Improve emscripten_futex_wait stub. NFC

The test for wasm workers with `emscripten_futex_wait` was added in back
in #21618 but AFAICT it was only even working when pthread support was
also enabled.

However because the stub was simply returning 0 in all cases it was
enough to make it seems as if the API was working when it wasn't.

Indeed the `test/webaudio/audioworklet_worker.c` test seems to have
been written under the assumption that this API was available for use
in wasm workers.

It seems better to return ENOTSUP unless the API is actually available.

Author: sbc100

system/lib/pthread/library_pthread_stub.c

@@ -21,10 +21,16 @@ bool emscripten_has_threading_support() { return false; }
 
 int emscripten_num_logical_cores() { return 1; }
 
-int emscripten_futex_wait(
-  volatile void /*uint32_t*/* addr, uint32_t val, double maxWaitMilliseconds) {
-  // nop
-  return 0; // success
+int emscripten_futex_wait(volatile void /*uint32_t*/* addr,
+                          uint32_t val,
+                          double maxWaitMilliseconds) {
+  if (!addr) {
+    return -EINVAL;
+  }
+  if (*(uint32_t*)addr != val) {
+    return -EWOULDBLOCK;
+  }
+  return -ENOTSUP;
 }
 
 int emscripten_futex_wake(volatile void /*uint32_t*/* addr, int count) {

test/codesize/test_codesize_hello_dylink_all.json

@@ -1,7 +1,7 @@
 {
   "a.out.js": 244691,
-  "a.out.nodebug.wasm": 577678,
-  "total": 822369,
+  "a.out.nodebug.wasm": 577699,
+  "total": 822390,
   "sent": [
     "IMG_Init",
     "IMG_Load",

test/test_browser.py

@@ -5039,9 +5039,11 @@ def test_wasm_worker_hello_wasm2js(self):
   def test_wasm_worker_hello_embedded(self):
     self.btest_exit('wasm_worker/hello_wasm_worker.c', cflags=['-sWASM_WORKERS=2'])
 
-  # Tests that it is possible to call emscripten_futex_wait() in Wasm Workers.
+  # Tests that it is possible to call emscripten_futex_wait() in Wasm Workers when pthreads
+  # are also enabled.
   @parameterized({
-    '': ([],),
+    # Without pthreads we expect the stub version of the futex API
+    '': (['-DEXPECT_STUB'],),
     'pthread': (['-pthread'],),
   })
   def test_wasm_worker_futex_wait(self, args):

test/wasm_worker/wasm_worker_futex_wait.c

@@ -1,5 +1,7 @@
 // Test that emscripten_futex_wait() works in a Wasm Worker.
 
+#include <assert.h>
+#include <errno.h>
 #include <emscripten.h>
 #include <emscripten/console.h>
 #include <emscripten/threading.h>
@@ -12,21 +14,37 @@ _Atomic uint32_t futex_value = 0;
 
 void wake_worker_after_delay(void *user_data) {
   futex_value = 1;
-  emscripten_futex_wake(&futex_value, INT_MAX);  
+  emscripten_futex_wake(&futex_value, INT_MAX);
 }
 
 void wake_worker() {
-  printf("Waking worker thread from futex wait.\n");
+  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");
+  printf("Worker sleeping on futex...\n");
+  double start = emscripten_performance_now();
+  int rc = emscripten_futex_wait(&futex_value, 0, 100);
+  double end = emscripten_performance_now();
+  printf("emscripten_futex_wait returned: %d after %.2fms\n", rc, end - start);
+#if EXPECT_STUB
+  // The stub implemenation returns -ENOTSUP
+  assert(rc == -ENOTSUP);
+#else
+  assert(rc == -ETIMEDOUT);
+  assert((end - start) >= 100);
+
+  printf("Worker sleeping on futex with wakeup...\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);
+  rc = emscripten_futex_wait(&futex_value, 0, INFINITY);
+  printf("emscripten_futex_wait returned: %d\n", rc);
+  assert(rc == 0);
+  assert(futex_value == 1);
+#endif
+
 #ifdef REPORT_RESULT
-  REPORT_RESULT(rc);
+  REPORT_RESULT(0);
 #endif
 }
 

test/webaudio/audioworklet_worker.c

@@ -2,14 +2,14 @@
 #include <emscripten/wasm_worker.h>
 #include <emscripten/threading.h>
 #include <assert.h>
+#include <stdbool.h>
 
 // Tests that
 // - audioworklets and workers can be used at the same time.
 // - an audioworklet can emscripten_futex_wake() a waiting worker.
 // - global values can be shared between audioworklets and workers.
 
-int workletToWorkerFutexLocation = 0;
-int workletToWorkerFlag = 0;
+_Atomic bool workletToWorkerFlag = false;
 EMSCRIPTEN_WEBAUDIO_T context;
 
 void do_exit() {
@@ -19,8 +19,10 @@ void do_exit() {
 }
 
 void run_in_worker() {
-  while (0 == emscripten_futex_wait(&workletToWorkerFutexLocation, 0, 30000)) {
-    if (workletToWorkerFlag == 1) {
+  // TODO: Convert to emscripten_futex_wait once it becomes available in Wasm
+  // Workers.
+  while (1) {
+    if (workletToWorkerFlag == true) {
       emscripten_out("Test success");
       emscripten_wasm_worker_post_function_v(EMSCRIPTEN_WASM_WORKER_ID_PARENT, &do_exit);
       break;
@@ -31,8 +33,7 @@ void run_in_worker() {
 // This event will fire on the audio worklet thread.
 void MessageReceivedInAudioWorkletThread() {
   assert(emscripten_current_thread_is_audio_worklet());
-  workletToWorkerFlag = 1;
-  emscripten_futex_wake(&workletToWorkerFutexLocation, 1);
+  workletToWorkerFlag = true;
 }
 
 void WebAudioWorkletThreadInitialized(EMSCRIPTEN_WEBAUDIO_T audioContext, bool success, void *userData) {