backend/multi: improve type-safety by exposing wlr_multi_backend

Expose an opaque wlr_multi_backend struct. That way functions
operating on a multi backend can take this struct instead of a
generic wlr_backend, without having to assert.

backend/backend.c

@@ -203,7 +203,7 @@ static struct wlr_backend *attempt_headless_backend(
 }
 
 static bool attempt_drm_backend(struct wl_display *display,
-		struct wlr_backend *backend, struct wlr_session *session) {
+		struct wlr_multi_backend *multi, struct wlr_session *session) {
 #if WLR_HAS_DRM_BACKEND
 	struct wlr_device *gpus[8];
 	ssize_t num_gpus = wlr_session_find_gpus(session, 8, gpus);
@@ -232,7 +232,7 @@ static bool attempt_drm_backend(struct wl_display *display,
 			primary_drm = drm;
 		}
 
-		wlr_multi_backend_add(backend, drm);
+		wlr_multi_backend_add(multi, drm);
 	}
 	if (!primary_drm) {
 		wlr_log(WLR_ERROR, "Could not successfully create backend on any GPU");
@@ -240,7 +240,7 @@ static bool attempt_drm_backend(struct wl_display *display,
 	}
 
 	if (getenv("WLR_DRM_DEVICES") == NULL) {
-		drm_backend_monitor_create(backend, primary_drm, session);
+		drm_backend_monitor_create(multi, primary_drm, session);
 	}
 
 	return true;
@@ -261,7 +261,7 @@ static struct wlr_backend *attempt_libinput_backend(struct wl_display *display,
 }
 
 static bool attempt_backend_by_name(struct wl_display *display,
-		struct wlr_backend *multi, char *name,
+		struct wlr_multi_backend *multi, char *name,
 		struct wlr_session **session_ptr) {
 	struct wlr_backend *backend = NULL;
 	if (strcmp(name, "wayland") == 0) {
@@ -304,7 +304,7 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
 	}
 
 	struct wlr_session *session = NULL;
-	struct wlr_backend *multi = wlr_multi_backend_create(display);
+	struct wlr_multi_backend *multi = wlr_multi_backend_create(display);
 	if (!multi) {
 		wlr_log(WLR_ERROR, "could not allocate multibackend");
 		return NULL;
@@ -387,10 +387,10 @@ success:
 	if (session_ptr != NULL) {
 		*session_ptr = session;
 	}
-	return multi;
+	return wlr_multi_backend_base(multi);
 
 error:
-	wlr_backend_destroy(multi);
+	wlr_backend_destroy(wlr_multi_backend_base(multi));
 #if WLR_HAS_SESSION
 	wlr_session_destroy(session);
 #endif

backend/drm/monitor.c

@@ -64,7 +64,7 @@ static void handle_multi_destroy(struct wl_listener *listener, void *data) {
 }
 
 struct wlr_drm_backend_monitor *drm_backend_monitor_create(
-		struct wlr_backend *multi,
+		struct wlr_multi_backend *multi,
 		struct wlr_backend *primary_drm,
 		struct wlr_session *session) {
 	struct wlr_drm_backend_monitor *monitor =
@@ -87,8 +87,9 @@ struct wlr_drm_backend_monitor *drm_backend_monitor_create(
 	monitor->primary_drm_destroy.notify = handle_primary_drm_destroy;
 	wl_signal_add(&primary_drm->events.destroy, &monitor->primary_drm_destroy);
 
+	struct wlr_backend *multi_base = wlr_multi_backend_base(multi);
 	monitor->multi_destroy.notify = handle_multi_destroy;
-	wl_signal_add(&multi->events.destroy, &monitor->multi_destroy);
+	wl_signal_add(&multi_base->events.destroy, &monitor->multi_destroy);
 
 	return monitor;
 }

backend/multi/backend.c

@@ -18,10 +18,10 @@ struct subbackend_state {
 	struct wl_list link;
 };
 
-static struct wlr_multi_backend *multi_backend_from_backend(
-		struct wlr_backend *wlr_backend) {
-	assert(wlr_backend_is_multi(wlr_backend));
-	return (struct wlr_multi_backend *)wlr_backend;
+static struct wlr_multi_backend *multi_backend_from_backend(struct wlr_backend *backend) {
+	struct wlr_multi_backend *multi = wlr_multi_backend_try_from(backend);
+	assert(multi != NULL);
+	return multi;
 }
 
 static bool multi_backend_start(struct wlr_backend *wlr_backend) {
@@ -125,7 +125,7 @@ static void handle_display_destroy(struct wl_listener *listener, void *data) {
 	multi_backend_destroy((struct wlr_backend*)backend);
 }
 
-struct wlr_backend *wlr_multi_backend_create(struct wl_display *display) {
+struct wlr_multi_backend *wlr_multi_backend_create(struct wl_display *display) {
 	struct wlr_multi_backend *backend =
 		calloc(1, sizeof(struct wlr_multi_backend));
 	if (!backend) {
@@ -142,11 +142,19 @@ struct wlr_backend *wlr_multi_backend_create(struct wl_display *display) {
 	backend->display_destroy.notify = handle_display_destroy;
 	wl_display_add_destroy_listener(display, &backend->display_destroy);
 
