backend: return wlr_session in wlr_backend_autocreate() call

Up until now, wlr_backend_autocreate() created the wlr_session and
then stuffed it into struct wlr_multi_backend so that compositors
can grab it later.

This is an abuse of wlr_multi_backend and the wlr_backend API:
wlr_backend_get_session() and wlr_multi_backend.session only exist
to accomodate the needs of wlr_backend_autocreate(). What's more,
the DRM and libinput backends don't implement
wlr_backend_impl.get_session.

Instead, return the struct wlr_session to the compositor in the
wlr_backend_autocreate() call. wlr_backend_get_session() will be
removed in the next commit.

backend/backend.c

@@ -245,7 +245,8 @@ static struct wlr_backend *attempt_drm_backend(struct wl_display *display,
 #endif
 
 static bool attempt_backend_by_name(struct wl_display *display,
-		struct wlr_multi_backend *multi, char *name) {
+		struct wlr_backend *multi, char *name,
+		struct wlr_session **session_ptr) {
 	struct wlr_backend *backend = NULL;
 	if (strcmp(name, "wayland") == 0) {
 		backend = attempt_wl_backend(display);
@@ -257,9 +258,9 @@ static bool attempt_backend_by_name(struct wl_display *display,
 		backend = attempt_headless_backend(display);
 	} else if (strcmp(name, "drm") == 0 || strcmp(name, "libinput") == 0) {
 		// DRM and libinput need a session
-		if (multi->session == NULL) {
-			multi->session = session_create_and_wait(display);
-			if (multi->session == NULL) {
+		if (*session_ptr == NULL) {
+			*session_ptr = session_create_and_wait(display);
+			if (*session_ptr == NULL) {
 				wlr_log(WLR_ERROR, "failed to start a session");
 				return false;
 			}
@@ -267,13 +268,13 @@ static bool attempt_backend_by_name(struct wl_display *display,
 
 		if (strcmp(name, "libinput") == 0) {
 #if WLR_HAS_LIBINPUT_BACKEND
-			backend = wlr_libinput_backend_create(display, multi->session);
+			backend = wlr_libinput_backend_create(display, *session_ptr);
 #endif
 		} else {
 #if WLR_HAS_DRM_BACKEND
 			// attempt_drm_backend adds the multi drm backends itself
-			return attempt_drm_backend(display, &multi->backend,
-					multi->session) != NULL;
+			return attempt_drm_backend(display, multi,
+				*session_ptr) != NULL;
 #endif
 		}
 	} else {
@@ -281,12 +282,17 @@ static bool attempt_backend_by_name(struct wl_display *display,
 		return false;
 	}
 
-	return wlr_multi_backend_add(&multi->backend, backend);
+	return wlr_multi_backend_add(multi, backend);
 }
 
-struct wlr_backend *wlr_backend_autocreate(struct wl_display *display) {
+struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
+		struct wlr_session **session_ptr) {
+	if (session_ptr != NULL) {
+		*session_ptr = NULL;
+	}
+
+	struct wlr_session *session = NULL;
 	struct wlr_backend *backend = wlr_multi_backend_create(display);
-	struct wlr_multi_backend *multi = (struct wlr_multi_backend *)backend;
 	if (!backend) {
 		wlr_log(WLR_ERROR, "could not allocate multibackend");
 		return NULL;
@@ -300,26 +306,23 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display) {
 		names = strdup(names);
 		if (names == NULL) {
 			wlr_log(WLR_ERROR, "allocation failed");
-			wlr_backend_destroy(backend);
-			return NULL;
+			goto error;
 		}
 
 		char *saveptr;
 		char *name = strtok_r(names, ",", &saveptr);
 		while (name != NULL) {
-			if (!attempt_backend_by_name(display, multi, name)) {
+			if (!attempt_backend_by_name(display, backend, name, &session)) {
 				wlr_log(WLR_ERROR, "failed to add backend '%s'", name);
-				wlr_session_destroy(multi->session);
-				wlr_backend_destroy(backend);
 				free(names);
-				return NULL;
+				goto error;
 			}
 
 			name = strtok_r(NULL, ",", &saveptr);
 		}
 
 		free(names);
-		return backend;
+		goto success;
 	}
 
 	if (getenv("WAYLAND_DISPLAY") || getenv("WAYLAND_SOCKET")) {
@@ -329,7 +332,7 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display) {
 		}
 
 		wlr_multi_backend_add(backend, wl_backend);
-		return backend;
+		goto success;
 	}
 
 #if WLR_HAS_X11_BACKEND
@@ -342,26 +345,23 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display) {
 		}
 
 		wlr_multi_backend_add(backend, x11_backend);
-		return backend;
+		goto success;
 	}
 #endif
 
 	// Attempt DRM+libinput
-	multi->session = session_create_and_wait(display);
-	if (!multi->session) {
+	session = session_create_and_wait(display);
+	if (!session) {
 		wlr_log(WLR_ERROR, "Failed to start a DRM session");
-		wlr_backend_destroy(backend);
-		return NULL;
+		goto error;
 	}
 
