feat(scheduler,supervisor,gen_server): graceful shutdown — request_shutdown, child Shutdown policy, handle_shutdown

Lift OTP's `exit(Pid, shutdown)` + child-spec `shutdown` wholesale.

scheduler / runtime
- `request_shutdown(pid)`: the polite stop. A target trapping exits gets an
  `ExitSignal { reason: DownReason::Shutdown }` on its trap inbox and keeps
  running; a non-trapping target is stopped as by `request_stop`, which is
  now documented as the hard stop (`exit(Pid, kill)`). Dead pid: no-op.
- `RuntimeHandle::request_shutdown` for the off-runtime (signal thread) path;
  `from == ROOT_PID` there.
- `DownReason::Shutdown` — appears only in ExitSignal, never in Down (a
  complying target exits *normally*).

supervisor
- `ChildSpec::shutdown(Shutdown::{BrutalKill, Timeout(d), Infinity})`,
  default Timeout(5s). Every supervisor-initiated stop (ordered shutdown and
  OneForAll/RestForOne sibling cycling) is: request_shutdown → await the
  child's Signal up to the grace → request_stop → await. Sequential, reverse
  start order.
- The supervisor traps exits; a Shutdown ExitSignal runs the ordered
  shutdown and `run()` returns normally, so `request_shutdown(root_sup)`
  tears a whole tree down top-down with each child's grace period.
- FIX: a hard `request_stop` on a supervisor previously orphaned its
  children (the ordered shutdown lived after the loop, and the unwind
  skipped it). `Live` (the by_pid map) now carries a drop guard that
  fire-and-forget hard-stops live children when unwinding.

gen_server
- `GenServerCtx::trap_exit()` opt-in in `init`; the trap inbox becomes arm 0
  of the loop's select. Shutdown ExitSignal → `handle_shutdown() ->
  ShutdownAction::{Exit, Continue}` (default Exit: loop breaks, `terminate`
  runs on the normal path and may block). Other ExitSignals →
  `handle_exit(sig)`.
- `GenServerCtx::stop_handle() -> StopHandle`, `stop()` ends the server
  after the current message with a *normal* exit — the missing
  `{stop, normal, State}`; `request_stop(self_pid())` was the only self-exit
  and it is abnormal (Transient restarts it).
- `GenServerRef::shutdown()` / `gen_server::shutdown(name)` now go through
  `request_shutdown`.

Tests: tests/shutdown.rs, tests/supervisor_shutdown.rs,
tests/gen_server_shutdown.rs. Full suite green; fmt + clippy --lib clean.

