Fix a deadlock issue with emscripten_lock_async_acquire()

src/lib/libwasm_worker.js

@@ -300,24 +300,21 @@ if (ENVIRONMENT_IS_WASM_WORKER
 
   emscripten_lock_async_acquire__deps: ['$polyfillWaitAsync'],
   emscripten_lock_async_acquire: (lock, asyncWaitFinished, userData, maxWaitMilliseconds) => {
-    let dispatch = (val, ret) => {
-      setTimeout(() => {
-        {{{ makeDynCall('vpiip', 'asyncWaitFinished') }}}(lock, val, /*waitResult=*/ret, userData);
-      }, 0);
-    };
     let tryAcquireLock = () => {
       do {
         var val = Atomics.compareExchange(HEAP32, {{{ getHeapOffset('lock', 'i32') }}}, 0/*zero represents lock being free*/, 1/*one represents lock being acquired*/);
-        if (!val) return dispatch(0, 0/*'ok'*/);
+        if (!val) return {{{ makeDynCall('vpiip', 'asyncWaitFinished') }}}(lock, 0, 0/*'ok'*/, userData);
         var wait = Atomics.waitAsync(HEAP32, {{{ getHeapOffset('lock', 'i32') }}}, val, maxWaitMilliseconds);
       } while (wait.value === 'not-equal');
 #if ASSERTIONS
       assert(wait.async || wait.value === 'timed-out');
 #endif
       if (wait.async) wait.value.then(tryAcquireLock);
-      else dispatch(val, 2/*'timed-out'*/);
+      else return {{{ makeDynCall('vpiip', 'asyncWaitFinished') }}}(lock, val, 2/*'timed-out'*/, userData);
     };
-    tryAcquireLock();
+    // Asynchronously dispatch acquiring the lock so that we have uniform control flow in both
+    // cases when the lock is acquired, and when it needs to wait.
+    setTimeout(tryAcquireLock);
   },
 
   emscripten_semaphore_async_acquire__deps: ['$polyfillWaitAsync'],

system/include/emscripten/wasm_worker.h

@@ -188,6 +188,10 @@ void emscripten_lock_busyspin_waitinf_acquire(emscripten_lock_t *lock __attribut
 // NOTE: This function can be called in both main thread and in Workers. If you
 //       use this API in Worker, you cannot utilise an infinite loop programming
 //       model.
+// NOTE 2: This function will always acquire the lock asynchronously. That is,
+//         the lock will only be attempted to acquire after current control flow
+//         yields back to the browser, so that the Wasm call stack is empty.
+//         This is to guarantee an uniform control flow.
 void emscripten_lock_async_acquire(emscripten_lock_t *lock __attribute__((nonnull)),
                                    emscripten_async_wait_volatile_callback_t asyncWaitFinished __attribute__((nonnull)),
                                    void *userData,

test/test_browser.py

@@ -5329,6 +5329,12 @@ def test_wasm_worker_lock_wait2(self):
   def test_wasm_worker_lock_async_acquire(self):
     self.btest_exit('wasm_worker/lock_async_acquire.c', cflags=['--closure=1', '-sWASM_WORKERS'])
 
+  # Tests emscripten_lock_async_acquire() function when lock is acquired both synchronously and asynchronously.
+  @also_with_minimal_runtime
+  @flaky('https://github.com/emscripten-core/emscripten/issues/25270')
+  def test_wasm_worker_lock_async_and_sync_acquire(self):
+    self.btest('wasm_worker/lock_async_and_sync_acquire.c', expected='1', cflags=['--closure=1', '-sWASM_WORKERS'])
+
   # Tests emscripten_lock_busyspin_wait_acquire() in Worker and main thread.
   @also_with_minimal_runtime
   def test_wasm_worker_lock_busyspin_wait(self):

test/wasm_worker/lock_async_and_sync_acquire.c

@@ -0,0 +1,29 @@
+#include <emscripten/wasm_worker.h>
+#include <emscripten/threading.h>
+#include <stdio.h>
+
+emscripten_lock_t lock = EMSCRIPTEN_LOCK_T_STATIC_INITIALIZER;
+
+int result = 0;
+
+void on_acquire(volatile void* address, uint32_t value,
+              ATOMICS_WAIT_RESULT_T waitResult, void* userData) {
+  printf("on_acquire: releasing lock.\n");
+  emscripten_lock_release(&lock);
+  printf("on_acquire: released lock.\n");
+#ifdef REPORT_RESULT
+  REPORT_RESULT(result);
+#endif
+}
+
+int main() {
+  printf("main: async acquiring lock.\n");
+  emscripten_lock_async_acquire(&lock, on_acquire, 0, 100);
+  printf("main: busy-spin acquiring lock.\n");
+  emscripten_lock_busyspin_waitinf_acquire(&lock);
+  printf("main: lock acquired.\n");
+  emscripten_lock_release(&lock);
+  printf("main: lock released.\n");
+  result += 1;
+  emscripten_exit_with_live_runtime();
+}