backend: destroy multi backend when primary backend is

This makes it easy for compositors to handle situations where the
DRM or libinput backend becomes unavailable. Compositors can listen
the destroy event of the multi backend returned by
wlr_backend_autocreate() and decide what to do.

backend/backend.c

@@ -146,6 +146,53 @@ static size_t parse_outputs_env(const char *name) {
 	return outputs;
 }
 
+/**
+ * Helper to destroy the multi backend when one of its nested backends is
+ * destroyed.
+ */
+struct wlr_auto_backend_monitor {
+	struct wlr_backend *multi;
+	struct wlr_backend *primary;
+
+	struct wl_listener multi_destroy;
+	struct wl_listener primary_destroy;
+};
+
+static void auto_backend_monitor_destroy(struct wlr_auto_backend_monitor *monitor) {
+	wl_list_remove(&monitor->multi_destroy.link);
+	wl_list_remove(&monitor->primary_destroy.link);
+	free(monitor);
+}
+
+static void monitor_handle_multi_destroy(struct wl_listener *listener, void *data) {
+	struct wlr_auto_backend_monitor *monitor = wl_container_of(listener, monitor, multi_destroy);
+	auto_backend_monitor_destroy(monitor);
+}
+
+static void monitor_handle_primary_destroy(struct wl_listener *listener, void *data) {
+	struct wlr_auto_backend_monitor *monitor = wl_container_of(listener, monitor, primary_destroy);
+	wlr_backend_destroy(monitor->multi);
+}
+
+static struct wlr_auto_backend_monitor *auto_backend_monitor_create(
+		struct wlr_backend *multi, struct wlr_backend *primary) {
+	struct wlr_auto_backend_monitor *monitor = calloc(1, sizeof(*monitor));
+	if (monitor == NULL) {
+		return NULL;
+	}
+
+	monitor->multi = multi;
+	monitor->primary = primary;
+
+	monitor->multi_destroy.notify = monitor_handle_multi_destroy;
+	wl_signal_add(&multi->events.destroy, &monitor->multi_destroy);
+
+	monitor->primary_destroy.notify = monitor_handle_primary_destroy;
+	wl_signal_add(&primary->events.destroy, &monitor->primary_destroy);
+
+	return monitor;
+}
+
 static struct wlr_backend *attempt_wl_backend(struct wl_display *display) {
 	struct wlr_backend *backend = wlr_wl_backend_create(display, NULL);
 	if (backend == NULL) {
@@ -335,8 +382,12 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
 		if (!wl_backend) {
 			goto error;
 		}
-
 		wlr_multi_backend_add(multi, wl_backend);
+
+		if (!auto_backend_monitor_create(multi, wl_backend)) {
+			goto error;
+		}
+
 		goto success;
 	}
 
@@ -347,8 +398,12 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
 		if (!x11_backend) {
 			goto error;
 		}
-
 		wlr_multi_backend_add(multi, x11_backend);
+
+		if (!auto_backend_monitor_create(multi, x11_backend)) {
+			goto error;
+		}
+
 		goto success;
 	}
 
@@ -362,6 +417,9 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
 	struct wlr_backend *libinput = attempt_libinput_backend(display, session);
 	if (libinput) {
 		wlr_multi_backend_add(multi, libinput);
+		if (!auto_backend_monitor_create(multi, libinput)) {
+			goto error;
+		}
 	} else if (env_parse_bool("WLR_LIBINPUT_NO_DEVICES")) {
 		wlr_log(WLR_INFO, "WLR_LIBINPUT_NO_DEVICES is set, "
 			"starting without libinput backend");
@@ -377,6 +435,10 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
 		goto error;
 	}
 
+	if (!auto_backend_monitor_create(multi, primary_drm)) {
+		goto error;
+	}
+
 success:
 	if (session_ptr != NULL) {
 		*session_ptr = session;

include/wlr/backend.h

@@ -37,6 +37,9 @@ struct wlr_backend {
  *
  * If session_ptr is not NULL, it's populated with the session which has been
  * created with the backend, if any.
+ *
+ * The multi-backend will be destroyed if one of the primary underlying
+ * backends is destroyed (e.g. if the primary DRM device is unplugged).
  */
 struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
 	struct wlr_session **session_ptr);