tests/gen_server_shutdown.rs

@@ -0,0 +1,216 @@
+//! gen_server graceful shutdown.
+//!
+//!  - A server that does not opt in (`ctx.trap_exit()` in `init`) is stopped
+//!    outright by `request_shutdown`, exactly as by `request_stop`.
+//!  - A trapping server receives the request as `handle_shutdown`. The default
+//!    returns `ShutdownAction::Exit`: the loop breaks and `terminate` runs on
+//!    the normal (non-unwind) path, so it may block. `Continue` keeps the loop
+//!    dispatching; the state later ends itself with a `StopHandle` — the only
+//!    way for a gen_server to exit *normally* on its own (`request_stop` on
+//!    self is an abnormal `Stopped`, which `Transient` restarts).
+//!  - Other exit signals (linked peers dying) reach a trapping server via
+//!    `handle_exit`.
+
+use smarm::gen_server::{
+    start, GenServer, GenServerBuilder, GenServerCtx, GenServerRef, ShutdownAction, StopHandle,
+};
+use smarm::supervisor::{ChildSpec, OneForOne, Restart};
+use smarm::{link, monitor, request_shutdown, run, self_pid, sleep, spawn, DownReason, ExitSignal};
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use std::sync::{Arc, Mutex};
+use std::time::Duration;
+
+#[derive(Default, Clone)]
+struct Log {
+    events: Arc<Mutex<Vec<&'static str>>>,
+}
+impl Log {
+    fn push(&self, e: &'static str) {
+        self.events.lock().unwrap().push(e);
+    }
+    fn get(&self) -> Vec<&'static str> {
+        self.events.lock().unwrap().clone()
+    }
+}
+
+/// A server with configurable shutdown behaviour.
+struct Srv {
+    log: Log,
+    trap: bool,
+    action: ShutdownAction,
+    stop: Option<StopHandle<Srv>>,
+    exits: Arc<Mutex<Vec<ExitSignal>>>,
+}
+
+impl Srv {
+    fn new(log: &Log, trap: bool, action: ShutdownAction) -> Self {
+        Srv {
+            log: log.clone(),
+            trap,
+            action,
+            stop: None,
+            exits: Default::default(),
+        }
+    }
+}
+
+enum Cast {
+    Note(&'static str),
+    StopNow,
+}
+
+impl GenServer for Srv {
+    type Call = ();
+    type Reply = ();
+    type Cast = Cast;
+    type Info = ();
+    type Timer = ();
+
+    fn init(&mut self, ctx: &GenServerCtx<Self>) {
+        if self.trap {
+            ctx.trap_exit();
+        }
+        self.stop = Some(ctx.stop_handle());
+    }
+    fn handle_call(&mut self, _: ()) {}
+    fn handle_cast(&mut self, c: Cast) {
+        match c {
+            Cast::Note(s) => self.log.push(s),
+            Cast::StopNow => self.stop.as_ref().unwrap().stop(),
+        }
+    }
+    fn handle_shutdown(&mut self) -> ShutdownAction {
+        self.log.push("handle_shutdown");
+        self.action
+    }
+    fn handle_exit(&mut self, sig: ExitSignal) {
+        self.log.push("handle_exit");
+        self.exits.lock().unwrap().push(sig);
+    }
+    fn terminate(&mut self) {
+        // Allowed to block on the graceful path.
+        if self.trap {
+            sleep(Duration::from_millis(10));
+        }
+        self.log.push("terminate");
+    }
+}
+
+fn spawn_settled<G: GenServer>(state: G) -> GenServerRef<G> {
+    let r = start(state);
+    sleep(Duration::from_millis(20)); // let init (trap_exit) run
+    r
+}
+
+#[test]
+fn non_trapping_server_is_stopped_outright() {
+    let log = Log::default();
+    let l = log.clone();
+    run(move || {
+        let r = spawn_settled(Srv::new(&l, false, ShutdownAction::Exit));
+        let mon = monitor(r.pid());
+        request_shutdown(r.pid());
+        let d = mon.rx.recv().unwrap();
+        assert_eq!(d.reason, DownReason::Stopped);
+    });
