main: Allow starting server from a bound socket fd

Instead of having wayvnc/neatvnc create a socket and listen() on it,
this patch allows listening on an already bound socket file descriptor
with the --external-listener-fd=FD option. This may be used to support
any kind of connection-based socket that isn't explicitly supported by
wayvnc in an option, such as ones using the AF_VSOCK address family, or
support cases where wayvnc is being activated by systemd (via a .socket
unit) or a super-server like inetd.

src/main.c

@@ -80,6 +80,7 @@ enum socket_type {
 	SOCKET_TYPE_TCP = 0,
 	SOCKET_TYPE_UNIX,
 	SOCKET_TYPE_WEBSOCKET,
+	SOCKET_TYPE_FROM_FD,
 };
 
 struct wayvnc {
@@ -885,7 +886,7 @@ static char* get_cfg_path(const struct cfg* cfg, char* dst, const char* src)
 }
 
 static int init_nvnc(struct wayvnc* self, const char* addr, uint16_t port,
-		enum socket_type socket_type)
+		int fd, enum socket_type socket_type)
 {
 	switch (socket_type) {
 		case SOCKET_TYPE_TCP:
@@ -897,15 +898,20 @@ static int init_nvnc(struct wayvnc* self, const char* addr, uint16_t port,
 		case SOCKET_TYPE_WEBSOCKET:
 			self->nvnc = nvnc_open_websocket(addr, port);
 			break;
+		case SOCKET_TYPE_FROM_FD:
+			self->nvnc = nvnc_open_from_fd(fd);
+			break;
 		default:
 			abort();
 	}
 	if (!self->nvnc) {
-		nvnc_log(NVNC_LOG_ERROR, "Failed to bind to address. Add -Ldebug to the argument list for more info.");
+		nvnc_log(NVNC_LOG_ERROR, "Failed to listen on socket or bind to its address. Add -Ldebug to the argument list for more info.");
 		return -1;
 	}
 	if (socket_type == SOCKET_TYPE_UNIX)
 		nvnc_log(NVNC_LOG_INFO, "Listening for connections on %s", addr);
+	else if (socket_type == SOCKET_TYPE_FROM_FD)
+		nvnc_log(NVNC_LOG_INFO, "Listening for connections on fd %d", fd);
 	else
 		nvnc_log(NVNC_LOG_INFO, "Listening for connections on %s:%d", addr, port);
 
@@ -1705,6 +1711,7 @@ int main(int argc, char* argv[])
 
 	const char* address = NULL;
 	int port = 0;
+	int external_listener_fd = -1;
 	bool use_unix_socket = false;
 	bool use_websocket = false;
 	bool start_detached = false;
@@ -1754,6 +1761,9 @@ int main(int argc, char* argv[])
 		  "Show performance counters." },
 		{ 'u', "unix-socket", NULL,
 		  "Create unix domain socket." },
+		{ 'x', "external-listener-fd", "<fd>",
+		  "Listen on a bound socket at <fd> instead of binding to an address.",
+		  .default_ = "-1" },
 		{ 'd', "disable-input", NULL,
 		  "Disable all remote input." },
 		{ 'D', "detached", NULL,
@@ -1796,6 +1806,7 @@ int main(int argc, char* argv[])
 	show_performance = !!option_parser_get_value(&option_parser, "performance");
 	use_unix_socket = !!option_parser_get_value(&option_parser, "unix-socket");
 	use_websocket = !!option_parser_get_value(&option_parser, "websocket");
+	external_listener_fd = atoi(option_parser_get_value(&option_parser, "external-listener-fd"));
 	disable_input = !!option_parser_get_value(&option_parser, "disable-input");
 	log_level = option_parser_get_value(&option_parser, "verbose")
 		? NVNC_LOG_INFO : NVNC_LOG_WARNING;
@@ -1831,6 +1842,16 @@ int main(int argc, char* argv[])
 		return 1;
 	}
 
+	if (external_listener_fd >= 0 && use_unix_socket) {
+		nvnc_log(NVNC_LOG_ERROR, "external-listener-fd and unix-socket are conflicting options");
+		return 1;
+	}
+
+	if (external_listener_fd >= 0 && use_websocket) {
+		nvnc_log(NVNC_LOG_ERROR, "external-listener-fd and websocket are conflicting options");
+		return 1;
+	}
+
 	if (use_transient_seat && disable_input) {
 		nvnc_log(NVNC_LOG_ERROR, "transient-seat and disable-input are conflicting options");
 		return 1;
@@ -1862,6 +1883,16 @@ int main(int argc, char* argv[])
 		if (!port) port = self.cfg.port;
 	}
 
+	if (external_listener_fd >= 0 && address) {
+		nvnc_log(NVNC_LOG_ERROR, "external-listener-fd and address are conflicting options");
+		return 1;
+	}
+
+	if (external_listener_fd >= 0 && port) {
+		nvnc_log(NVNC_LOG_ERROR, "external-listener-fd and port are conflicting options");
+		return 1;
+	}
+
 	if (!address) address = DEFAULT_ADDRESS;
 	if (!port) port = DEFAULT_PORT;
 
@@ -1928,8 +1959,10 @@ int main(int argc, char* argv[])
 		socket_type = SOCKET_TYPE_UNIX;
 	else if (use_websocket)
 		socket_type = SOCKET_TYPE_WEBSOCKET;
+	else if (external_listener_fd >= 0)
+		socket_type = SOCKET_TYPE_FROM_FD;
 
-	if (init_nvnc(&self, address, port, socket_type) < 0)
+	if (init_nvnc(&self, address, port, external_listener_fd, socket_type) < 0)
 		goto nvnc_failure;
 
 	if (!start_detached) {

wayvnc.scd

@@ -55,6 +55,11 @@ wayvnc - A VNC server for wlroots based Wayland compositors.
 *-w, --websocket*
 	Create a websocket.
 
+*-x, --external-listener-fd=<fd>*
+	Have the VNC server listen on an already bound socket at this file
+	descriptor which should be inherited from wayvnc's parent process.
+	This is incompatible with all other server socket-related options.
+
 *-L, --log-level*
 	Set log level. The levels are: error, warning, info, debug, trace and
 	quiet.