-	return &backend->backend;
+	return backend;
 }
 
-bool wlr_backend_is_multi(struct wlr_backend *b) {
-	return b->impl == &backend_impl;
+struct wlr_multi_backend *wlr_multi_backend_try_from(struct wlr_backend *backend) {
+	if (backend->impl != &backend_impl) {
+		return NULL;
+	}
+	struct wlr_multi_backend *multi = wl_container_of(backend, multi, backend);
+	return multi;
+}
+
+struct wlr_backend *wlr_multi_backend_base(struct wlr_multi_backend *backend) {
+	return &backend->backend;
 }
 
 static void new_input_reemit(struct wl_listener *listener, void *data) {
@@ -178,12 +186,9 @@ static struct subbackend_state *multi_backend_get_subbackend(struct wlr_multi_ba
 	return NULL;
 }
 
-bool wlr_multi_backend_add(struct wlr_backend *_multi,
-		struct wlr_backend *backend) {
-	assert(_multi && backend);
-	assert(_multi != backend);
-
-	struct wlr_multi_backend *multi = multi_backend_from_backend(_multi);
+bool wlr_multi_backend_add(struct wlr_multi_backend *multi, struct wlr_backend *backend) {
+	assert(multi && backend);
+	assert(&multi->backend != backend);
 
 	if (multi_backend_get_subbackend(multi, backend)) {
 		// already added
@@ -213,29 +218,20 @@ bool wlr_multi_backend_add(struct wlr_backend *_multi,
 	return true;
 }
 
-void wlr_multi_backend_remove(struct wlr_backend *_multi,
-		struct wlr_backend *backend) {
-	struct wlr_multi_backend *multi = multi_backend_from_backend(_multi);
-
-	struct subbackend_state *sub =
-		multi_backend_get_subbackend(multi, backend);
-
+void wlr_multi_backend_remove(struct wlr_multi_backend *multi, struct wlr_backend *backend) {
+	struct subbackend_state *sub = multi_backend_get_subbackend(multi, backend);
 	if (sub) {
 		wl_signal_emit_mutable(&multi->events.backend_remove, backend);
 		subbackend_state_destroy(sub);
 	}
 }
 
-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;
+bool wlr_multi_backend_is_empty(struct wlr_multi_backend *backend) {
 	return wl_list_length(&backend->backends) < 1;
 }
 
-void wlr_multi_for_each_backend(struct wlr_backend *_backend,
+void wlr_multi_backend_for_each(struct wlr_multi_backend *backend,
 		void (*callback)(struct wlr_backend *backend, void *data), void *data) {
-	assert(wlr_backend_is_multi(_backend));
-	struct wlr_multi_backend *backend = (struct wlr_multi_backend *)_backend;
 	struct subbackend_state *sub;
 	wl_list_for_each(sub, &backend->backends, link) {
 		callback(sub->backend, data);

include/backend/drm/monitor.h

@@ -7,7 +7,7 @@
  * Helper to create new DRM sub-backends on GPU hotplug.
  */
 struct wlr_drm_backend_monitor {
-	struct wlr_backend *multi;
+	struct wlr_multi_backend *multi;
 	struct wlr_backend *primary_drm;
 	struct wlr_session *session;
 
@@ -18,7 +18,7 @@ struct wlr_drm_backend_monitor {
 };
 
 struct wlr_drm_backend_monitor *drm_backend_monitor_create(
-	struct wlr_backend *multi, struct wlr_backend *primary_drm,
+	struct wlr_multi_backend *multi, struct wlr_backend *primary_drm,
 	struct wlr_session *session);
 
 #endif

include/wlr/backend/multi.h

@@ -11,25 +11,31 @@
 
 #include <wlr/backend.h>
 
+struct wlr_multi_backend;
+
 /**
  * Creates a multi-backend. Multi-backends wrap an arbitrary number of backends
  * and aggregate their new_output/new_input signals.
  */
-struct wlr_backend *wlr_multi_backend_create(struct wl_display *display);
+struct wlr_multi_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.
  */
-bool wlr_multi_backend_add(struct wlr_backend *multi,
+bool wlr_multi_backend_add(struct wlr_multi_backend *multi,
 	struct wlr_backend *backend);
 
-void wlr_multi_backend_remove(struct wlr_backend *multi,
+void wlr_multi_backend_remove(struct wlr_multi_backend *multi,
 	struct wlr_backend *backend);
 
-bool wlr_backend_is_multi(struct wlr_backend *backend);
-bool wlr_multi_is_empty(struct wlr_backend *backend);
+bool wlr_multi_backend_is_empty(struct wlr_multi_backend *backend);
 
-void wlr_multi_for_each_backend(struct wlr_backend *backend,
+void wlr_multi_backend_for_each(struct wlr_multi_backend *backend,
 		void (*callback)(struct wlr_backend *backend, void *data), void *data);
 
+struct wlr_multi_backend *wlr_multi_backend_try_from(struct wlr_backend *backend);
+
+struct wlr_backend *wlr_multi_backend_base(struct wlr_multi_backend *backend);
+
 #endif

types/wlr_drm_lease_v1.c

@@ -704,9 +704,10 @@ struct wlr_drm_lease_v1_manager *wlr_drm_lease_v1_manager_create(
 	wl_list_init(&manager->devices);
 	manager->display = display;
 
-	if (wlr_backend_is_multi(backend)) {
+	struct wlr_multi_backend *multi = wlr_multi_backend_try_from(backend);
+	if (multi != NULL) {
 		/* TODO: handle backends added after the manager is created */
-		wlr_multi_for_each_backend(backend, multi_backend_cb, manager);
+		wlr_multi_backend_for_each(multi, multi_backend_cb, manager);
 	} else if (wlr_backend_is_drm(backend)) {
 		drm_lease_device_v1_create(manager, backend);
 	}