Android: app freezes while waiting for al_acknowledge_drawing_halt()

[alemart]

Summary

An Android app may freeze while AllegroSurface.nativeOnDestroy() waits for a call to al_acknowledge_drawing_halt().

How to reproduce the issue

Open ex_android and keep switching the app in and out of context very quickly. Eventually the app will freeze. The issue is reproducible with other examples too.

Details

As can be seen below, method AllegroSurface.nativeOnDestroy() in android_display.c emits an ALLEGRO_EVENT_DISPLAY_HALT_DRAWING just before waiting for a condition variable that is signaled in android_acknowledge_drawing_halt(). The drawing halt event is handled in a different thread. If the order of operations is such that the event is handled (and the condition variable is signaled) before the other thread is waiting, the app will freeze. The signal is lost and does nothing. The freeze can be reproduced easily by adding an al_rest() call before al_wait_cond().

allegro5/src/android/android_display.c

Lines 88 to 101 in 9ced31b

	if (_al_event_source_needs_to_generate_event(&d->es)) {
	ALLEGRO_EVENT event;
	event.display.type = ALLEGRO_EVENT_DISPLAY_HALT_DRAWING;
	event.display.timestamp = al_current_time();
	_al_event_source_emit_event(&d->es, &event);
	}
	ALLEGRO_DEBUG("unlocking display event source");
	_al_event_source_unlock(&d->es);
	// wait for acknowledge_drawing_halt
	al_lock_mutex(display->mutex);
	al_wait_cond(display->cond, display->mutex);
	al_unlock_mutex(display->mutex);

[dos1]

Sounds like a duplicate of #959