked only after death — must remain unknowable.
+    marked_late_reason: Option<DownReason>,
+    panic_reason: Option<DownReason>,
+    stopped_reason: Option<DownReason>,
+    live_reason: Option<DownReason>,
+    live_resolution_is_live: bool,
+    unknown_resolution: NameResolution,
+    /// First resolve after the named target's panic — must be Corpse(old pid).
+    corpse_resolution_matches: bool,
+    /// Second resolve — the Corpse arm pruned, so the name has healed.
+    resolution_after_prune: NameResolution,
+    /// Read AFTER the prune above: the record is slot-side, not registry-side.
+    corpse_reason_after_prune: Option<DownReason>,
+    /// Record survives the slot being re-tenanted (new tenant still alive).
+    corpse_reason_after_reuse: Option<DownReason>,
+    /// ... and dies with the next tenancy's death (overwritten).
+    corpse_reason_after_tenant_death: Option<DownReason>,
+    tenant_reason: Option<DownReason>,
+}
+
+#[test]
+fn terminal_record_recovers_the_reason_a_raced_watch_lost() {
+    let out: Arc<Mutex<Option<Observed>>> = Arc::new(Mutex::new(None));
+    let out_w = out.clone();
+
+    // Tiny slab: prompt slot recycling for the re-tenancy phase.
+    init(Config::exact(2).max_actors(32)).run(move || {
+        // --- Registered plain actors: one record per way of dying. The
+        // record is named-tenancy-only, so each actor self-registers a
+        // throwaway channel before dying; the anonymous control below pins
+        // the complement.
+        let h = smarm::spawn(|| {
+            let (tx, _rx) = smarm::channel::<()>();
+            let _ = smarm::register(smarm::Name::<()>::new("terminal_probe_exit"), tx);
+        });
+        let pid_exit = h.pid();
+        let _ = h.join();
+        let exit_reason = terminal_reason(pid_exit);
+
+        let h = smarm::spawn(|| {
+            let (tx, _rx) = smarm::channel::<()>();
+            let _ = smarm::register(smarm::Name::<()>::new("terminal_probe_panic"), tx);
+            panic!("induced");
+        });
+        let pid_panic = h.pid();
+        let _ = h.join();
+        let panic_reason = terminal_reason(pid_panic);
+
+        let h = smarm::spawn(|| {
+            let (tx, _rx) = smarm::channel::<()>();
+            let _ = smarm::register(smarm::Name::<()>::new("terminal_probe_stop"), tx);
+            loop {
+                smarm::sleep(Duration::from_millis(2));
+            }
+        });
+        let pid_stop = h.pid();
+        request_stop(pid_stop);
+        let _ = h.join();
+        let stopped_reason = terminal_reason(pid_stop);
+
+        // --- Anonymous control: an unregistered death must NOT stamp (nor
+        // evict) — the free list is LIFO, so green-thread churn would
+        // otherwise overwrite a watchable record faster than any race
+        // window this exists to cover.
+        let h = smarm::spawn(|| panic!("anonymous"));
+        let pid_anon = h.pid();
+        let _ = h.join();
+        let anon_reason = terminal_reason(pid_anon);
+
+        // --- mark_watchable: the bridge's export-seam eligibility (sig 5).
+        // An anonymous actor marked while alive stamps like a named one ...
+        let h = smarm::spawn(|| loop {
+            smarm::sleep(Duration::from_millis(2));
+        });
+        let pid_marked = h.pid();
+        mark_watchable(pid_marked);
+        request_stop(pid_marked);
+        let _ = h.join();
+        let marked_reason = terminal_reason(pid_marked);
+
+        // ... while marking a pid whose tenancy already ended is a no-op:
+        // the history is honestly unknowable, not retroactively invented.
+        mark_watchable(pid_anon);
+        let marked_late_reason = terminal_reason(pid_anon);
+
+        // --- The named target: live readings first. -----------------------
+        let target = GenServerBuilder::new(Target)
+            .named(TARGET)
+            .start()
+            .expect("name free at test start");
+        let old_pid = target.pid();
+        let live_reason = terminal_reason(old_pid);
+        let live_resolution_is_live =
+            resolve_name(TARGET.as_str()) == NameResolution::Live(old_pid.erase());
+        let unknown_resolution = resolve_name("terminal_never_bound");
+
+        // --- Kill it by panic; confirm death via the ref, NEVER the name
+        // (any name reader would take the prune arm and destroy the corpse
