Only use futex_wait_busy on the main browser thread
#18402
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When I ported this code from JS back in #15742 I mistakenly changed the semantics here. This change restores the previous behaviour of using atomic.wait except in on the main browser thread. The `futex_wait_busy` can clearly only be used a single main thread and is not appropriate for using in cases where there are other threads in the system that also don't support `atomic.wait` (such as IIUC audio worklets). To support that case we would need a different busy wait function that didn't depend on some global singleton like _emscripten_main_thread_futex.
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When I ported this code from JS back in #15742 I mistakenly changed the semantics here. This change restores the previous behaviour of using atomic.wait except in on the main browser thread.
The
futex_wait_busycan clearly only be used a single main thread and is not appropriate for using in cases where there are other threads in the system that also don't supportatomic.wait(such as IIUC audio worklets). To support that case we would need a different busy wait function that didn't depend on some global singleton like _emscripten_main_thread_futex.