Fix Wasm Workers calling emscripten_futex_wait() when building with pthreads support enabled. Fixes #21549. (#21618)

Would benefit from a test, I'll look into this shortly.

Author: juj

src/library_wasm_worker.js

@@ -32,6 +32,11 @@
 #error "-sPROXY_TO_WORKER is not supported with -sWASM_WORKERS"
 #endif
 
+{{{
+  globalThis.workerSupportsFutexWait = () => AUDIO_WORKLET ? "typeof AudioWorkletGlobalScope === 'undefined'" : '1';
+  null;
+}}}
+
 #endif // ~WASM_WORKERS
 
 
@@ -95,8 +100,9 @@ addToLibrary({
     // Run the C side Worker initialization for stack and TLS.
     __emscripten_wasm_worker_initialize(m['sb'], m['sz']);
 #if PTHREADS
-    // Record that this Wasm Worker supports synchronous blocking in emscripten_futex_wake().
-    ___set_thread_state(/*thread_ptr=*/0, /*is_main_thread=*/0, /*is_runtime_thread=*/0, /*supports_wait=*/0);
+    // Record the pthread configuration, and whether this Wasm Worker supports synchronous blocking in emscripten_futex_wait().
+    // (regular Wasm Workers do, AudioWorklets don't)
+    ___set_thread_state(/*thread_ptr=*/0, /*is_main_thread=*/0, /*is_runtime_thread=*/0, /*supports_wait=*/ {{{ workerSupportsFutexWait() }}});
 #endif
 #if STACK_OVERFLOW_CHECK >= 2
     // Fix up stack base. (TLS frame is created at the bottom address end of the stack)

system/lib/pthread/emscripten_futex_wait.c

@@ -21,7 +21,10 @@ static int futex_wait_main_browser_thread(volatile void* addr,
                                           uint32_t val,
                                           double timeout) {
   // Atomics.wait is not available in the main browser thread, so simulate it
-  // via busy spinning.
+  // via busy spinning. Only the main browser thread is allowed to call into
+  // this function. It is not thread-safe to be called from any other thread.
+  assert(emscripten_is_main_browser_thread());
+
   double now = emscripten_get_now();
   double end = now + timeout;
 

test/test_browser.py

@@ -4946,6 +4946,14 @@ def test_wasm_worker_hello_wasm2js(self):
   def test_wasm_worker_embedded(self):
     self.btest('wasm_worker/hello_wasm_worker.c', expected='0', args=['-sWASM_WORKERS=2'])
 
+  # Tests that it is possible to call emscripten_futex_wait() in Wasm Workers.
+  @parameterized({
+    '': ([],),
+    'pthread': (['-pthread'],),
+  })
+  def test_wasm_worker_futex_wait(self, args):
+    self.btest('wasm_worker/wasm_worker_futex_wait.c', expected='0', args=['-sWASM_WORKERS=1', '-sASSERTIONS'] + args)
+
   # Tests Wasm Worker thread stack setup
   @also_with_minimal_runtime
   def test_wasm_worker_thread_stack(self):

test/wasm_worker/wasm_worker_futex_wait.c

@@ -0,0 +1,35 @@
+// Test that emscripten_futex_wait() works in a Wasm Worker.
+
+#include <emscripten.h>
+#include <emscripten/console.h>
+#include <emscripten/threading.h>
+#include <emscripten/wasm_worker.h>
+#include <limits.h>
+#include <stdio.h>
+#include <math.h>
+
+_Atomic uint32_t futex_value = 0;
+
+void wake_worker_after_delay(void *user_data) {
+  futex_value = 1;
+  emscripten_futex_wake(&futex_value, INT_MAX);  
+}
+
+void wake_worker() {
+  printf("Waking worker thread from futex wait.\n");
+  emscripten_set_timeout(wake_worker_after_delay, 500, 0);
+}
+
+void worker_main() {
+  printf("Worker sleeping for futex wait.\n");
+  emscripten_wasm_worker_post_function_v(0, wake_worker);
+  int rc = emscripten_futex_wait(&futex_value, 0, INFINITY);
+  printf("emscripten_futex_wait returned with code %d.\n", rc);
+#ifdef REPORT_RESULT
+  REPORT_RESULT(rc);
+#endif
+}
+
+int main() {
+  emscripten_wasm_worker_post_function_v(emscripten_malloc_wasm_worker(1024), worker_main);
+}