Gracefully handle inert wl_output resources

Closes: https://github.com/swaywm/wlroots/issues/2088

include/wlr/types/wlr_output.h

@@ -369,6 +369,11 @@ bool wlr_output_set_gamma(struct wlr_output *output, size_t size,
 	const uint16_t *r, const uint16_t *g, const uint16_t *b);
 bool wlr_output_export_dmabuf(struct wlr_output *output,
 	struct wlr_dmabuf_attributes *attribs);
+/**
+ * Returns the wlr_output matching the provided wl_output resource. If the
+ * resource isn't a wl_output, it aborts. If the resource is inert (because the
+ * wlr_output has been destroyed), NULL is returned.
+ */
 struct wlr_output *wlr_output_from_resource(struct wl_resource *resource);
 /**
  * Locks the output to only use rendering instead of direct scan-out. This is

types/wlr_gamma_control_v1.c

@@ -161,6 +161,13 @@ static void gamma_control_manager_get_gamma_control(struct wl_client *client,
 	wl_resource_set_implementation(gamma_control->resource, &gamma_control_impl,
 		gamma_control, gamma_control_handle_resource_destroy);
 
+	if (output == NULL) {
+		wl_resource_set_user_data(gamma_control->resource, NULL);
+		zwlr_gamma_control_v1_send_failed(gamma_control->resource);
+		free(gamma_control);
+		return;
+	}
+
 	wl_signal_add(&output->events.destroy,
 		&gamma_control->output_destroy_listener);
 	gamma_control->output_destroy_listener.notify =

types/wlr_screencopy_v1.c

@@ -379,13 +379,19 @@ static void capture_output(struct wl_client *wl_client,
 		wl_client_post_no_memory(wl_client);
 		return;
 	}
+	wl_resource_set_implementation(frame->resource, &frame_impl, frame,
+		frame_handle_resource_destroy);
+
+	if (output == NULL) {
+		wl_resource_set_user_data(frame->resource, NULL);
+		zwlr_screencopy_frame_v1_send_failed(frame->resource);
+		free(frame);
+		return;
+	}
 
 	frame->client = client;
 	client->ref++;
 
-	wl_resource_set_implementation(frame->resource, &frame_impl, frame,
-		frame_handle_resource_destroy);
-
 	wl_list_insert(&client->manager->frames, &frame->link);
 
 	wl_list_init(&frame->output_precommit.link);

types/wlr_xdg_output_v1.c

@@ -96,6 +96,20 @@ static void output_manager_handle_get_xdg_output(struct wl_client *client,
 	struct wlr_output_layout *layout = manager->layout;
 	struct wlr_output *output = wlr_output_from_resource(output_resource);
 
+	struct wl_resource *xdg_output_resource = wl_resource_create(client,
+			&zxdg_output_v1_interface, wl_resource_get_version(resource), id);
+	if (!xdg_output_resource) {
+		wl_client_post_no_memory(client);
+		return;
+	}
+	wl_resource_set_implementation(xdg_output_resource, &output_implementation,
+		NULL, output_handle_resource_destroy);
+
+	if (output == NULL) {
+		wl_list_init(wl_resource_get_link(xdg_output_resource));
+		return;
+	}
+
 	struct wlr_output_layout_output *layout_output =
 		wlr_output_layout_get(layout, output);
 	assert(layout_output);
@@ -109,14 +123,6 @@ static void output_manager_handle_get_xdg_output(struct wl_client *client,
 	}
 	assert(xdg_output);
 
-	struct wl_resource *xdg_output_resource = wl_resource_create(client,
-			&zxdg_output_v1_interface, wl_resource_get_version(resource), id);
-	if (!xdg_output_resource) {
-		wl_client_post_no_memory(client);
-		return;
-	}
-	wl_resource_set_implementation(xdg_output_resource, &output_implementation,
-		NULL, output_handle_resource_destroy);
 
 	wl_list_insert(&xdg_output->resources,
 		wl_resource_get_link(xdg_output_resource));