+        // precondition — the same trap stale_name_slot_reuse.rs documents).
+        let _ = target.cast(());
+        loop {
+            match target.call(()) {
+                Err(CallError::ServerDown) => break,
+                Ok(()) => smarm::sleep(Duration::from_millis(2)),
+            }
+        }
+
+        let corpse_resolution_matches =
+            resolve_name(TARGET.as_str()) == NameResolution::Corpse(old_pid.erase());
+        let resolution_after_prune = resolve_name(TARGET.as_str());
+        let corpse_reason_after_prune = terminal_reason(old_pid);
+
+        // --- Re-tenant the freed slot; the record must outlive the install
+        // and die only with the next tenancy's death.
+        let mut fillers = Vec::new();
+        let mut tenant = None;
+        for i in 0..24 {
+            let name: &'static str = Box::leak(format!("terminal_filler_{i}").into_boxed_str());
+            let f = GenServerBuilder::new(Filler)
+                .named(GenServerName::<Filler>::new(name))
+                .start()
+                .expect("filler names are fresh");
+            let fp = f.pid();
+            let landed = fp.index() == old_pid.index();
+            fillers.push(f);
+            if landed {
+                tenant = Some((fillers.len() - 1, fp));
+                break;
+            }
+        }
+        let (tenant_at, tenant_pid) = tenant.expect(
+            "precondition: the freed slot must be re-tenanted within the tiny slab \
+             (slots are recycled; every filler is held alive)",
+        );
+        let corpse_reason_after_reuse = terminal_reason(old_pid);
+
+        request_stop(tenant_pid);
+        loop {
+            match fillers[tenant_at].call(()) {
+                Err(CallError::ServerDown) => break,
+                Ok(()) => smarm::sleep(Duration::from_millis(2)),
+            }
+        }
+        let corpse_reason_after_tenant_death = terminal_reason(old_pid);
+        let tenant_reason = terminal_reason(tenant_pid);
+
+        *out_w.lock().unwrap() = Some(Observed {
+            exit_reason,
+            anon_reason,
+            panic_reason,
+            stopped_reason,
+            live_reason,
+            live_resolution_is_live,
+            unknown_resolution,
+            corpse_resolution_matches,
+            resolution_after_prune,
+            marked_reason,
+            marked_late_reason,
+            corpse_reason_after_prune,
+            corpse_reason_after_reuse,
+            corpse_reason_after_tenant_death,
+            tenant_reason,
+        });
+    });
+
+    let o = out.lock().unwrap().take().expect("runtime body completed");
+    assert_eq!(o.exit_reason, Some(DownReason::Exit), "{o:?}");
+    assert_eq!(
+        o.anon_reason, None,
+        "anonymous deaths must not stamp: {o:?}"
+    );
+    assert_eq!(o.panic_reason, Some(DownReason::Panic), "{o:?}");
+    assert_eq!(
+        o.marked_reason,
+        Some(DownReason::Stopped),
+        "mark_watchable while alive must make the death stamp: {o:?}"
+    );
+    assert_eq!(
+        o.marked_late_reason, None,
+        "marking a dead tenancy must not invent history: {o:?}"
+    );
+    assert_eq!(o.stopped_reason, Some(DownReason::Stopped), "{o:?}");
+    assert_eq!(
+        o.live_reason, None,
+        "live tenancy must have no record: {o:?}"
+    );
+    assert!(o.live_resolution_is_live, "{o:?}");
+    assert_eq!(o.unknown_resolution, NameResolution::Unbound, "{o:?}");
+    assert!(
+        o.corpse_resolution_matches,
+        "first post-death resolve must carry the corpse: {o:?}"
+    );
+    assert_eq!(
+        o.resolution_after_prune,
+        NameResolution::Unbound,
+        "the Corpse arm prunes — the name heals: {o:?}"
+    );
+    assert_eq!(
+        o.corpse_reason_after_prune,
+        Some(DownReason::Panic),
+        "the record is slot-side; registry pruning must not touch it: {o:?}"
+    );
+    assert_eq!(
+        o.corpse_reason_after_reuse,
+        Some(DownReason::Panic),
+        "a new tenant's install must leave the previous tenancy's record: {o:?}"
+    );
+    assert_eq!(
+        o.corpse_reason_after_tenant_death, None,
+        "the next death overwrites — the old generation no longer matches: {o:?}"
+    );
+    assert_eq!(o.tenant_reason, Some(DownReason::Stopped), "{o:?}");
+}