+    assert_eq!(log.get(), vec!["terminate"]);
+}
+
+#[test]
+fn trapping_server_exits_normally_via_handle_shutdown() {
+    let log = Log::default();
+    let l = log.clone();
+    run(move || {
+        let r = spawn_settled(Srv::new(&l, true, ShutdownAction::Exit));
+        let mon = monitor(r.pid());
+        request_shutdown(r.pid());
+        let d = mon.rx.recv().unwrap();
+        assert_eq!(d.reason, DownReason::Exit);
+    });
+    assert_eq!(log.get(), vec!["handle_shutdown", "terminate"]);
+}
+
+#[test]
+fn continue_keeps_dispatching_until_stop_handle() {
+    let log = Log::default();
+    let l = log.clone();
+    run(move || {
+        let r = spawn_settled(Srv::new(&l, true, ShutdownAction::Continue));
+        let mon = monitor(r.pid());
+        request_shutdown(r.pid());
+        sleep(Duration::from_millis(20));
+        r.cast(Cast::Note("after-shutdown-request")).unwrap();
+        r.cast(Cast::StopNow).unwrap();
+        let d = mon.rx.recv().unwrap();
+        assert_eq!(d.reason, DownReason::Exit);
+    });
+    assert_eq!(
+        log.get(),
+        vec!["handle_shutdown", "after-shutdown-request", "terminate"]
+    );
+}
+
+#[test]
+fn stop_handle_is_a_normal_exit_that_transient_does_not_restart() {
+    let starts = Arc::new(AtomicUsize::new(0));
+    let s = starts.clone();
+    run(move || {
+        let s2 = s.clone();
+        let sup = spawn(move || {
+            let s3 = s2.clone();
+            OneForOne::new()
+                .child(ChildSpec::new(Restart::Transient, move || {
+                    s3.fetch_add(1, Ordering::SeqCst);
+                    let log = Log::default();
+                    let r = GenServerBuilder::new(Srv::new(&log, false, ShutdownAction::Exit))
+                        .under(self_pid())
+                        .start();
+                    r.cast(Cast::StopNow).unwrap();
+                    // Block until the server is gone; a bare spawn parent
+                    // returning would not itself end the server.
+                    let mon = monitor(r.pid());
+                    let _ = mon.rx.recv();
+                }))
+                .run();
+        });
+        sup.join().unwrap(); // returns only if the child was not restarted forever
+    });
+    assert_eq!(starts.load(Ordering::SeqCst), 1);
+}
+
+#[test]
+fn linked_peer_death_reaches_handle_exit() {
+    let log = Log::default();
+    let l = log.clone();
+    let alive = Arc::new(AtomicBool::new(false));
+    let a = alive.clone();
+    run(move || {
+        let r = spawn_settled(Srv::new(&l, true, ShutdownAction::Exit));
+        let pid = r.pid();
+        let peer = spawn(move || {
+            link(pid);
+            panic!("peer dies");
+        });
+        let _ = peer.join();
+        sleep(Duration::from_millis(20));
+        r.cast(Cast::Note("still-serving")).unwrap();
+        sleep(Duration::from_millis(20));
+        a.store(true, Ordering::SeqCst);
+        let mon = monitor(r.pid());
+        drop(r); // inbox closes → clean exit
+        let _ = mon.rx.recv(); // don't let the root-exit sweep race terminate
+    });
+    assert!(alive.load(Ordering::SeqCst));
+    assert_eq!(log.get(), vec!["handle_exit", "still-serving", "terminate"]);
+}
+
+#[test]
+fn gen_server_ref_shutdown_is_graceful_for_a_trapping_server() {
+    let log = Log::default();
+    let l = log.clone();
+    run(move || {
+        let r = spawn_settled(Srv::new(&l, true, ShutdownAction::Exit));
+        r.shutdown();
+    });
+    assert_eq!(log.get(), vec!["handle_shutdown", "terminate"]);
+}

