smarm/tests/link.rs

//! Link + trap_exit tests (roadmap item #3).
//!
//! A *link* is bidirectional and persistent (contrast a monitor, which is
//! unidirectional and one-shot). When a linked actor dies *abnormally* — a
//! panic or a cooperative `request_stop` — the death propagates to the peer:
//!
//!   - a peer that has NOT trapped exits is cooperatively `request_stop`'d, so
//!     a crash fate-shares across the link set (Erlang's "let it crash");
//!   - a peer that HAS called `trap_exit()` instead receives an [`ExitSignal`]
//!     *message* on its trap inbox and survives.
//!
//! A *normal* exit never propagates — not even to a trapping peer. Linking an
//! already-dead pid delivers an immediate exit signal (`NoProc`): a message to
//! a trapping caller, a `request_stop` to a non-trapping one.
//!
//! These cases are deterministic under the single-thread runtime: an actor
//! runs until it parks/yields, so the relative order of link setup, the
//! triggering death, and the propagated signal is fixed.

use smarm::{
    channel, link, monitor, run, self_pid, spawn, trap_exit, unlink, yield_now, DownReason,
};
use std::sync::atomic::{AtomicBool, Ordering};
use std::sync::Arc;

/// Sets its flag when dropped — proves a propagated stop unwound the peer's
/// stack (running Drop) rather than leaking it.
struct DropFlag(Arc<AtomicBool>);
impl Drop for DropFlag {
    fn drop(&mut self) {
        self.0.store(true, Ordering::SeqCst);
    }
}

/// Abnormal death of one linked actor cooperatively stops its non-trapping
/// peer, and the peer's Drop guards run during the propagated unwind.
#[test]
fn linked_pair_one_panics_other_is_stopped() {
    let dropped = Arc::new(AtomicBool::new(false));
    let saw_stopped = Arc::new(AtomicBool::new(false));
    let (d, s) = (dropped.clone(), saw_stopped.clone());
    run(move || {
        // B parks on recv (kept open by a retained sender) so it is alive and
        // suspended when the propagated stop arrives.
        let hb = spawn(move || {
            let _g = DropFlag(d);
            let (tx, rx) = channel::<u8>();
            let _keep = tx;
            let _ = rx.recv(); // parks until the link-propagated stop unwinds us
        });
        let b = hb.pid();
        let down_b = monitor(b);

        // A links B, then panics. The panic propagates along the link to B.
        let ha = spawn(move || {
            link(b);
            panic!("boom");
        });

        let dn = down_b.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);
        }
        drop(ha);
        let _ = hb.join();
    });
    assert!(
        saw_stopped.load(Ordering::SeqCst),
        "peer should be Stopped by the propagated abnormal exit"
    );
    assert!(
        dropped.load(Ordering::SeqCst),
        "peer's Drop guard must run during the propagated cancellation unwind"
    );
}

/// A trapping peer receives an abnormal death as an `ExitSignal` message and
/// keeps running instead of being torn down.
#[test]
fn trap_exit_turns_abnormal_death_into_a_message() {
    let ok = Arc::new(AtomicBool::new(false));
    let o = ok.clone();
    run(move || {
        let h = spawn(move || {
            let inbox = trap_exit();
            // Spawn the peer from here so we control ordering: it is enqueued
            // behind us and runs once we park on `inbox.recv()`.
            let ha = spawn(|| panic!("peer down"));
            let a = ha.pid();
            link(a);
            // Parks here; A then runs, panics, and the trap message wakes us.
            let sig = inbox.recv().expect("trap inbox closed without a signal");
            if sig.from == a && matches!(sig.reason, DownReason::Panic) {
                o.store(true, Ordering::SeqCst);
            }
            drop(ha);
        });
        let _ = h.join();
    });
    assert!(
        ok.load(Ordering::SeqCst),
        "trapping peer should receive ExitSignal{{from, Panic}} and survive"
    );
}

