RFC 016 Chunk 1: runtime introspection read primitive

snapshot() / actor_info() return owned ActorInfo over the slab: pid,
names, fine-grained scheduling state, parent edge, trap flag, mailbox
depth, and monitor/link/joiner counts. Pure reads, no hot-path change.

- ActorState maps the packed slot word (no new storage); introspect.rs.
- D1: RuntimeSnapshot carries SNAPSHOT_FORMAT_VERSION from day one.
- D2: per-slot tearing (ps semantics); actor_info coherent per actor.
- D3: mailbox depth included. Registry channels now stored behind an
  ErasedSender trait (downcast for clone_sender + type-erased
  queued_len); depth summed over published channels under the registry
  Leaf (Leaf -> Channel), kept out of the cold-lock pass so no two
  Leaves are ever held at once. Depth covers published channels only.
- Done slots surface as root-less tombstones.

src/channel.rs

@@ -137,6 +137,14 @@ 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.
+    pub(crate) fn queued_len(&self) -> usize {
+        self.inner.lock().queue.len()
+    }
+
     pub fn send(&self, value: T) -> Result<(), SendError<T>> {
         let unpark = {
             let mut g = self.inner.lock();

src/introspect.rs

@@ -0,0 +1,180 @@
+//! RFC 016 — runtime introspection (Chunk 1: the read primitive).
+//!
+//! 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.
+//!
+//! ## Consistency (DECISION D2 — per-slot tearing, `ps` semantics)
+//!
+//! [`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.
+//!
+//! ## Locking
+//!
+//! 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.
+
+use crate::pid::Pid;
+use crate::registry::MailboxInfo;
+use crate::runtime::{Slot, ROOT_PID};
+use crate::scheduler::with_runtime;
+use crate::slot_state::{
+    word_gen, word_state, ST_DONE, ST_PARKED, ST_QUEUED, ST_RUNNING, ST_RUNNING_NOTIFIED,
+};
+
+/// 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.
+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.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ActorState {
+    Queued,
+    Running,
+    Notified,
+    Parked,
+    Done,
+}
+
+/// Classify a packed state word. `None` for a Vacant slot (skipped by the scan)
+/// — the only state that is not an actor.
+fn classify(w: u64) -> Option<ActorState> {
+    Some(match word_state(w) {
+        ST_QUEUED => ActorState::Queued,
+        ST_RUNNING => ActorState::Running,
+        ST_RUNNING_NOTIFIED => ActorState::Notified,
+        ST_PARKED => ActorState::Parked,
+        ST_DONE => ActorState::Done,
+        _ => return None, // ST_VACANT
+    })
+}
+
+/// Owned, point-in-time view of one actor — no borrows of runtime internals, so
+/// it is safe to hand to any consumer.
+#[derive(Debug, Clone)]
+pub struct ActorInfo {
+    pub pid: Pid,
+    /// Registered names, inverted from the registry (usually 0 or 1).
+    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).
+    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.
+    pub mailbox_depth: u32,
+}
+
+/// A whole-runtime snapshot. See the module docs for the D2 tearing model.
+#[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.
+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).
+        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).
+        let mut actors = Vec::new();
+        for (idx, slot) in inner.slots.iter().enumerate() {
+            let idx = idx as u32;
+            if let Some(info) = read_slot(slot, idx, mail.get(&idx)) {
+                actors.push(info);
+            }
+        }
+        RuntimeSnapshot { format_version: SNAPSHOT_FORMAT_VERSION, actors }
+    })
+}
+
+/// 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)?;
+        let mail = inner.registry.lock().introspect_one(pid.index());
+        let info = read_slot(slot, pid.index(), mail.as_ref())?;
+        // read_slot keys on the slab's *current* generation; reject if that
+        // isn't the incarnation the caller asked about.
+        (info.pid.generation() == pid.generation()).then_some(info)
+    })
+}
+
+/// 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.
+fn read_slot(slot: &Slot, idx: u32, mail: Option<&MailboxInfo>) -> Option<ActorInfo> {
+    let w = slot.state_word();
+    let state = classify(w)?;
+    let gen = word_gen(w);
+    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 word_gen(slot.state_word()) != gen {
+        return None;
+    }
+    let (supervisor, trap_exit) = match cold.actor.as_ref() {
+        // Live incarnation: parent + trap live on the Actor.
+        Some(actor) => (actor.supervisor, actor.trap.is_some()),
+        // Done tombstone: the Actor was taken at finalize and the collections
+        // cleared, so report it root-less with empty counts.
+        None => (ROOT_PID, false),
+    };
+    let monitors = cold.monitors.len() as u32;
+    let links = cold.links.len() as u32;
+    let joiners = cold.waiters.len() as u32;
+    drop(cold);
+
+    // Names + depth belong to this incarnation only if the registry mailbox's
+    // pid matches the slab generation; a stale registry entry (dead prior
+    // occupant, not yet pruned) contributes nothing.
+    let (names, mailbox_depth) = match mail {
+        Some(mi) if mi.pid.generation() == gen => (mi.names.clone(), mi.depth),
+        _ => (Vec::new(), 0),
+    };
+
+    Some(ActorInfo {
+        pid,
+        names,
+        state,
+        supervisor,
+        trap_exit,
+        monitors,
+        links,
+        joiners,
+        mailbox_depth,
+    })
+}

