[pthreads] Fix relaying of mailbox notifications to main thread#27336
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In emscripten-core#27018, `d.targetThread && d.targetThread != _pthread_self()` in `worker.onmessage` on the main thread was changed to only check `d.targetThread` before relaying or asserting. However, when `waitAsyncPolyfilled` is active or `Atomics.waitAsync` is unsupported (e.g., Firefox before 145), `thread->waiting_async` is not set when `_emscripten_thread_mailbox_await` runs. In this fallback mode, `emscripten_thread_mailbox_send` calls `_emscripten_notify_mailbox_postmessage`, which sends a `postMessage` from the worker with `targetThread` set to the main thread. When `worker.onmessage` on the main thread receives this message, `d.targetThread == _pthread_self()`. Without checking `d.targetThread != _pthread_self()`, the main thread either triggers an assertion failure (`assert(d.targetThread != _pthread_self())`) or drops the message when looking up `PThread.pthreads[targetThread]`. Restoring `d.targetThread != _pthread_self()` ensures that mailbox notifications addressed to the main thread fall through and are processed directly (`checkMailbox()`) instead of being forwarded. Also add a browser test (`browser.test_pthread_no_waitasync`) that disables `Atomics.waitAsync` in a `--pre-js` script to verify this fallback behavior. Fixes: emscripten-core#27037
dschuff
approved these changes
Jul 14, 2026
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In #27018,
d.targetThread && d.targetThread != _pthread_self()inworker.onmessageon the main thread was changed to only checkd.targetThreadbefore relaying or asserting.However, when
waitAsyncPolyfilledis active orAtomics.waitAsyncis unsupported (e.g., Firefox before 145),thread->waiting_asyncis not set when_emscripten_thread_mailbox_awaitruns. In this fallback mode,emscripten_thread_mailbox_sendcalls_emscripten_notify_mailbox_postmessage, which sends apostMessagefrom the worker withtargetThreadset to the main thread.When
worker.onmessageon the main thread receives this message,d.targetThread == _pthread_self(). Without checkingd.targetThread != _pthread_self(), the main thread either triggers an assertion failure (assert(d.targetThread != _pthread_self())) or drops the message when looking upPThread.pthreads[targetThread].Restoring
d.targetThread != _pthread_self()ensures that mailbox notifications addressed to the main thread fall through and are processed directly (checkMailbox()) instead of being forwarded.Also add a browser test (
browser.test_pthread_no_waitasync) that disablesAtomics.waitAsyncin a--pre-jsscript to verify this fallback behavior.Fixes: #27037