sway/server: implement wayland socket handover

This commit implements the compositor side of the Wayland socket
handover protocol as described in the [KDE Wiki]. The CLI options are
chosen so that they are compatible with Kwin.

[KDE Wiki]: https://invent.kde.org/plasma/kwin/-/wikis/Restarting

include/sway/server.h

@@ -155,7 +155,7 @@ extern struct sway_debug debug;
 
 extern bool allow_unsupported_gpu;
 
-bool server_init(struct sway_server *server);
+bool server_init(struct sway_server *server, const char * socket_name, int socket_fd);
 void server_fini(struct sway_server *server);
 bool server_start(struct sway_server *server);
 void server_run(struct sway_server *server);

sway/main.c

@@ -196,6 +196,8 @@ static const struct option long_options[] = {
 	{"verbose", no_argument, NULL, 'V'},
 	{"get-socketpath", no_argument, NULL, 'p'},
 	{"unsupported-gpu", no_argument, NULL, 'u'},
+	{"socket", required_argument, NULL, 's'},
+	{"wayland-fd", required_argument, NULL, 'S'},
 	{0, 0, 0, 0}
 };
 
@@ -209,12 +211,16 @@ static const char usage[] =
 	"  -v, --version          Show the version number and quit.\n"
 	"  -V, --verbose          Enables more verbose logging.\n"
 	"      --get-socketpath   Gets the IPC socket path and prints it, then exits.\n"
+	"      --socket <name>    Sets the Wayland socket name (for Wayland socket handover)\n"
+	"      --wayland-fd <fd>  Sets the Wayland socket fd (for Wayland socket handover)\n"
 	"\n";
 
 int main(int argc, char **argv) {
 	static bool verbose = false, debug = false, validate = false;
 
 	char *config_path = NULL;
+	char *socket_name = NULL;
+	int socket_fd = -1;
 
 	int c;
 	while (1) {
@@ -260,6 +266,13 @@ int main(int argc, char **argv) {
 				exit(EXIT_FAILURE);
 			}
 			break;
+		case 's': // --socket
+			free(socket_name);
+			socket_name = strdup(optarg);
+			break;
+		case 'S': // --wayland-fd
+			socket_fd = atoi(optarg);
+			break;
 		default:
 			fprintf(stderr, "%s", usage);
 			exit(EXIT_FAILURE);
@@ -331,7 +344,7 @@ int main(int argc, char **argv) {
 
 	sway_log(SWAY_INFO, "Starting sway version " SWAY_VERSION);
 
-	if (!server_init(&server)) {
+	if (!server_init(&server, socket_name, socket_fd)) {
 		return 1;
 	}
 

sway/server.c

@@ -2,6 +2,7 @@
 #include <stdbool.h>
 #include <stdlib.h>
 #include <string.h>
+#include <fcntl.h>
 #include <wayland-server-core.h>
 #include <wlr/backend.h>
 #include <wlr/backend/headless.h>
@@ -215,7 +216,7 @@ static void handle_renderer_lost(struct wl_listener *listener, void *data) {
 	wlr_renderer_destroy(old_renderer);
 }
 
-bool server_init(struct sway_server *server) {
+bool server_init(struct sway_server *server, const char * socket_name, int socket_fd) {
 	sway_log(SWAY_DEBUG, "Initializing Wayland server");
 	server->wl_display = wl_display_create();
 	server->wl_event_loop = wl_display_get_event_loop(server->wl_display);
@@ -406,6 +407,13 @@ bool server_init(struct sway_server *server) {
 
 	wl_list_init(&server->pending_launcher_ctxs);
 
+	if (socket_name != NULL && socket_fd != -1) {
+		// Add the passed socket
+		fcntl(socket_fd, F_SETFD, FD_CLOEXEC);
+		if (wl_display_add_socket_fd(server->wl_display, socket_fd) >= 0) {
+			server->socket = strdup(socket_name);
+		}
+	} else {
 		// Avoid using "wayland-0" as display socket
 		char name_candidate[16];
 		for (unsigned int i = 1; i <= 32; ++i) {
@@ -415,6 +423,7 @@ bool server_init(struct sway_server *server) {
 				break;
 			}
 		}
+	}
 
 	if (!server->socket) {
 		sway_log(SWAY_ERROR, "Unable to open wayland socket");

sway/sway.1.scd

@@ -31,6 +31,12 @@ sway - An i3-compatible Wayland compositor
 *--get-socketpath*
 	Gets the IPC socket path and prints it, then exits.
 
+*--socket NAME*
+	Sets the Wayland socket name (for Wayland socket handover)
+
+*--wayland-fd FD*
+	Sets the Wayland socket file descriptor (for Wayland socket handover)
+
 # DESCRIPTION
 
 sway was created to fill the need of an i3-like window manager for Wayland. The