thread: Add support for posting async task

This uses the queue that was introduced when migrating impl task
management from MetaThread to MetaThreadImpl, with the exception that
it's now fully used as an actual queue. It now has a GSource that sits
on the right GMainContext that is dispatched whenever there are tasks to
execute.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2777>

src/tests/native-thread.c

@@ -193,6 +193,48 @@ add_idle_func (MetaThreadImpl  *thread_impl,
   return GINT_TO_POINTER (TRUE);
 }
 
+typedef struct
+{
+  MetaThread *thread;
+  GMainLoop *loop;
+
+  GMutex mutex;
+  int state;
+} AsyncData;
+
+static gpointer
+async_func (MetaThreadImpl  *thread_impl,
+            gpointer         user_data,
+            GError         **error)
+{
+  AsyncData *async_data = user_data;
+
+  meta_assert_in_thread_impl (async_data->thread);
+
+  g_mutex_lock (&async_data->mutex);
+  g_assert_cmpint (async_data->state, ==, 0);
+  async_data->state = 1;
+  g_mutex_unlock (&async_data->mutex);
+
+  return GINT_TO_POINTER (TRUE);
+}
+
+static void
+async_feedback_func (gpointer      retval,
+                     const GError *error,
+                     gpointer      user_data)
+{
+  AsyncData *async_data = user_data;
+
+  meta_assert_not_in_thread_impl (async_data->thread);
+
+  g_mutex_lock (&async_data->mutex);
+  g_assert_cmpint (async_data->state, ==, 1);
+  async_data->state = 2;
+  g_main_loop_quit (async_data->loop);
+  g_mutex_unlock (&async_data->mutex);
+}
+
 static void
 run_thread_tests (MetaThread *thread)
 {
@@ -204,6 +246,7 @@ run_thread_tests (MetaThread *thread)
   int buf;
   PipeData pipe_data;
   IdleData idle_data;
+  AsyncData async_data;
 
   meta_assert_not_in_thread_impl (thread);
 
@@ -254,6 +297,24 @@ run_thread_tests (MetaThread *thread)
   g_main_loop_run (idle_data.loop);
   g_assert_cmpint (idle_data.state, ==, 3);
   g_main_loop_unref (idle_data.loop);
+
+  /* Test async tasks */
+  g_debug ("Test async task");
+  async_data = (AsyncData) { 0 };
+  g_mutex_init (&async_data.mutex);
+  async_data.thread = thread;
+  async_data.loop = g_main_loop_new (NULL, FALSE);
+  g_mutex_lock (&async_data.mutex);
+  meta_thread_post_impl_task (thread, async_func, &async_data,
+                              async_feedback_func, &async_data);
+  g_assert_cmpint (async_data.state, ==, 0);
+  g_mutex_unlock (&async_data.mutex);
+  g_main_loop_run (async_data.loop);
+  g_mutex_lock (&async_data.mutex);
+  g_assert_cmpint (async_data.state, ==, 2);
+  g_mutex_unlock (&async_data.mutex);
+  g_main_loop_unref (async_data.loop);
+  g_mutex_clear (&async_data.mutex);
 }
 
 static void