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fix race in JoinFuture::poll when waker changes between notified check and waiters list update #104

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fix race in JoinFuture::poll when waker changes between notified check and waiters list update #104

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14 changes: 14 additions & 0 deletions src/sync_placeholder.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -558,6 +558,20 @@ impl<
let new_waker = cx.waker();
if !waker.will_wake(new_waker) {
let mut state = shared.state.write();
// Re-check notified after acquiring the lock. A concurrent
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@arthurprs arthurprs Mar 6, 2026

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Could you try merging this match arm with the one above, maybe

JoinFutureState::Pending { waker, shared } => loop {
    if this.notified.load(Ordering::Acquire) {
       ..
    }
    ..
    let state = shared.state.write();
    if this.notified.load(Ordering::Acquire) {
        continue;
    }
    ...
}

// insert may have drained the waiters list between the
// notified check in the match guard above and this point.
if this.notified.load(Ordering::Acquire) {
drop(state);
let JoinFutureState::Pending { shared, .. } =
mem::replace(&mut this.state, JoinFutureState::Done)
else {
unsafe { unreachable_unchecked() }
};
return Poll::Ready(PlaceholderGuard::handle_notification(
lifecycle, shard, shared,
));
}
if let Some(w) = state
.waiters
.iter_mut()
Expand Down