backend/wayland: use a separate event queue for busy loops

This avoids processing events which we're not interested in.
Specifically, this fixes a case where output_commit() could be
indirectly called from itself either from import_dmabuf() or while
waiting for a configure event when enabling the output.

backend/wayland/backend.c

@@ -555,6 +555,7 @@ static void backend_destroy(struct wlr_backend *backend) {
 	wl_compositor_destroy(wl->compositor);
 	wl_registry_destroy(wl->registry);
 	wl_display_flush(wl->remote_display);
+	wl_event_queue_destroy(wl->busy_loop_queue);
 	if (wl->own_remote_display) {
 		wl_display_disconnect(wl->remote_display);
 	}
@@ -610,10 +611,16 @@ struct wlr_backend *wlr_wl_backend_create(struct wl_event_loop *loop,
 		wl->own_remote_display = true;
 	}
 
+	wl->busy_loop_queue = wl_display_create_queue(wl->remote_display);
+	if (wl->busy_loop_queue == NULL) {
+		wlr_log_errno(WLR_ERROR, "Could not create a Wayland event queue");
+		goto error_display;
+	}
+
 	wl->registry = wl_display_get_registry(wl->remote_display);
 	if (!wl->registry) {
 		wlr_log_errno(WLR_ERROR, "Could not obtain reference to remote registry");
-		goto error_display;
+		goto error_queue;
 	}
 	wl_registry_add_listener(wl->registry, &registry_listener, wl);
 
@@ -715,6 +722,8 @@ error_registry:
 		xdg_wm_base_destroy(wl->xdg_wm_base);
 	}
 	wl_registry_destroy(wl->registry);
+error_queue:
+	wl_event_queue_destroy(wl->busy_loop_queue);
 error_display:
 	if (wl->own_remote_display) {
 		wl_display_disconnect(wl->remote_display);

backend/wayland/output.c

@@ -243,15 +243,24 @@ static struct wl_buffer *import_dmabuf(struct wlr_wl_backend *wl,
 	zwp_linux_buffer_params_v1_add_listener(params, &dmabuf_listener, &data);
 	zwp_linux_buffer_params_v1_create(params, dmabuf->width, dmabuf->height, dmabuf->format, 0);
 
+	struct wl_event_queue *display_queue =
+		wl_proxy_get_queue((struct wl_proxy *)wl->remote_display);
+	wl_proxy_set_queue((struct wl_proxy *)params, wl->busy_loop_queue);
+
 	while (!data.done) {
-		if (wl_display_dispatch(wl->remote_display) < 0) {
+		if (wl_display_dispatch_queue(wl->remote_display, wl->busy_loop_queue) < 0) {
-			wlr_log(WLR_ERROR, "wl_display_dispatch() failed");
+			wlr_log(WLR_ERROR, "wl_display_dispatch_queue() failed");
 			break;
 		}
 	}
 
+	struct wl_buffer *buffer = data.wl_buffer;
+	if (buffer != NULL) {
+		wl_proxy_set_queue((struct wl_proxy *)buffer, display_queue);
+	}
+
 	zwp_linux_buffer_params_v1_destroy(params);
-	return data.wl_buffer;
+	return buffer;
 }
 
 static struct wl_buffer *import_shm(struct wlr_wl_backend *wl,
@@ -747,13 +756,25 @@ static bool output_commit(struct wlr_output *wlr_output, const struct wlr_output
 		wl_surface_commit(output->surface);
 		output->initialized = true;
 
+		struct wl_event_queue *display_queue =
+			wl_proxy_get_queue((struct wl_proxy *)wl->remote_display);
+		wl_proxy_set_queue((struct wl_proxy *)output->xdg_surface, wl->busy_loop_queue);
+		wl_proxy_set_queue((struct wl_proxy *)output->xdg_toplevel, wl->busy_loop_queue);
+
 		wl_display_flush(wl->remote_display);
 		while (!output->configured) {
-			if (wl_display_dispatch(wl->remote_display) == -1) {
+			if (wl_display_dispatch_queue(wl->remote_display, wl->busy_loop_queue) == -1) {
-				wlr_log(WLR_ERROR, "wl_display_dispatch() failed");
+				wlr_log(WLR_ERROR, "wl_display_dispatch_queue() failed");
-				return false;
+				break;
 			}
 		}
+
+		wl_proxy_set_queue((struct wl_proxy *)output->xdg_surface, display_queue);
+		wl_proxy_set_queue((struct wl_proxy *)output->xdg_toplevel, display_queue);
+
+		if (!output->configured) {
+			return false;
+		}
 	}
 
 	struct wlr_wl_buffer *buffer = NULL;

include/backend/wayland.h

@@ -22,6 +22,7 @@ struct wlr_wl_backend {
 	/* local state */
 	bool started;
 	struct wl_event_loop *event_loop;
+	struct wl_event_queue *busy_loop_queue;
 	struct wl_list outputs;
 	int drm_fd;
 	struct wl_list buffers; // wlr_wl_buffer.link