tests/terminal_wake.rs

@@ -35,7 +35,10 @@ impl PipePair {
         let mut fds: [libc::c_int; 2] = [0; 2];
         let r = unsafe { libc::pipe2(fds.as_mut_ptr(), libc::O_CLOEXEC | libc::O_NONBLOCK) };
         assert_eq!(r, 0, "pipe2 failed");
-        PipePair { read: fds[0], write: fds[1] }
+        PipePair {
+            read: fds[0],
+            write: fds[1],
+        }
     }
 }
 
@@ -67,9 +70,9 @@ fn run_with_watchdog(limit: Duration, body: impl FnOnce() + Send + 'static) {
         rt.run(body);
         let _ = done_tx.send(());
     });
-    done_rx
-        .recv_timeout(limit)
-        .expect("Runtime::run did not return: idle scheduler thread was never woken at termination");
+    done_rx.recv_timeout(limit).expect(
+        "Runtime::run did not return: idle scheduler thread was never woken at termination",
+    );
 }
 
 /// Permanent-hang variant: sibling blocked in `poll_wake(wake_fd, None)`

tests/timer.rs

@@ -166,14 +166,19 @@ fn timers_only_pop_entries_whose_deadline_has_passed() {
 #[test]
 fn timers_mix_sleep_and_wait_timeout_reasons() {
     let mut t = Timers::new();
-    let target = Arc::new(RecordingTarget { calls: Mutex::new(Vec::new()) });
+    let target = Arc::new(RecordingTarget {
+        calls: Mutex::new(Vec::new()),
+    });
     let now = Instant::now();
 
     t.insert_sleep(now + Duration::from_millis(5), Pid::new(0, 0), 1);
     t.insert(
         now + Duration::from_millis(10),
         Pid::new(1, 0),
-        Reason::WaitTimeout { target: target.clone(), epoch: 42 },
+        Reason::WaitTimeout {
+            target: target.clone(),
+            epoch: 42,
+        },
     );
 
     let due = t.pop_due(now + Duration::from_millis(20));
@@ -238,7 +243,10 @@ fn armed_send_timer_is_returned_and_fires() {
 
     let mut due = t.pop_due(now + Duration::from_millis(20));
     assert_eq!(due.len(), 1, "an armed send timer should pop when due");
-    assert!(!fired.load(Ordering::SeqCst), "pop must not fire on its own");
+    assert!(
+        !fired.load(Ordering::SeqCst),
+        "pop must not fire on its own"
+    );
     run_fire(due.pop().unwrap());
     assert!(fired.load(Ordering::SeqCst), "running the thunk delivers");
     assert!(t.is_empty());
@@ -282,7 +290,11 @@ fn cancel_after_fire_returns_false() {
 fn cancel_unknown_id_returns_false() {
     let mut t = Timers::new();
     let now = Instant::now();
-    let id = t.insert_send(now + Duration::from_millis(5), Pid::new(0, 0), Box::new(|| {}));
+    let id = t.insert_send(
+        now + Duration::from_millis(5),
+        Pid::new(0, 0),
+        Box::new(|| {}),
+    );
     assert!(t.cancel(id));
     // Second cancel of the same id: already gone.
     assert!(!t.cancel(id));
@@ -293,7 +305,11 @@ fn send_timers_interleave_with_sleep_in_deadline_order() {
     let mut t = Timers::new();
     let now = Instant::now();
     t.insert_sleep(now + Duration::from_millis(30), Pid::new(0, 0), 1);
-    let _id = t.insert_send(now + Duration::from_millis(10), Pid::new(1, 0), Box::new(|| {}));
+    let _id = t.insert_send(
+        now + Duration::from_millis(10),
+        Pid::new(1, 0),
+        Box::new(|| {}),
+    );
     t.insert_sleep(now + Duration::from_millis(20), Pid::new(2, 0), 1);
 
