rootston: handle backend creation failed

backend/multi/backend.c

@@ -42,7 +42,6 @@ static void subbackend_state_destroy(struct subbackend_state *sub) {
 
 static void multi_backend_destroy(struct wlr_backend *wlr_backend) {
 	struct wlr_multi_backend *backend = (struct wlr_multi_backend *)wlr_backend;
-	wl_list_remove(&backend->display_destroy.link);
 	struct subbackend_state *sub, *next;
 	wl_list_for_each_safe(sub, next, &backend->backends, link) {
 		// XXX do we really want to take ownership over added backends?
@@ -190,3 +189,9 @@ struct wlr_session *wlr_multi_get_session(struct wlr_backend *_backend) {
 	}
 	return NULL;
 }
+
+bool wlr_multi_is_empty(struct wlr_backend *_backend) {
+	assert(wlr_backend_is_multi(_backend));
+	struct wlr_multi_backend *backend = (struct wlr_multi_backend *)_backend;
+	return wl_list_length(&backend->backends) < 1;
+}

include/backend/multi.h

@@ -10,8 +10,6 @@ struct wlr_multi_backend {
 	struct wlr_backend backend;
 
 	struct wl_list backends;
-
-	struct wl_listener display_destroy;
 };
 
 #endif

include/wlr/backend/multi.h

@@ -16,4 +16,6 @@ bool wlr_backend_is_multi(struct wlr_backend *backend);
 
 struct wlr_session *wlr_multi_get_session(struct wlr_backend *base);
 
+bool wlr_multi_is_empty(struct wlr_backend *backend);
+
 #endif

rootston/main.c

@@ -5,6 +5,7 @@
 #include <wayland-server.h>
 #include <wlr/backend.h>
 #include <wlr/backend/headless.h>
+#include <wlr/backend/multi.h>
 #include <wlr/render.h>
 #include <wlr/render/gles2.h>
 #include <wlr/util/log.h>
@@ -32,6 +33,12 @@ int main(int argc, char **argv) {
 
 	assert(server.backend = wlr_backend_autocreate(server.wl_display));
 
+	if (wlr_multi_is_empty(server.backend)) {
+		wlr_log(L_ERROR, "could not start backend");
+		wlr_backend_destroy(server.backend);
+		return 1;
+	}
+
 	assert(server.renderer = wlr_gles2_renderer_create(server.backend));
 	server.data_device_manager =
 		wlr_data_device_manager_create(server.wl_display);