input/cursor: handle image surface destroy

This adds a listener for the destroy event of the cursor image surface.
This prevents a use-after-free when the last visible image surface is
freed, there has not been a new cursor set, and the cursor is reshown.

include/sway/input/cursor.h

@@ -58,6 +58,7 @@ struct sway_cursor {
 	uint32_t tool_buttons;
 
 	struct wl_listener request_set_cursor;
+	struct wl_listener image_surface_destroy;
 
 	struct wl_listener constraint_commit;
 

sway/input/cursor.c

@@ -789,6 +789,15 @@ static void handle_pointer_swipe_end(struct wl_listener *listener, void *data) {
 			cursor->pointer_gestures, cursor->seat->wlr_seat,
 			event->time_msec, event->cancelled);
 }
+
+static void handle_image_surface_destroy(struct wl_listener *listener,
+		void *data) {
+	struct sway_cursor *cursor =
+		wl_container_of(listener, cursor, image_surface_destroy);
+	cursor_set_image(cursor, NULL, cursor->image_client);
+	cursor_rebase(cursor);
+}
+
 void cursor_set_image(struct sway_cursor *cursor, const char *image,
 		struct wl_client *client) {
 	if (!(cursor->seat->wlr_seat->capabilities & WL_SEAT_CAPABILITY_POINTER)) {
@@ -797,7 +806,9 @@ void cursor_set_image(struct sway_cursor *cursor, const char *image,
 
 	const char *current_image = cursor->image;
 	cursor->image = image;
+	wl_list_remove(&cursor->image_surface_destroy.link);
 	cursor->image_surface = NULL;
+	wl_list_init(&cursor->image_surface_destroy.link);
 	cursor->hotspot_x = cursor->hotspot_y = 0;
 	cursor->image_client = client;
 
@@ -821,7 +832,10 @@ void cursor_set_image_surface(struct sway_cursor *cursor,
 	}
 
 	cursor->image = NULL;
+	wl_list_remove(&cursor->image_surface_destroy.link);
 	cursor->image_surface = surface;
+	wl_signal_add(&cursor->image_surface->events.destroy,
+			&cursor->image_surface_destroy);
 	cursor->hotspot_x = hotspot_x;
 	cursor->hotspot_y = hotspot_y;
 	cursor->image_client = client;
@@ -840,6 +854,7 @@ void sway_cursor_destroy(struct sway_cursor *cursor) {
 
 	wl_event_source_remove(cursor->hide_source);
 
+	wl_list_remove(&cursor->image_surface_destroy.link);
 	wl_list_remove(&cursor->pinch_begin.link);
 	wl_list_remove(&cursor->pinch_update.link);
 	wl_list_remove(&cursor->pinch_end.link);
@@ -885,6 +900,9 @@ struct sway_cursor *sway_cursor_create(struct sway_seat *seat) {
 	cursor->hide_source = wl_event_loop_add_timer(server.wl_event_loop,
 			hide_notify, cursor);
 
+	wl_list_init(&cursor->image_surface_destroy.link);
+	cursor->image_surface_destroy.notify = handle_image_surface_destroy;
+
 	cursor->pointer_gestures = wlr_pointer_gestures_v1_create(server.wl_display);
 	cursor->pinch_begin.notify = handle_pointer_pinch_begin;
 	wl_signal_add(&wlr_cursor->events.pinch_begin, &cursor->pinch_begin);