     let due = t.pop_due(now + Duration::from_millis(50));
@@ -308,7 +324,11 @@ fn send_timers_interleave_with_sleep_in_deadline_order() {
 fn clear_drops_armed_send_timers() {
     let mut t = Timers::new();
     let now = Instant::now();
-    let id = t.insert_send(now + Duration::from_millis(10), Pid::new(0, 0), Box::new(|| {}));
+    let id = t.insert_send(
+        now + Duration::from_millis(10),
+        Pid::new(0, 0),
+        Box::new(|| {}),
+    );
     t.clear();
     assert!(t.is_empty());
     // The arm record is gone too: cancelling reports nothing to cancel.

tests/try_spawn.rs

@@ -0,0 +1,185 @@
+//! Non-panicking spawn at slab capacity (`try_spawn`).
+//!
+//! Covers: parity with `spawn` when slots are free; `Err(AtCapacity)` instead
+//! of a panic on a full slab (the spawning actor survives — the crash-loop
+//! from the motivating slowloris incident cannot start); self-heal (a freed
+//! slot makes the next `try_spawn` succeed); and exact claim-or-report
+//! accounting under a multi-thread race for the last slots (no TOCTOU
+//! overshoot, no panic).
+
+use smarm::runtime::Config;
+use smarm::{spawn, try_spawn, try_spawn_under_with, yield_now, SpawnError, SpawnOpts};
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use std::sync::Arc;
+
+/// A child that holds its slot until `release` flips, without parking
+/// machinery: busy-yield keeps the scheduler moving and the slot occupied.
+fn holder(release: Arc<AtomicBool>) -> impl FnOnce() + Send + 'static {
+    move || {
+        while !release.load(Ordering::Acquire) {
+            yield_now();
+        }
+    }
+}
+
+#[test]
+fn try_spawn_is_spawn_when_slots_free() {
+    smarm::runtime::init(Config::exact(1)).run(|| {
+        let ran = Arc::new(AtomicBool::new(false));
+        let flag = ran.clone();
+        let h = try_spawn(move || flag.store(true, Ordering::Release))
+            .expect("slots free — must behave exactly like spawn");
+        h.join().unwrap();
+        assert!(ran.load(Ordering::Acquire));
+    });
+}
+
+#[test]
+fn at_capacity_is_err_not_panic_and_accounting_is_exact() {
+    const MAX: usize = 8;
+    smarm::runtime::init(Config::exact(1).max_actors(MAX)).run(|| {
+        let release = Arc::new(AtomicBool::new(false));
+        // Fill the slab from the initial actor: slots are claimed at spawn
+        // time, so children need not have run yet. Count until refusal.
+        let mut held = Vec::new();
+        loop {
+            match try_spawn(holder(release.clone())) {
+                Ok(h) => held.push(h),
+                Err(e) => {
+                    assert_eq!(e, SpawnError::AtCapacity);
+                    break;
+                }
+            }
+        }
+        // Initial actor occupies one slot; the rest were spawnable.
+        assert_eq!(held.len(), MAX - 1, "slab accounting must be exact");
+        // Still refusing (and still not panicking) on repeat.
+        assert!(matches!(try_spawn(|| ()), Err(SpawnError::AtCapacity)));
+        // The `_with` surface refuses identically — a custom shape must not
+        // reach stack allocation when there is no slot for it.
+        let opts = SpawnOpts {
+            stack_reserve: Some(1024 * 1024),
+            ..SpawnOpts::default()
+        };
+        assert!(matches!(
+            try_spawn_under_with(smarm::self_pid(), opts, || ()),
+            Err(SpawnError::AtCapacity)
+        ));
+
+        // Self-heal: free the slots, join, and the next try_spawn succeeds.
+        release.store(true, Ordering::Release);
+        for h in held {
+            h.join().unwrap();
+        }
+        let h = try_spawn(|| ()).expect("slots freed — must succeed again");
+        h.join().unwrap();
+    });
+}
+
+#[test]
+fn plain_spawn_still_panics_at_capacity() {
+    // The existing invariant-check semantics of `spawn` are untouched: at a
+    // full slab it panics, the panic is caught at the actor isolation
