Fix a data race between the feeder thread and attaching of the stream…

… to the mixer.

This is done by by ensuring the stream is pre-filled before attaching is
possible.

Possibly related to #1561

addons/acodec/helper.c

@@ -11,19 +11,23 @@ void _al_acodec_start_feed_thread(ALLEGRO_AUDIO_STREAM *stream)
    stream->feed_thread_started_cond = al_create_cond();
    stream->feed_thread_started_mutex = al_create_mutex();
    al_start_thread(stream->feed_thread);
-}
-
-void _al_acodec_stop_feed_thread(ALLEGRO_AUDIO_STREAM *stream)
-{
-   ALLEGRO_EVENT quit_event;
 
    /* Need to wait for the thread to start, otherwise the quit event may be
-    * sent before the event source is registered with the queue. */
+    * sent before the event source is registered with the queue.
+    *
+    * This also makes the pre-fill system thread safe, as it needs to operate
+    * before the mutexes are set up.
+    */
    al_lock_mutex(stream->feed_thread_started_mutex);
    while (!stream->feed_thread_started) {
       al_wait_cond(stream->feed_thread_started_cond, stream->feed_thread_started_mutex);
    }
    al_unlock_mutex(stream->feed_thread_started_mutex);
+}
+
+void _al_acodec_stop_feed_thread(ALLEGRO_AUDIO_STREAM *stream)
+{
+   ALLEGRO_EVENT quit_event;
 
    quit_event.type = _KCM_STREAM_FEEDER_QUIT_EVENT_TYPE;
    al_emit_user_event(al_get_audio_stream_event_source(stream), &quit_event, NULL);

addons/audio/kcm_stream.c

@@ -692,11 +692,6 @@ void *_al_kcm_feed_stream(ALLEGRO_THREAD *self, void *vstream)
    queue = al_create_event_queue();
    al_register_event_source(queue, &stream->spl.es);
 
-   al_lock_mutex(stream->feed_thread_started_mutex);
-   stream->feed_thread_started = true;
-   al_broadcast_cond(stream->feed_thread_started_cond);
-   al_unlock_mutex(stream->feed_thread_started_mutex);
-
    stream->quit_feed_thread = false;
 
    while (!stream->quit_feed_thread) {
@@ -708,7 +703,6 @@ void *_al_kcm_feed_stream(ALLEGRO_THREAD *self, void *vstream)
 
       if ((prefill || event.type == ALLEGRO_EVENT_AUDIO_STREAM_FRAGMENT)
           && !stream->is_draining) {
-         prefill = false;
          unsigned long bytes;
          unsigned long bytes_written;
          ALLEGRO_MUTEX *stream_mutex;
@@ -784,6 +778,13 @@ void *_al_kcm_feed_stream(ALLEGRO_THREAD *self, void *vstream)
          fin_event.user.timestamp = al_get_time();
          al_emit_user_event(&stream->spl.es, &fin_event, NULL);
       }
+      if (prefill) {
+         al_lock_mutex(stream->feed_thread_started_mutex);
+         stream->feed_thread_started = true;
+         al_broadcast_cond(stream->feed_thread_started_cond);
+         al_unlock_mutex(stream->feed_thread_started_mutex);
+      }
+      prefill = false;
    }
 
    al_destroy_event_queue(queue);