select(&[&dyn Selectable]) -> usize registers (pid, epoch) in every arm
under one wait epoch, parks once, and returns the first ready index in
documented priority order (BEAM-style; no fairness promise). A closed arm
counts as ready — and stays ready forever, so callers drop it from the set
once observed. Losing arms need no cancellation pass: the winning wake
consumed the epoch, so their registrations die at their wakers' failed CAS
or are overwritten by the receiver's next wait on that channel (the
single-receiver debug_asserts relax to 'none or own pid' accordingly).
The one genuinely new protocol piece is the no-park exit: returning with an
arm ready at registration time leaves earlier arms holding LIVE-epoch
registrations, whose wakes could fault a later one-shot park as a pending
notification. scheduler::retire_wait closes it — bump the epoch (in-flight
wakes die at their CAS), eat a notification that already landed
(StateWord::clear_notify), then re-observe the stop flag, in that order.
Proved by the new loom theorem retire_eats_late_arm_notification; the
integration probes in tests/select.rs fire stale loser-arm wakes and assert
a subsequent sleep's one-shot park holds its full duration.
Word repacks to (gen << 32) | (epoch << 8) | state. begin_wait opens a wait
identity; every successful wake consumes it, so at most one wake lands per
wait by construction. unpark grows an Option<epoch> match; RuntimeInner gains
unpark_at. Claim/yield/park transitions become epoch-preserving CAS loops.
Loom: all four prior theorems re-proved on the new word, plus
consumed_epoch_unpark_never_lands — a late waker stamped with a consumed
epoch can neither enqueue nor notify, in every interleaving (the property
select's loser arms and every one-shot park site lean on).
No callers stamped yet; behaviour identical. Two transient dead_code
warnings (begin_wait, unpark_at) are consumed by the next commit.
State machine extracted to src/slot_state.rs as a standalone unit (StateWord:
publish_queued / try_claim / yield_return / park_return / unpark / set_done /
reclaim, plus the Status view for cold paths). runtime.rs keeps the protocol
rationale and consumes the mechanism; every transition self-asserts its
precondition per the assert-the-invariants rule.
src/sync_shim.rs: std vs loom indirection (atomics + UnsafeCell with the
with/with_mut access API) for the two loom-modeled modules. Loom models run
the PRODUCTION code, not a replica:
- slot_state: no-lost-wakeup (park vs unpark), two-unparkers-one-enqueue,
stale-unpark-never-hits-reused-slot (the ABA theorem), unpark-vs-claim.
- run_queue: mpmc exactly-once through a lap wraparound, push/pop race,
striped two-producer drain.
RUSTFLAGS="--cfg loom" cargo test --lib --release — 7 models, all pass.
RawMutex deliberately not loom-modeled (futexes can't be; textbook mutex3
with stress + unwind coverage).
Assertion sweep (invariants now checked at the point of reliance):
- enqueue debug-asserts the word reads EXACTLY (gen, Queued) — the
at-most-once-enqueued invariant the ring capacity proof leans on.
- RawMutex enforces the leaf rule mechanically: debug-build thread-local
held-count, panics at the acquisition that violates it.
- live_actors underflow (double finalize) asserted.
- StateWord transition preconditions asserted (yield/park/done/reclaim/
publish/claim).
Audit: with_runtime, RawMutex guards, run-queue ops, and trace::record all
gate preemption (and thereby the stop sentinel) for their span; trace was
already self-gating. Sole remaining exception is the channel std MutexGuard
— the documented first post-v0.5 fast-follow.
Review fixes to the branched-in work:
- slot_state::status_for mapped (matching gen, Vacant) to Stale instead of
unreachable!: Pid::new is public, so a forged/never-issued pid (e.g.
monitor(Pid::new(5,0)) on a fresh slab) could reach it — "no such actor"
is the correct total answer; issued pids still can't get there.
- Tightened enqueue's assert from Status::Live to the exact (gen, Queued)
word its own comment argues for.
Validated: 22 suites in debug (all asserts + leaf counter live) for
rq-mutex/rq-mpmc/rq-striped, release for rq-mutex, smarm-trace build +
stress, loom 7/7.
smarm