add --ready-fd to signal readiness

This adds a new commandline option, --ready-fd, to send a notification
when sway is ready to run.

This allows a process supervisor to notice sway's startup is complete
which allows, for example, the session manager to start Wayland
programs in the right order. Without this signal, users have to go
through horrible hacks to order services.

For example, I've been using `NotifyAccess=all` in the `sway.service`
file and `exec systemd-notify --ready` in my sway config to emulate
this. Problem is it's racy and error-prone.

A particularly nasty bug triggered by `NotifyAccess=all` is when
`podman` starts and then terminates a container. In that context,
`conmon(8)` ends up notifying systemd it's the session master and
takes over thee `MainPID` field in systemd. When it dies, systemd
believes the session is over and proceeds to kill the entire
session. This is explained in more details in:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1039857

This might not actually be the right place to do this in the sway
startup sequence, that said. We call `server_run` right after, and
maybe somewhere below that would be a better place. But `server_run`
only calls `wl_display_run` and that's part of the core wayland
library, so that seems a little too far down. I'm not sure Wayland
itself is a place to do this, so for now I'm scratching my own itch
and doing this in Sway itself.

Note that this approach was taken instead of using the proper
`sd_notify` library call, as that approach was refused in #7659. The
`--ready-fd` approach was accepted in swaywm/swaylock#281 so it is
hoped it will be seen as acceptable here.

An alternative implementation would be to instead check the
`NOTIFY_SOCKET` environment variable and use that, if present. That
variable is used by systemd and at least the s6 supervisor to receive
readiness notifications, so it might be less disruptive.

sway/main.c

@@ -201,6 +201,7 @@ static const struct option long_options[] = {
 	{"verbose", no_argument, NULL, 'V'},
 	{"get-socketpath", no_argument, NULL, 'p'},
 	{"unsupported-gpu", no_argument, NULL, 'u'},
+	{"ready-fd", required_argument, NULL, 'R'},
 	{0, 0, 0, 0}
 };
 
@@ -214,17 +215,19 @@ 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"
+	"  -R  --ready-fd <fd>    Send readiness notification to the given file descriptor.\n"
 	"\n";
 
 int main(int argc, char **argv) {
 	static bool verbose = false, debug = false, validate = false;
 
 	char *config_path = NULL;
+	int ready_fd;
 
 	int c;
 	while (1) {
 		int option_index = 0;
-		c = getopt_long(argc, argv, "hCdD:vVc:", long_options, &option_index);
+		c = getopt_long(argc, argv, "hCdD:vVc:R:", long_options, &option_index);
 		if (c == -1) {
 			break;
 		}
@@ -265,6 +268,9 @@ int main(int argc, char **argv) {
 				exit(EXIT_FAILURE);
 			}
 			break;
+		case 'R': // --ready-fd
+			ready_fd = strtol(optarg, NULL, 10);
+			break;
 		default:
 			fprintf(stderr, "%s", usage);
 			exit(EXIT_FAILURE);
@@ -371,6 +377,17 @@ int main(int argc, char **argv) {
 		swaynag_show(&config->swaynag_config_errors);
 	}
 
+	if (ready_fd >= 0) {
+		// s6 wants a newline and ignores any text before that, systemd wants
+		// READY=1, so use the least common denominator
+		const char ready_str[] = "READY=1\n";
+		if (write(ready_fd, ready_str, strlen(ready_str)) != (ssize_t) strlen(ready_str)) {
+			sway_log(SWAY_INFO, "Failed to send readiness notification");
+		}
+		close(ready_fd);
+		ready_fd = -1;
+	}
+
 	server_run(&server);
 
 shutdown: