Explicitly allow spurious wakeups in emscripten_futex_wait - #27760
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In emscripten-core#26735, `emscripten_futex_wait` was updated to return `-EINTR` when woken via side-channel notifications (`_emscripten_thread_notify`) or when a timer fired. However, side-channel notifications are not POSIX signals, and returning `-EINTR` creates the false expectation that a return value of `0` guarantees the underlying condition was met. In practice, spurious wakeups returning `0` can still occur (e.g. via races in `_emscripten_thread_notify`, collateral wakeups on shared wait addresses, or async wait cancellations). Furthermore, returning `-EINTR` could cause functions like `sem_timedwait` to fail prematurely. Revert the `-EINTR` reporting so that `emscripten_futex_wait` matches the Linux `futex(2)` contract: any non-error wakeup returns `0`, and callers must always verify their application condition in a loop. This change has to land after emscripten-core#27755. Fixes: emscripten-core#27759
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I make some further cleanups here. PTAL. |
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In #26735,
emscripten_futex_waitwas updated to return-EINTRwhenwoken via side-channel notifications (
_emscripten_thread_notify) orwhen a timer fired.
In practice, however, spurious wakeups returning
0can still occur(e.g. via races in
_emscripten_thread_notify, collateral wakeups onshared wait addresses, or async wait cancellations). Furthermore,
returning
-EINTRcould cause functions likesem_timedwaitto failprematurely (and in musl's
__timedwait_cp,EINTRis alreadyconverted to
0because__eintr_valid_flagis always0).Revert the
-EINTRreporting so thatemscripten_futex_waitmatchesthe Linux
futex(2)contract: any non-error wakeup returns0, andcallers must always verify their application condition in a loop. When
max_wait_msis shortened on the main runtime thread to service_emscripten_next_timer(), track this viatimeout_adjustedso that anearly wakeup returns
0rather than-ETIMEDOUT.Finally, since no callers check the return value of
_emscripten_yieldanymore, change
_emscripten_yieldand_emscripten_check_timersbackto returning
void.Fixes: #27759