render/drm_syncobj: Add a callback when ready

The old approach of using a signal is fundamentally broken for a common
usecase: When the waiter is ready, it's common to immediately finish and
free any resources associated with it.
Because of the semantics of wl_signal_emit_mutable() this is UB.
wl_signal_emit_mutable() always excepts that the waiter hasn't been freed
until the signal has finished being emitted.

Instead of over engineering the solution, let's just add a callback required
by wlr_drm_syncobj_timeline_waiter_init(). In this callback, the implementation
is free to finish() or free() any resource it likes.

backend/wayland/output.c

@@ -135,7 +135,6 @@ static const struct wp_presentation_feedback_listener
 };
 
 static void buffer_remove_drm_syncobj_waiter(struct wlr_wl_buffer *buffer) {
-	wl_list_remove(&buffer->drm_syncobj_ready.link);
 	wlr_drm_syncobj_timeline_waiter_finish(&buffer->drm_syncobj_waiter);
 	buffer->has_drm_syncobj_waiter = false;
 }
@@ -177,8 +176,8 @@ static const struct wl_buffer_listener buffer_listener = {
 	.release = buffer_handle_release,
 };
 
-static void buffer_handle_drm_syncobj_ready(struct wl_listener *listener, void *data) {
+static void buffer_handle_drm_syncobj_ready(struct wlr_drm_syncobj_timeline_waiter *waiter) {
-	struct wlr_wl_buffer *buffer = wl_container_of(listener, buffer, drm_syncobj_ready);
+	struct wlr_wl_buffer *buffer = wl_container_of(waiter, buffer, drm_syncobj_waiter);
 	buffer_remove_drm_syncobj_waiter(buffer);
 	buffer_release(buffer);
 }
@@ -819,14 +818,11 @@ static bool output_commit(struct wlr_output *wlr_output,
 			signal_timeline->wl, signal_point_hi, signal_point_lo);
 
 		if (!wlr_drm_syncobj_timeline_waiter_init(&buffer->drm_syncobj_waiter,
-				signal_timeline->base, signal_point, 0, output->backend->event_loop)) {
+				signal_timeline->base, signal_point, 0, output->backend->event_loop,
+				buffer_handle_drm_syncobj_ready)) {
 			return false;
 		}
 		buffer->has_drm_syncobj_waiter = true;
-
-		buffer->drm_syncobj_ready.notify = buffer_handle_drm_syncobj_ready;
-		wl_signal_add(&buffer->drm_syncobj_waiter.events.ready,
-			&buffer->drm_syncobj_ready);
 	} else {
 		if (output->drm_syncobj_surface_v1 != NULL) {
 			wp_linux_drm_syncobj_surface_v1_destroy(output->drm_syncobj_surface_v1);

include/backend/wayland.h

@@ -64,7 +64,6 @@ struct wlr_wl_buffer {
 
 	bool has_drm_syncobj_waiter;
 	struct wlr_drm_syncobj_timeline_waiter drm_syncobj_waiter;
-	struct wl_listener drm_syncobj_ready;
 
 	struct wlr_drm_syncobj_timeline *fallback_signal_timeline;
 	uint64_t fallback_signal_point;

include/wlr/render/drm_syncobj.h

@@ -37,6 +37,11 @@ struct wlr_drm_syncobj_timeline {
 	} WLR_PRIVATE;
 };
 
+struct wlr_drm_syncobj_timeline_waiter;
+
+typedef void (*wlr_drm_syncobj_timeline_ready_callback)(
+	struct wlr_drm_syncobj_timeline_waiter *waiter);
+
 struct wlr_drm_syncobj_timeline_waiter {
 	struct {
 		struct wl_signal ready;
@@ -45,6 +50,7 @@ struct wlr_drm_syncobj_timeline_waiter {
 	struct {
 		int ev_fd;
 		struct wl_event_source *event_source;
+		wlr_drm_syncobj_timeline_ready_callback callback;
 	} WLR_PRIVATE;
 };
 