/// A normal exit never propagates — a trapping linked peer sees no message and
/// stays alive.
#[test]
fn normal_exit_does_not_propagate() {
    let empty = Arc::new(AtomicBool::new(false));
    let e = empty.clone();
    run(move || {
        let h = spawn(move || {
            let inbox = trap_exit();
            let ha = spawn(|| { /* exits normally, immediately */ });
            let a = ha.pid();
            link(a);
            yield_now(); // let A run to completion and finalize
            // A exited normally: nothing should have landed on the inbox.
            if let Ok(None) = inbox.try_recv() {
                e.store(true, Ordering::SeqCst);
            }
            drop(ha);
        });
        let _ = h.join();
    });
    assert!(
        empty.load(Ordering::SeqCst),
        "normal exit must not deliver any ExitSignal to a trapping peer"
    );
}

/// Linking an already-dead pid from a non-trapping actor stops the caller
/// (no silent no-op).
#[test]
fn link_to_dead_pid_stops_a_nontrapping_caller() {
    let dropped = Arc::new(AtomicBool::new(false));
    let saw_stopped = Arc::new(AtomicBool::new(false));
    let (d, s) = (dropped.clone(), saw_stopped.clone());
    run(move || {
        let ha = spawn(|| {});
        let a = ha.pid();
        ha.join().unwrap(); // A finishes and is reclaimed → `a` is now stale

        let hb = spawn(move || {
            let _g = DropFlag(d);
            link(a); // dead target, not trapping → request_stop(self)
            loop {
                smarm::check!(); // observation point to realize the stop
            }
        });
        let b = hb.pid();
        let down_b = monitor(b);
        let dn = down_b.recv().expect("monitor channel closed before Down");
        if matches!(dn.reason, DownReason::Stopped) {
            s.store(true, Ordering::SeqCst);
        }
        let _ = hb.join();
    });
    assert!(
        saw_stopped.load(Ordering::SeqCst),
        "linking a dead pid (non-trapping) should stop the caller"
    );
    assert!(dropped.load(Ordering::SeqCst), "Drop guard must run");
}

/// Linking an already-dead pid from a trapping actor delivers an immediate
/// `NoProc` exit message instead of killing it.
#[test]
fn link_to_dead_pid_messages_a_trapping_caller() {
    let ok = Arc::new(AtomicBool::new(false));
    let o = ok.clone();
    run(move || {
        let ha = spawn(|| {});
        let a = ha.pid();
        ha.join().unwrap(); // `a` is now stale

        let hb = spawn(move || {
            let inbox = trap_exit();
            link(a); // dead target, trapping → immediate NoProc message
            let sig = inbox.recv().expect("trap inbox closed without a signal");
            if sig.from == a && matches!(sig.reason, DownReason::NoProc) {
                o.store(true, Ordering::SeqCst);
            }
        });
        let _ = hb.join();
    });
    assert!(
        ok.load(Ordering::SeqCst),
        "linking a dead pid (trapping) should deliver a NoProc ExitSignal"
    );
}

/// `unlink` removes the relationship in both directions: an abnormal death no
/// longer reaches the formerly-linked peer.
#[test]
fn unlink_prevents_propagation() {
    let survived = Arc::new(AtomicBool::new(false));
    let sv = survived.clone();
    run(move || {
        let h = spawn(move || {
            let b = self_pid();
            let inbox = trap_exit();
            let ha = spawn(move || {
                link(b);
                unlink(b);
                panic!("boom"); // abnormal, but the link is gone
            });
            yield_now(); // let A link, unlink, and panic
            // Unlinked before death → no ExitSignal should have arrived.
            if let Ok(None) = inbox.try_recv() {
                sv.store(true, Ordering::SeqCst);
            }
            drop(ha);
        });
        let _ = h.join();
    });
    assert!(
        survived.load(Ordering::SeqCst),
        "unlinked peer must not receive a propagated exit"
    );
}