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Special-case emscripten without atomics to take no-threads path#156366

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Special-case emscripten without atomics to take no-threads path#156366
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juntyr:emscripten-no-threads-sync

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@juntyr juntyr commented May 9, 2026 •
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Fixes #156350

Emscripten's pthread stub, which is used when compiling Emscripten without pthreads/atomics, has an unsound mutex implementation. As a quick fix, explicitly make Emscripten without atomics take the no-threads path.

@hoodmane Do we support Emscripten tests in CI here? How should we test this?

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels May 9, 2026
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rustbot commented May 9, 2026

r? @Mark-Simulacrum

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@juntyr juntyr force-pushed the emscripten-no-threads-sync branch from 3eabd84 to aeabcda Compare May 9, 2026 15:50
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Mark-Simulacrum commented May 10, 2026

As a quick fix, explicitly make Emscripten without atomics take the no-threads path

Does Emscripten without atomics mean Emscripten without threads? It's not obvious to me that is true.

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curiousdannii commented May 10, 2026 •
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Or the reverse - atomics could be enabled but pthreads disabled.

If Emscripten pthreads needs RUSTFLAGS="-Clink-arg=-pthread" then is it even something that could be tested with cfg_select!?

Also for a Rust staticlib there might not be any link args?

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juntyr commented May 11, 2026

As a quick fix, explicitly make Emscripten without atomics take the no-threads path

Does Emscripten without atomics mean Emscripten without threads? It's not obvious to me that is true.

@sbc100 Could you answer this one?

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Mutex::try_lock doesn't error in wasm32-emscripten

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