@@ -92,10 +98,12 @@ bool wlr_drm_syncobj_timeline_check(struct wlr_drm_syncobj_timeline *timeline,
  * Asynchronously wait for a timeline point.
  *
  * See wlr_drm_syncobj_timeline_check() for a definition of flags.
+ *
+ * A callback must be provided that will be invoked when the waiter has finished.
  */
 bool wlr_drm_syncobj_timeline_waiter_init(struct wlr_drm_syncobj_timeline_waiter *waiter,
 	struct wlr_drm_syncobj_timeline *timeline, uint64_t point, uint32_t flags,
-	struct wl_event_loop *loop);
+	struct wl_event_loop *loop, wlr_drm_syncobj_timeline_ready_callback callback);
 /**
  * Cancel a timeline waiter.
  */

render/drm_syncobj.c

@@ -193,12 +193,15 @@ static int handle_eventfd_ready(int ev_fd, uint32_t mask, void *data) {
 	}
 
 	wl_signal_emit_mutable(&waiter->events.ready, NULL);
+	waiter->callback(waiter);
 	return 0;
 }
 
 bool wlr_drm_syncobj_timeline_waiter_init(struct wlr_drm_syncobj_timeline_waiter *waiter,
 		struct wlr_drm_syncobj_timeline *timeline, uint64_t point, uint32_t flags,
-		struct wl_event_loop *loop) {
+		struct wl_event_loop *loop, wlr_drm_syncobj_timeline_ready_callback callback) {
+	assert(callback);
+
 	int ev_fd;
 #if HAVE_EVENTFD
 	ev_fd = eventfd(0, EFD_CLOEXEC);
@@ -235,6 +238,7 @@ bool wlr_drm_syncobj_timeline_waiter_init(struct wlr_drm_syncobj_timeline_waiter
 	*waiter = (struct wlr_drm_syncobj_timeline_waiter){
 		.ev_fd = ev_fd,
 		.event_source = source,
+		.callback = callback,
 	};
 	wl_signal_init(&waiter->events.ready);
 	return true;

types/wlr_linux_drm_syncobj_v1.c

@@ -29,7 +29,6 @@ struct wlr_linux_drm_syncobj_surface_v1_commit {
 	struct wlr_drm_syncobj_timeline_waiter waiter;
 	uint32_t cached_seq;
 
-	struct wl_listener waiter_ready;
 	struct wl_listener surface_destroy;
 };
 
@@ -194,14 +193,13 @@ static struct wlr_linux_drm_syncobj_surface_v1 *surface_from_wlr_surface(
 static void surface_commit_destroy(struct wlr_linux_drm_syncobj_surface_v1_commit *commit) {
 	wlr_surface_unlock_cached(commit->surface->surface, commit->cached_seq);
 	wl_list_remove(&commit->surface_destroy.link);
-	wl_list_remove(&commit->waiter_ready.link);
 	wlr_drm_syncobj_timeline_waiter_finish(&commit->waiter);
 	free(commit);
 }
 
-static void surface_commit_handle_waiter_ready(struct wl_listener *listener, void *data) {
+static void surface_commit_handle_waiter_ready(struct wlr_drm_syncobj_timeline_waiter *waiter) {
 	struct wlr_linux_drm_syncobj_surface_v1_commit *commit =
-		wl_container_of(listener, commit, waiter_ready);
+		wl_container_of(waiter, commit, waiter);
 	surface_commit_destroy(commit);
 }
 
@@ -233,7 +231,7 @@ static bool lock_surface_commit(struct wlr_linux_drm_syncobj_surface_v1 *surface
 	struct wl_display *display = wl_client_get_display(client);
 	struct wl_event_loop *loop = wl_display_get_event_loop(display);
 	if (!wlr_drm_syncobj_timeline_waiter_init(&commit->waiter, timeline, point,
-			flags, loop)) {
+			flags, loop, surface_commit_handle_waiter_ready)) {
 		free(commit);
 		return false;
 	}
@@ -241,9 +239,6 @@ static bool lock_surface_commit(struct wlr_linux_drm_syncobj_surface_v1 *surface
 	commit->surface = surface;
 	commit->cached_seq = wlr_surface_lock_pending(surface->surface);
 
-	commit->waiter_ready.notify = surface_commit_handle_waiter_ready;
-	wl_signal_add(&commit->waiter.events.ready, &commit->waiter_ready);
-
 	commit->surface_destroy.notify = surface_commit_handle_surface_destroy;
 	wl_signal_add(&surface->surface->events.destroy, &commit->surface_destroy);