Fix race condition in browser.test_audioworklet_worker (#26839)

Fix race condition in browser.test_audioworklet_worker where worker
might miss the wakeup signal that audio worklet has posted.

Update documentation of `emscripten_futex_wait()` for the different
return values the function can have.

Author: juj

system/include/emscripten/threading_primitives.h

@@ -189,11 +189,16 @@ void emscripten_condvar_signal(emscripten_condvar_t * _Nonnull condvar, uint32_t
 // If the given memory address contains value val, puts the calling thread to
 // sleep waiting for that address to be notified. Like the linux futex syscall
 // this function returns negative errno values on failure.
-// Returns -EINVAL if addr is null.
-// Returns -ETIMEOUT if the maxWaitMilliseconds timeout was exceeded.
-// Returns -EINTR if the operation was interrupted (e.g. a timer fired, or an
-// async signal was received).
-// Returns 0 on success (i.e. another thread signaled this address)
+// Pass maxWaitMilliseconds = INFINITY (or __builtin_inf()) to sleep indefinitely.
+// Returns:
+// * negative value -EINVAL if addr is null.
+// * negative value -ETIMEDOUT if the maxWaitMilliseconds timeout was exceeded.
+// * negative value -EINTR if the operation was interrupted (e.g. a timer fired, or an
+//   async signal was received).
+// * negative value -EWOULDBLOCK if the value of the memory address 'addr' was
+//   not equal to 'val' to begin with.
+// * negative value -ECANCELED if the calling thread has been canceled.
+// * the value 0 on success (i.e. another thread signaled this address)
 int emscripten_futex_wait(volatile void/*uint32_t*/ * _Nonnull addr, uint32_t val, double maxWaitMilliseconds);
 
 // Wakes the given number of threads waiting on a location. Pass count ==

test/webaudio/audioworklet_worker.c

@@ -21,13 +21,9 @@ void do_exit() {
 void run_in_worker() {
   double start = emscripten_performance_now();
   emscripten_outf("run_in_worker");
-  while (emscripten_futex_wait(&workletToWorkerFlag, 0, 30000) == 0) {
-    if (workletToWorkerFlag == true) {
-      emscripten_outf("Test success (waited %.fms)", emscripten_performance_now() - start);
-      emscripten_wasm_worker_post_function_v(EMSCRIPTEN_WASM_WORKER_ID_PARENT, &do_exit);
-      break;
-    }
-  }
+  emscripten_futex_wait(&workletToWorkerFlag, 0, __builtin_inf());
+  emscripten_outf("Test success (waited %.fms)", emscripten_performance_now() - start);
+  emscripten_wasm_worker_post_function_v(EMSCRIPTEN_WASM_WORKER_ID_PARENT, &do_exit);
 }
 
 // This event will fire on the audio worklet thread.