backend/multi: disallow multiple renderers at the same time

backend/multi/backend.c

@@ -130,20 +130,29 @@ static struct subbackend_state *multi_backend_get_subbackend(struct wlr_multi_ba
 	return NULL;
 }
 
-void wlr_multi_backend_add(struct wlr_backend *_multi,
+bool wlr_multi_backend_add(struct wlr_backend *_multi,
 		struct wlr_backend *backend) {
 	assert(wlr_backend_is_multi(_multi));
 	struct wlr_multi_backend *multi = (struct wlr_multi_backend *)_multi;
 
 	if (multi_backend_get_subbackend(multi, backend)) {
 		// already added
-		return;
+		return true;
 	}
 
-	struct subbackend_state *sub;
-	if (!(sub = calloc(1, sizeof(struct subbackend_state)))) {
+	struct wlr_renderer *multi_renderer =
+		multi_backend_get_renderer(&multi->backend);
+	struct wlr_renderer *backend_renderer = wlr_backend_get_renderer(backend);
+	if (multi_renderer != NULL && backend_renderer != NULL) {
+		wlr_log(L_ERROR, "Could not add backend: multiple renderers at the "
+			"same time aren't supported");
+		return false;
+	}
+
+	struct subbackend_state *sub = calloc(1, sizeof(struct subbackend_state));
+	if (sub == NULL) {
 		wlr_log(L_ERROR, "Could not add backend: allocation failed");
-		return;
+		return false;
 	}
 	wl_list_insert(&multi->backends, &sub->link);
 
@@ -160,6 +169,7 @@ void wlr_multi_backend_add(struct wlr_backend *_multi,
 	sub->new_output.notify = new_output_reemit;
 
 	wlr_signal_emit_safe(&multi->events.backend_add, backend);
+	return true;
 }
 
 void wlr_multi_backend_remove(struct wlr_backend *_multi,

include/wlr/backend/multi.h

@@ -13,7 +13,7 @@ struct wlr_backend *wlr_multi_backend_create(struct wl_display *display);
  * Adds the given backend to the multi backend. This should be done before the
  * new backend is started.
  */
-void wlr_multi_backend_add(struct wlr_backend *multi,
+bool wlr_multi_backend_add(struct wlr_backend *multi,
 	struct wlr_backend *backend);
 
 void wlr_multi_backend_remove(struct wlr_backend *multi,