Add --ready-fd

This implements a readiness notification mechanism which works on
both systemd and s6.

References: swaywm#42
References: swaywm#275

include/swaylock.h

@@ -65,6 +65,7 @@ struct swaylock_args {
 	bool hide_keyboard_layout;
 	bool show_failed_attempts;
 	bool daemonize;
+	int ready_fd;
 	bool indicator_idle_visible;
 	char *plugin_command;
 };

main.c

@@ -820,6 +820,7 @@ static int parse_options(int argc, char **argv, struct swaylock_state *state,
 		{"debug", no_argument, NULL, 'd'},
 		{"ignore-empty-password", no_argument, NULL, 'e'},
 		{"daemonize", no_argument, NULL, 'f'},
+		{"ready-fd", required_argument, NULL, 'R'},
 		{"help", no_argument, NULL, 'h'},
 		{"image", required_argument, NULL, 'i'},
 		{"disable-caps-lock-text", no_argument, NULL, 'L'},
@@ -887,6 +888,8 @@ static int parse_options(int argc, char **argv, struct swaylock_state *state,
 			"Show current count of failed authentication attempts.\n"
 		"  -f, --daemonize                  "
 			"Detach from the controlling terminal after locking.\n"
+		"  -R, --ready-fd <fd>              "
+			"File descriptor to send readiness notifications to.\n"
 		"  -h, --help                       "
 			"Show help message and quit.\n"
 		"  -i, --image [[<output>]:]<path>  "
@@ -998,7 +1001,7 @@ static int parse_options(int argc, char **argv, struct swaylock_state *state,
 	optind = 1;
 	while (1) {
 		int opt_idx = 0;
-		c = getopt_long(argc, argv, "c:deFfhi:kKLlnrs:tuvC:", long_options,
+		c = getopt_long(argc, argv, "c:deFfhi:kKLlnrs:tuvC:R:", long_options,
 				&opt_idx);
 		if (c == -1) {
 			break;
@@ -1032,6 +1035,11 @@ static int parse_options(int argc, char **argv, struct swaylock_state *state,
 				state->args.daemonize = true;
 			}
 			break;
+		case 'R':
+			if (state) {
+				state->args.ready_fd = strtol(optarg, NULL, 10);
+			}
+			break;
 		case 'i':
 			if (state) {
 				load_image(optarg, state);
@@ -1738,6 +1746,7 @@ int main(int argc, char **argv) {
 		.hide_keyboard_layout = false,
 		.show_failed_attempts = false,
 		.indicator_idle_visible = false,
+		.ready_fd = -1,
 		.plugin_command = NULL,
 	};
 	wl_list_init(&state.images);
@@ -1870,6 +1879,17 @@ int main(int argc, char **argv) {
 		state.locked = true;
 	}
 
+	if (state.args.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(state.args.ready_fd, ready_str, strlen(ready_str)) != strlen(ready_str)) {
+			swaylock_log(LOG_ERROR, "Failed to send readiness notification");
+			return 2;
+		}
+		close(state.args.ready_fd);
+		state.args.ready_fd = -1;
+	}
 	if (state.args.daemonize) {
 		daemonize();
 	}