Add emscripten_queue_microtask

This change adds a new `emscripten_queue_microtask` API to match the
long standing web API (also available in node).

See https://developer.mozilla.org/en-US/docs/Web/API/Window/queueMicrotask

For now, I'm adding this without a polyfill so usage of the API will
crash on very old targets.  We could potentially polyfill using
something like setTimeout, but the semantics are a little different so
I'm not sure it worth doing.

Split out from emscripten-core#24481

Author: sbc100

src/lib/libeventloop.js

@@ -85,6 +85,15 @@ LibraryJSEventLoop = {
   $emClearImmediate_deps: ['$emSetImmediate'],
   $emClearImmediate: undefined,
 
+  emscripten_queue_microtask__deps: ['$emSetImmediate', '$callUserCallback'],
+  emscripten_queue_microtask: (cb, userData) => {
+    {{{ runtimeKeepalivePush(); }}}
+    return queueMicrotask(() => {
+      {{{ runtimeKeepalivePop(); }}}
+      callUserCallback(() => {{{ makeDynCall('vp', 'cb') }}}(userData));
+    });
+  },
+
   emscripten_set_immediate__deps: ['$emSetImmediate', '$callUserCallback'],
   emscripten_set_immediate: (cb, userData) => {
     {{{ runtimeKeepalivePush(); }}}

src/lib/libsigs.js

@@ -733,6 +733,7 @@ sigs = {
   emscripten_promise_race__sig: 'ppp',
   emscripten_promise_resolve__sig: 'vpip',
   emscripten_promise_then__sig: 'ppppp',
+  emscripten_queue_microtask__sig: 'ipp',
   emscripten_random__sig: 'f',
   emscripten_request_animation_frame__sig: 'ipp',
   emscripten_request_animation_frame_loop__sig: 'vpp',

system/include/emscripten/eventloop.h

@@ -26,6 +26,8 @@ void emscripten_set_immediate_loop(bool (*cb)(void *user_data), void *user_data)
 int emscripten_set_interval(void (*cb)(void *user_data) __attribute__((nonnull)), double interval_ms, void *user_data);
 void emscripten_clear_interval(int id);
 
+int emscripten_queue_microtask(void (*cb)(void *user_data) __attribute__((nonnull)), void *user_data);
+
 void emscripten_runtime_keepalive_push(void);
 void emscripten_runtime_keepalive_pop(void);
 bool emscripten_runtime_keepalive_check(void);

test/other/test_queue_microtask.c

@@ -0,0 +1,26 @@
+#include <assert.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <emscripten/eventloop.h>
+
+bool got_timeout = false;
+bool got_microtask = false;
+
+void callback_timeout(void* user_data) {
+  assert(got_microtask);
+  printf("callback_timeout: %ld\n", (intptr_t)user_data);
+  got_timeout = true;
+}
+
+void callback_microtask(void* user_data) {
+  assert(!got_timeout);
+  printf("callback_microtask: %ld\n", (intptr_t)user_data);
+  got_microtask = true;
+}
+
+int main() {
+  emscripten_set_timeout(callback_timeout, 0, (void*)42);
+  emscripten_queue_microtask(callback_microtask, (void*)43);
+  printf("done main\n");
+  return 0;
+}

test/other/test_queue_microtask.out

@@ -0,0 +1,3 @@
+done main
+callback_microtask: 43
+callback_timeout: 42

test/test_other.py

@@ -16186,3 +16186,6 @@ def test_unsupported_min_version_when_unsupported_env(self, env):
     self.assertContainedIf(f'var MIN_CHROME_VERSION = {unsupported};', src, env == 'node')
     self.assertContainedIf(f'var MIN_SAFARI_VERSION = {unsupported};', src, env == 'node')
     self.assertContainedIf(f'var MIN_FIREFOX_VERSION = {unsupported};', src, env == 'node')
+
+  def test_queue_microtask(self):
+    self.do_other_test('test_queue_microtask.c')