tests/shutdown.rs

@@ -0,0 +1,122 @@
+//! Graceful shutdown — `request_shutdown` (OTP `exit(Pid, shutdown)`).
+//!
+//! `request_stop` is `exit(Pid, kill)`: an uncatchable unwind at the target's
+//! next observation point. `request_shutdown` is the polite form:
+//!   - a target that is NOT trapping exits is stopped exactly as by
+//!     `request_stop` (OTP's rule: don't trap, you die);
+//!   - a target that IS trapping receives an `ExitSignal { reason: Shutdown }`
+//!     on its trap inbox and keeps running — it is expected to wind down and
+//!     exit normally on its own.
+
+use smarm::{monitor, request_shutdown, run, self_pid, sleep, spawn, trap_exit, DownReason, Pid};
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::{mpsc, Arc};
+use std::thread;
+use std::time::Duration;
+
+const WATCHDOG: Duration = Duration::from_secs(10);
+
+#[test]
+fn request_shutdown_stops_a_non_trapping_actor() {
+    run(|| {
+        let h = spawn(|| sleep(Duration::from_secs(3600)));
+        let mon = monitor(h.pid());
+        request_shutdown(h.pid());
+        let down = mon.rx.recv().expect("down");
+        assert_eq!(down.reason, DownReason::Stopped);
+    });
+}
+
+#[test]
+fn request_shutdown_is_a_message_to_a_trapping_actor() {
+    let unwound = Arc::new(AtomicBool::new(false));
+    let u = unwound.clone();
+    run(move || {
+        struct Unwound(Arc<AtomicBool>);
+        impl Drop for Unwound {
+            fn drop(&mut self) {
+                if std::thread::panicking() {
+                    self.0.store(true, Ordering::SeqCst);
+                }
+            }
+        }
+        let (tx, rx) = smarm::channel::<(Pid, DownReason)>();
+        let (ready_tx, ready_rx) = smarm::channel::<()>();
+        let h = spawn(move || {
+            let _g = Unwound(u);
+            let inbox = trap_exit();
+            let _ = ready_tx.send(());
+            let sig = inbox.recv().expect("exit signal");
+            let _ = tx.send((sig.from, sig.reason));
+            // Keep doing work after the request: shutdown is advisory.
+            sleep(Duration::from_millis(20));
+        });
+        // Trapping is set by the target itself; a request that beats it is a
+        // plain stop (same window as OTP's exit-before-process_flag).
+        ready_rx.recv().expect("ready");
+        let me = self_pid();
+        let mon = monitor(h.pid());
+        request_shutdown(h.pid());
+        let (from, reason) = rx.recv().expect("relayed");
+        assert_eq!(from, me);
+        assert_eq!(reason, DownReason::Shutdown);
+        let down = mon.rx.recv().expect("down");
+        assert_eq!(
+            down.reason,
+            DownReason::Exit,
+            "target exited normally, not stopped"
+        );
+    });
+    assert!(
+        !unwound.load(Ordering::SeqCst),
+        "trapping target must not be unwound"
+    );
+}
+
+#[test]
+fn request_shutdown_on_dead_pid_is_a_no_op() {
+    run(|| {
+        let h = spawn(|| {});
+        let pid = h.pid();
+        let _ = h.join();
+        request_shutdown(pid); // must not panic
+    });
+}
+
+#[test]
+fn handle_request_shutdown_from_foreign_thread() {
+    let rt = smarm::init(smarm::Config::exact(2));
+    let handle = rt.handle();
+
+    let (pid_tx, pid_rx) = mpsc::channel::<Pid>();
+    let requester = thread::spawn(move || {
+        let pid = pid_rx.recv().expect("pid");
+        thread::sleep(Duration::from_millis(50));
+        handle.request_shutdown(pid);
+    });
+
+    let (done_tx, done_rx) = mpsc::channel();
+    thread::spawn(move || {
+        rt.run(move || {
+            let (tx, rx) = smarm::channel::<DownReason>();
+            let (ready_tx, ready_rx) = smarm::channel::<()>();
+            let h = spawn(move || {
+                let inbox = trap_exit();
+                let _ = ready_tx.send(());
+                let sig = inbox.recv().expect("exit signal");
+                let _ = tx.send(sig.reason);
+            });
+            ready_rx.recv().expect("ready");
+            pid_tx.send(h.pid()).expect("send pid");
+            let reason = rx.recv().expect("relayed");
+            assert_eq!(reason, DownReason::Shutdown);
+            let _ = h.join();
+        });
+        let _ = done_tx.send(());
+    });
+
+    done_rx
+        .recv_timeout(WATCHDOG)
+        .expect("run did not return: foreign-thread request_shutdown never reached the target");
+    requester.join().expect("requester thread");
+}