src/lib.rs

@@ -27,6 +27,7 @@ pub mod registry;
 pub mod pg;
 pub mod link;
 pub mod gen_server;
+pub mod introspect;
 pub mod runtime;
 pub(crate) mod raw_mutex;
 pub(crate) mod slot_state;
@@ -54,6 +55,9 @@ pub use gen_server::{
     call, cast, shutdown, whereis_server, CallError, CallTimeoutError, CastError, GenServer,
     NamedServerBuilder, ServerBuilder, ServerCtx, ServerName, ServerRef, TimerHandle, Watcher,
 };
+pub use introspect::{
+    actor_info, snapshot, ActorInfo, ActorState, RuntimeSnapshot, SNAPSHOT_FORMAT_VERSION,
+};
 pub use link::{link, trap_exit, unlink, ExitSignal};
 pub use monitor::{demonitor, monitor, Down, DownReason, Monitor, MonitorId};
 pub use mutex::{LockTimeout, Mutex, MutexGuard};

src/registry.rs

@@ -148,11 +148,30 @@ impl<M> std::fmt::Display for SendError<M> {
 
 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.
+trait ErasedSender: Send {
+    fn as_any(&self) -> &dyn Any;
+    fn queued_len(&self) -> usize;
+}
+
+impl<M: Send + 'static> ErasedSender for Sender<M> {
+    fn as_any(&self) -> &dyn Any {
+        self
+    }
+    fn queued_len(&self) -> usize {
+        Sender::queued_len(self)
+    }
+}
+
 /// 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.
 struct Channel {
-    sender: Box<dyn Any + Send>,
+    sender: Box<dyn ErasedSender>,
     msg_type: &'static str,
 }
 
@@ -176,6 +195,7 @@ impl Mailbox {
         let ch = self.channels.get(&TypeId::of::<M>())?;
         let tx = ch
             .sender
+            .as_any()
             .downcast_ref::<Sender<M>>()
             .expect("channel keyed by TypeId but downcast to its own type failed — smarm bug");
         debug_assert_eq!(ch.msg_type, type_name::<M>(), "msg_type / TypeId disagree");
@@ -183,6 +203,18 @@ impl Mailbox {
     }
 }
 
+/// Per-actor registry view handed to RFC 016 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
+/// actor that holds only a private `channel()` receiver is invisible here and
+/// reports depth 0.
+pub(crate) struct MailboxInfo {
+    pub(crate) pid: Pid,
+    pub(crate) names: Vec<&'static str>,
+    pub(crate) depth: u32,
+}
+
 /// The directory. Invariant (held under the registry lock): every value in
 /// `by_name` is the index of a [`Mailbox`] present in `by_index`, and that
 /// mailbox's `pid.index()` equals the key. Stale entries (dead actors) violate
@@ -204,6 +236,49 @@ impl Registry {
         self.by_index.remove(&index);
         self.by_name.retain(|_, idx| *idx != 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 that dangle (point at no mailbox) are
+    /// dropped — they violate no invariant and get pruned on next contact.
+    pub(crate) fn introspect_map(&self) -> HashMap<u32, MailboxInfo> {
+        let mut names: HashMap<u32, Vec<&'static str>> = HashMap::new();
+        for (&name, &idx) in &self.by_name {
+            names.entry(idx).or_default().push(name);
+        }
+        let mut out: HashMap<u32, MailboxInfo> = HashMap::with_capacity(self.by_index.len());
+        for (&idx, mb) in &self.by_index {
+            let depth: usize = mb.channels.values().map(|c| c.sender.queued_len()).sum();
+            out.insert(
+                idx,
+                MailboxInfo {
+                    pid: mb.pid,
+                    names: names.remove(&idx).unwrap_or_default(),
+                    depth: depth.min(u32::MAX as usize) as u32,
+                },
+            );
+        }
+        out
+    }
+
+    /// 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.
+    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();
+        let names = self
+            .by_name
+            .iter()
+            .filter_map(|(&n, &i)| (i == idx).then_some(n))
+            .collect();
+        Some(MailboxInfo { pid: mb.pid, names, depth: depth.min(u32::MAX as usize) as u32 })
+    }
 }
 
 /// Is `pid` a live actor right now? Atomic slot-word read; no lock.

src/runtime.rs

@@ -457,6 +457,14 @@ impl Slot {
         self.word.generation()
     }
 
+    /// Raw packed state word, for introspection's lock-free classify
+    /// (`introspect.rs`). The coarse `status_for` only distinguishes
+    /// Live/Done/Stale; the snapshot needs the fine scheduling state.
+    #[inline]
+    pub(crate) fn state_word(&self) -> u64 {
+        self.word.load()
+    }
+
     /// A pid's-eye snapshot of the slot. Cold paths re-read this under the
     /// cold lock (generation can't change while it is held).
     #[inline]