Only use futex_wait_busy on the main browser thread

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.

Author: sbc100

system/lib/pthread/emscripten_futex_wait.c

@@ -15,9 +15,9 @@
 
 extern void* _emscripten_main_thread_futex;
 
-int _emscripten_thread_supports_atomics_wait(void);
-
-static int futex_wait_busy(volatile void *addr, uint32_t val, double timeout) {
+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.
   double now = emscripten_get_now();
@@ -119,11 +119,10 @@ int emscripten_futex_wait(volatile void *addr, uint32_t val, double max_wait_ms)
   int ret;
   emscripten_conditional_set_current_thread_status(EM_THREAD_STATUS_RUNNING, EM_THREAD_STATUS_WAITFUTEX);
 
-  // For threads that cannot block (i.e. the main browser thread) we can't use
-  // __builtin_wasm_memory_atomic_wait32 so we call out the JS function that
-  // will busy wait.
-  if (!_emscripten_thread_supports_atomics_wait()) {
-    ret = futex_wait_busy(addr, val, max_wait_ms);
+  // For the main browser thread we can't use
+  // __builtin_wasm_memory_atomic_wait32 so we have busy wait instead.
+  if (emscripten_is_main_browser_thread()) {
+    ret = futex_wait_main_browser_thread(addr, val, max_wait_ms);
     emscripten_conditional_set_current_thread_status(EM_THREAD_STATUS_WAITFUTEX, EM_THREAD_STATUS_RUNNING);
     return ret;
   }

test/core/pthread/emscripten_futexes.c

@@ -2,20 +2,21 @@
 #include <emscripten/threading.h>
 #include <errno.h>
 #include <limits.h>
+#include <math.h>
 #include <stdio.h>
 
 int main() {
   // This should return -EWOULDBLOCK.
   int futex_value = 42;
-  int rc = emscripten_futex_wait(&futex_value, futex_value + 1, 0);
+  int rc = emscripten_futex_wait(&futex_value, futex_value + 1, INFINITY);
   assert(rc == -EWOULDBLOCK);
 
   // This should return -ETIMEDOUT.
   rc = emscripten_futex_wait(&futex_value, futex_value, 1);
   assert(rc == -ETIMEDOUT);
 
   // Check that this thread has removed itself from the wait queue.
-  rc = emscripten_futex_wake(&futex_value, INT_MAX);
+  rc = emscripten_futex_wake(&futex_value, INFINITY);
   assert(rc == 0);
 
   // Check that the wait address is checked for validity.