tests/supervisor_shutdown.rs

@@ -0,0 +1,303 @@
+//! Supervisor shutdown — the OTP child-spec `shutdown` policy.
+//!
+//! A supervisor traps exits. A `request_shutdown` reaching it (from its parent
+//! supervisor, or from the app via `request_shutdown`/`RuntimeHandle`) runs
+//! the ordered shutdown: children are stopped in reverse start order, each
+//! per its `Shutdown` policy — `request_shutdown`, wait up to the timeout for
+//! its termination signal, `request_stop` if it overstays — and then `run()`
+//! returns normally. Every supervisor-initiated child stop (ordered shutdown,
+//! OneForAll/RestForOne sibling cycling) goes through the same policy.
+//!
+//! A *hard* `request_stop` on a supervisor unwinds it; a drop guard then
+//! hard-stops its live children so the subtree is never orphaned.
+
+use smarm::supervisor::{ChildSpec, OneForOne, Restart, Shutdown, Strategy};
+use smarm::{
+    monitor, request_shutdown, request_stop, run, sleep, spawn, trap_exit, DownReason, JoinHandle,
+};
+use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use std::sync::{Arc, Mutex};
+use std::time::{Duration, Instant};
+
+/// A child that traps exits, records the order it was shut down in, and exits
+/// normally on the request (after `delay`). Ignores the request if `comply`
+/// is false — a straggler that must be hard-stopped.
+fn polite_child(
+    tag: usize,
+    log: &Arc<Mutex<Vec<usize>>>,
+    delay: Duration,
+    comply: bool,
+) -> impl Fn() + Send + Sync + 'static {
+    let log = log.clone();
+    move || {
+        let inbox = trap_exit();
+        loop {
+            let sig = match inbox.recv() {
+                Ok(s) => s,
+                Err(_) => return,
+            };
+            if sig.reason == DownReason::Shutdown {
+                log.lock().unwrap().push(tag);
+                if comply {
+                    sleep(delay);
+                    return;
+                }
+                // Not complying: keep running until hard-stopped.
+                loop {
+                    sleep(Duration::from_millis(5));
+                }
+            }
+        }
+    }
+}
+
+/// Spawn `sup`, let its children reach `trap_exit`, return the handle.
+fn spawn_settled(sup: OneForOne) -> JoinHandle {
+    let h = spawn(move || sup.run());
+    sleep(Duration::from_millis(30));
+    h
+}
+
+#[test]
+fn shutdown_stops_children_in_reverse_order_and_returns_normally() {
+    let log = Arc::new(Mutex::new(Vec::new()));
+    let l = log.clone();
+    run(move || {
+        let sup = OneForOne::new()
+            .child(ChildSpec::new(
+                Restart::Permanent,
+                polite_child(1, &l, Duration::ZERO, true),
+            ))
+            .child(ChildSpec::new(
+                Restart::Permanent,
+                polite_child(2, &l, Duration::ZERO, true),
+            ))
+            .child(ChildSpec::new(
+                Restart::Permanent,
+                polite_child(3, &l, Duration::ZERO, true),
+            ));
+        let h = spawn_settled(sup);
+        let mon = monitor(h.pid());
+        request_shutdown(h.pid());
+        let down = mon.rx.recv().expect("down");
+        assert_eq!(
+            down.reason,
+            DownReason::Exit,
+            "supervisor exits normally after shutdown"
+        );
+    });
+    assert_eq!(*log.lock().unwrap(), vec![3, 2, 1]);
+}
+
+#[test]
+fn non_trapping_child_is_simply_stopped() {
+    let dropped = Arc::new(AtomicBool::new(false));
+    let d = dropped.clone();
+    run(move || {
+        struct G(Arc<AtomicBool>);
+        impl Drop for G {
+            fn drop(&mut self) {
+                self.0.store(true, Ordering::SeqCst);
+            }
+        }
+        let sup = OneForOne::new().child(ChildSpec::new(Restart::Permanent, move || {
+            let _g = G(d.clone());
+            loop {
+                sleep(Duration::from_millis(5));
+            }
+        }));
+        let h = spawn_settled(sup);
+        request_shutdown(h.pid());
+        h.join().expect("sup");
+    });
+    assert!(dropped.load(Ordering::SeqCst));
+}
+
+#[test]
+fn straggler_is_hard_stopped_after_timeout() {
+    let log = Arc::new(Mutex::new(Vec::new()));
+    let l = log.clone();
+    run(move || {
+        let sup = OneForOne::new().child(
+            ChildSpec::new(
+                Restart::Permanent,
+                polite_child(1, &l, Duration::ZERO, false),
+            )
+            .shutdown(Shutdown::Timeout(Duration::from_millis(50))),
+        );
+        let h = spawn_settled(sup);
+        let t0 = Instant::now();
+        request_shutdown(h.pid());
+        h.join().expect("sup");
+        let took = t0.elapsed();
+        assert!(
+            took >= Duration::from_millis(50),
+            "returned before the grace period: {took:?}"
+        );
+        assert!(
+            took < Duration::from_secs(2),
+            "did not fall back to a hard stop: {took:?}"
+        );
+    });
+    assert_eq!(
+        *log.lock().unwrap(),
+        vec![1],
+        "the straggler did receive the request"
+    );
+}
+
+#[test]
+fn infinity_waits_for_a_slow_but_compliant_child() {
+    let log = Arc::new(Mutex::new(Vec::new()));