+    // boundary, and it surfaces as a join error — exactly as before. The
+    // bomb actor is spawned into the LAST slot (so the slab is full only
+    // once the bomb itself is live) and the panic lands inside the bomb,
+    // not the initial actor.
+    const MAX: usize = 6;
+    smarm::runtime::init(Config::exact(1).max_actors(MAX)).run(|| {
+        let release = Arc::new(AtomicBool::new(false));
+        let mut held = Vec::new();
+        for _ in 0..MAX - 2 {
+            held.push(spawn(holder(release.clone())));
+        }
+        let armed = Arc::new(AtomicBool::new(false));
+        let armed2 = armed.clone();
+        let bomb = spawn(move || {
+            armed2.store(true, Ordering::Release);
+            // Slab is now full (initial + MAX−2 holders + this actor); the
+            // plain spawn must panic this actor.
+            let _ = spawn(|| ());
+            unreachable!("allocate_slot must have panicked");
+        });
+        let err = bomb
+            .join()
+            .expect_err("bomb must die by panic, not run through");
+        assert!(armed.load(Ordering::Acquire), "bomb must have actually run");
+        // The panic message is a formatted String (panic! with args).
+        let msg = err
+            .payload
+            .downcast_ref::<String>()
+            .cloned()
+            .unwrap_or_else(|| "<non-string payload>".into());
+        assert!(
+            msg.contains("slot table exhausted"),
+            "panic must be the slab-exhaustion invariant message, got: {msg}"
+        );
+        release.store(true, Ordering::Release);
+        for h in held {
+            h.join().unwrap();
+        }
+    });
+}
+
+#[test]
+fn racing_try_spawns_claim_exactly_the_free_slots() {
+    // 4 scheduler threads, 4 spawner actors hammering try_spawn for a small
+    // pool of remaining slots. Claim-or-report must hand out exactly the
+    // free slots across all racers — no overshoot (TOCTOU), no panic.
+    const MAX: usize = 32;
+    const SPAWNERS: usize = 4;
+    smarm::runtime::init(Config::exact(4).max_actors(MAX)).run(|| {
+        let release = Arc::new(AtomicBool::new(false));
+        let won = Arc::new(AtomicUsize::new(0));
+        let done = Arc::new(AtomicUsize::new(0));
+
+        // Occupy some slots up front so the racers fight over a remainder.
+        let mut pre = Vec::new();
+        for _ in 0..8 {
+            pre.push(spawn(holder(release.clone())));
+        }
+        // Free slots now: MAX − 1 (initial) − 8 (pre) − SPAWNERS.
+        let up_for_grabs = MAX - 1 - 8 - SPAWNERS;
+
+        let mut spawners = Vec::new();
+        for _ in 0..SPAWNERS {
+            let release = release.clone();
+            let won = won.clone();
+            let done = done.clone();
+            spawners.push(spawn(move || {
+                loop {
+                    match try_spawn(holder(release.clone())) {
+                        Ok(h) => {
+                            won.fetch_add(1, Ordering::AcqRel);
+                            drop(h); // detached; slot held by the holder
+                        }
+                        Err(SpawnError::AtCapacity) => break,
+                        Err(_) => unreachable!("non_exhaustive future-proofing"),
+                    }
+                }
+                done.fetch_add(1, Ordering::AcqRel);
+            }));
+        }
+        // Wait for every racer to hit AtCapacity.
+        while done.load(Ordering::Acquire) < SPAWNERS {
+            yield_now();
+        }
+        assert_eq!(won.load(Ordering::Acquire), up_for_grabs);
+
+        release.store(true, Ordering::Release);
+        for h in pre.into_iter().chain(spawners) {
+            h.join().unwrap();
+        }
+    });
+}
+
+#[test]
+fn spawn_error_is_a_real_error() {
+    let e = SpawnError::AtCapacity;
+    let msg = format!("{e}");
+    assert!(
+        msg.contains("capacity"),
+        "Display should name the condition: {msg}"
+    );
+    let _: &dyn std::error::Error = &e;
+}