 #if WLR_HAS_LIBINPUT_BACKEND
-	struct wlr_backend *libinput = wlr_libinput_backend_create(display,
-		multi->session);
+	struct wlr_backend *libinput =
+		wlr_libinput_backend_create(display, session);
 	if (!libinput) {
 		wlr_log(WLR_ERROR, "Failed to start libinput backend");
-		wlr_session_destroy(multi->session);
-		wlr_backend_destroy(backend);
-		return NULL;
+		goto error;
 	}
 	wlr_multi_backend_add(backend, libinput);
 #else
@@ -372,28 +372,31 @@ struct wlr_backend *wlr_backend_autocreate(struct wl_display *display) {
 		wlr_log(WLR_ERROR, "libinput support is not compiled in, "
 			"refusing to start");
 		wlr_log(WLR_ERROR, "Set WLR_LIBINPUT_NO_DEVICES=1 to suppress this check");
-		wlr_session_destroy(multi->session);
-		wlr_backend_destroy(backend);
-		return NULL;
+		goto error;
 	}
 #endif
 
 #if WLR_HAS_DRM_BACKEND
 	struct wlr_backend *primary_drm =
-		attempt_drm_backend(display, backend, multi->session);
+		attempt_drm_backend(display, backend, session);
 	if (!primary_drm) {
 		wlr_log(WLR_ERROR, "Failed to open any DRM device");
-		wlr_session_destroy(multi->session);
-		wlr_backend_destroy(backend);
-		return NULL;
+		goto error;
 	}
 
-	drm_backend_monitor_create(backend, primary_drm, multi->session);
-
-	return backend;
+	drm_backend_monitor_create(backend, primary_drm, session);
 #endif
 
+success:
+	struct wlr_multi_backend *multi = wl_container_of(backend, multi, backend);
+	multi->session = session;
+	if (session_ptr != NULL) {
+		*session_ptr = session;
+	}
+	return backend;
+
 error:
 	wlr_backend_destroy(backend);
+	wlr_session_destroy(session);
 	return NULL;
 }

examples/fullscreen-shell.c

@@ -205,7 +205,7 @@ int main(int argc, char *argv[]) {
 
 	struct fullscreen_server server = {0};
 	server.wl_display = wl_display_create();
-	server.backend = wlr_backend_autocreate(server.wl_display);
+	server.backend = wlr_backend_autocreate(server.wl_display, NULL);
 	server.renderer = wlr_renderer_autocreate(server.backend);
 	wlr_renderer_init_wl_display(server.renderer, server.wl_display);
 	server.allocator = wlr_allocator_autocreate(server.backend,

examples/multi-pointer.c

@@ -272,7 +272,7 @@ int main(int argc, char *argv[]) {
 		.clear_color = { 0.25f, 0.25f, 0.25f, 1 },
 		.display = display,
 	};
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/output-layout.c

@@ -268,7 +268,7 @@ int main(int argc, char *argv[]) {
 	state.layout = wlr_output_layout_create();
 	clock_gettime(CLOCK_MONOTONIC, &state.ts_last);
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/pointer.c

@@ -333,7 +333,7 @@ int main(int argc, char *argv[]) {
 		.display = display
 	};
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/quads.c

@@ -189,7 +189,7 @@ int main(int argc, char *argv[]) {
 	};
 	wl_list_init(&state.outputs);
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/rotation.c

@@ -239,7 +239,7 @@ int main(int argc, char *argv[]) {
 	};
 	wl_list_init(&state.outputs);
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/scene-graph.c

@@ -157,7 +157,7 @@ int main(int argc, char *argv[]) {
 	struct server server = {0};
 	server.surface_offset = 0;
 	server.display = wl_display_create();
-	server.backend = wlr_backend_autocreate(server.display);
+	server.backend = wlr_backend_autocreate(server.display, NULL);
 	server.scene = wlr_scene_create();
 
 	server.renderer = wlr_renderer_autocreate(server.backend);

examples/simple.c

@@ -172,7 +172,7 @@ int main(void) {
 		.last_frame = { 0 },
 		.display = display
 	};
-	struct wlr_backend *backend = wlr_backend_autocreate(display);
+	struct wlr_backend *backend = wlr_backend_autocreate(display, NULL);
 	if (!backend) {
 		exit(1);
 	}

examples/tablet.c

@@ -355,7 +355,7 @@ int main(int argc, char *argv[]) {
 	};
 	wl_list_init(&state.tablet_pads);
 	wl_list_init(&state.tablet_tools);
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

examples/touch.c

@@ -248,7 +248,7 @@ int main(int argc, char *argv[]) {
 	wl_list_init(&state.touch_points);
 	wl_list_init(&state.touch);
 
-	struct wlr_backend *wlr = wlr_backend_autocreate(display);
+	struct wlr_backend *wlr = wlr_backend_autocreate(display, NULL);
 	if (!wlr) {
 		exit(1);
 	}

include/wlr/backend.h

@@ -34,8 +34,12 @@ struct wlr_backend {
  * Automatically initializes the most suitable backend given the environment.
  * Will always return a multi-backend. The backend is created but not started.
  * Returns NULL on failure.
+ *
+ * If session_ptr is not NULL, it's populated with the session which has been
+ * created with the backend, if any.
  */
-struct wlr_backend *wlr_backend_autocreate(struct wl_display *display);
+struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
+	struct wlr_session **session_ptr);
 /**
  * Start the backend. This may signal new_input or new_output immediately, but
  * may also wait until the display's event loop begins. Returns false on

tinywl/tinywl.c

@@ -828,7 +828,7 @@ int main(int argc, char *argv[]) {
 	 * output hardware. The autocreate option will choose the most suitable
 	 * backend based on the current environment, such as opening an X11 window
 	 * if an X11 server is running. */
-	server.backend = wlr_backend_autocreate(server.wl_display);
+	server.backend = wlr_backend_autocreate(server.wl_display, NULL);
 	if (server.backend == NULL) {
 		wlr_log(WLR_ERROR, "failed to create wlr_backend");
 		return 1;