+    let l = log.clone();
+    let finished = Arc::new(AtomicBool::new(false));
+    let f = finished.clone();
+    run(move || {
+        let f2 = f.clone();
+        let l2 = l.clone();
+        let sup = OneForOne::new().child(
+            ChildSpec::new(Restart::Permanent, move || {
+                let inbox = trap_exit();
+                let _ = inbox.recv();
+                l2.lock().unwrap().push(1);
+                sleep(Duration::from_millis(150));
+                f2.store(true, Ordering::SeqCst); // only reached if not hard-stopped
+            })
+            .shutdown(Shutdown::Infinity),
+        );
+        let h = spawn_settled(sup);
+        request_shutdown(h.pid());
+        h.join().expect("sup");
+    });
+    assert!(
+        finished.load(Ordering::SeqCst),
+        "Infinity must not hard-stop a compliant child"
+    );
+}
+
+#[test]
+fn brutal_kill_skips_the_request() {
+    let log = Arc::new(Mutex::new(Vec::new()));
+    let l = log.clone();
+    run(move || {
+        let sup = OneForOne::new().child(
+            ChildSpec::new(
+                Restart::Permanent,
+                polite_child(1, &l, Duration::ZERO, true),
+            )
+            .shutdown(Shutdown::BrutalKill),
+        );
+        let h = spawn_settled(sup);
+        request_shutdown(h.pid());
+        h.join().expect("sup");
+    });
+    assert!(
+        log.lock().unwrap().is_empty(),
+        "a BrutalKill child never sees the request"
+    );
+}
+
+#[test]
+fn hard_stop_of_supervisor_does_not_orphan_children() {
+    let alive = Arc::new(AtomicUsize::new(0));
+    let a = alive.clone();
+    run(move || {
+        struct Alive(Arc<AtomicUsize>);
+        impl Drop for Alive {
+            fn drop(&mut self) {
+                self.0.fetch_sub(1, Ordering::SeqCst);
+            }
+        }
+        let mk = |a: Arc<AtomicUsize>| {
+            move || {
+                a.fetch_add(1, Ordering::SeqCst);
+                let _g = Alive(a.clone());
+                loop {
+                    sleep(Duration::from_millis(5));
+                }
+            }
+        };
+        let sup = OneForOne::new()
+            .child(ChildSpec::new(Restart::Permanent, mk(a.clone())))
+            .child(ChildSpec::new(Restart::Permanent, mk(a.clone())));
+        let h = spawn_settled(sup);
+        assert_eq!(a.load(Ordering::SeqCst), 2);
+        let mon = monitor(h.pid());
+        request_stop(h.pid());
+        let _ = mon.rx.recv();
+        sleep(Duration::from_millis(50));
+        assert_eq!(
+            a.load(Ordering::SeqCst),
+            0,
+            "children orphaned by a hard supervisor stop"
+        );
+    });
+}
+
+#[test]
+fn nested_shutdown_reaches_grandchildren() {
+    let log = Arc::new(Mutex::new(Vec::new()));
+    let l = log.clone();
+    run(move || {
+        let l_inner = l.clone();
+        let inner = move || {
+            OneForOne::new()
+                .child(ChildSpec::new(
+                    Restart::Permanent,
+                    polite_child(10, &l_inner, Duration::ZERO, true),
+                ))
+                .child(ChildSpec::new(
+                    Restart::Permanent,
+                    polite_child(11, &l_inner, Duration::ZERO, true),
+                ))
+                .run()
+        };
+        let sup = OneForOne::new()
+            .child(ChildSpec::new(
+                Restart::Permanent,
+                polite_child(1, &l, Duration::ZERO, true),
+            ))
+            .child(ChildSpec::new(Restart::Permanent, inner).shutdown(Shutdown::Infinity));
+        let h = spawn_settled(sup);
+        request_shutdown(h.pid());
+        h.join().expect("sup");
+    });
+    assert_eq!(*log.lock().unwrap(), vec![11, 10, 1]);
+}
+
+#[test]
+fn sibling_cycling_uses_graceful_shutdown() {
+    // OneForAll: when child A dies, sibling B (trapping) must receive a
+    // Shutdown request rather than a bare stop.
+    let log = Arc::new(Mutex::new(Vec::new()));
+    let l = log.clone();
+    let a_runs = Arc::new(AtomicUsize::new(0));
+    let ar = a_runs.clone();
+    run(move || {
+        let ar2 = ar.clone();
+        let sup = OneForOne::new()
+            .strategy(Strategy::OneForAll)
+            .intensity(5, Duration::from_secs(60))
+            .child(ChildSpec::new(Restart::Transient, move || {
+                let n = ar2.fetch_add(1, Ordering::SeqCst) + 1;
+                sleep(Duration::from_millis(30));
+                if n == 1 {
+                    panic!("first run dies");
+                }
+                // Second run: park until shut down.
+                let inbox = trap_exit();
+                let _ = inbox.recv();
+            }))
+            .child(ChildSpec::new(
+                Restart::Permanent,
+                polite_child(2, &l, Duration::ZERO, true),
+            ));
+        let h = spawn(move || sup.run());
+        sleep(Duration::from_millis(150));
+        request_shutdown(h.pid());
+        h.join().expect("sup");
+    });
+    // B was shut down once by the cycle and once by the final shutdown.
+    assert_eq!(*log.lock().unwrap(), vec![2, 2]);
+    assert_eq!(a_runs.load(Ordering::SeqCst), 2);
+}