Add optional daemonization

If the `-f` argument is passed to swayidle it will fork and daemonize
after initialization. This allows proper ordering of other programs
after swayidle is fully running, including Type=forking in systemd units

main.c

@@ -68,6 +68,8 @@ static const char *verbosity_colors[] = {
 	[LOG_DEBUG ] = "\x1B[1;30m",
 };
 
+static bool daemonize = false;
+
 static enum log_importance log_importance = LOG_INFO;
 
 void swayidle_log_init(enum log_importance verbosity) {
@@ -827,7 +829,7 @@ static int parse_idlehint(int argc, char **argv) {
 
 static int parse_args(int argc, char *argv[], char **config_path) {
 	int c;
-	while ((c = getopt(argc, argv, "C:hdwS:")) != -1) {
+	while ((c = getopt(argc, argv, "C:hdfwS:")) != -1) {
 		switch (c) {
 		case 'C':
 			free(*config_path);
@@ -836,6 +838,9 @@ static int parse_args(int argc, char *argv[], char **config_path) {
 		case 'd':
 			swayidle_log_init(LOG_DEBUG);
 			break;
+		case 'f':
+			daemonize = true;
+			break;
 		case 'w':
 			state.wait = true;
 			break;
@@ -848,6 +853,7 @@ static int parse_args(int argc, char *argv[], char **config_path) {
 			printf("  -h\tthis help menu\n");
 			printf("  -C\tpath to config file\n");
 			printf("  -d\tdebug\n");
+			printf("  -f\tdaemonize after initialization\n");
 			printf("  -w\twait for command to finish\n");
 			printf("  -S\tpick the seat to work with\n");
 			return 1;
@@ -1016,6 +1022,25 @@ static int load_config(const char *config_path) {
 	return 0;
 }
 
+int do_daemonize(void) {
+	// don't close stdin, stdout, stderr
+	// just fork and setsid
+	pid_t child;
+
+	child = fork();
+	if (child < 0) {
+		return -1;
+	}
+
+	if (child == 0) {
+		if (setsid() == -1)
+			return -1;
+		return 0;
+	} else {
+		swayidle_log(LOG_DEBUG, "Child forked, pid: %d", child);
+		_exit(0);
+	}
+}
 
 int main(int argc, char *argv[]) {
 	swayidle_init();
@@ -1118,6 +1143,13 @@ int main(int argc, char *argv[]) {
 		display_event, NULL);
 	wl_event_source_check(source);
 
+	if (daemonize) {
+		if (do_daemonize() != 0 ) {
+			swayidle_log_errno(LOG_ERROR, "Failed to daemonize, will exit!");
+			sway_terminate(1);
+		}
+	}
+
 	while (wl_event_loop_dispatch(state.event_loop, -1) != 1) {
 		// This space intentionally left blank
 	}

swayidle.1.scd

@@ -21,6 +21,9 @@ swayidle - Idle manager for Wayland
 *-d*
 	Enable debug output.
 
+*-f*
+	Fork and daemonize after fully initialized.
+
 *-w*
 	Wait for command to finish executing before continuing, helpful for ensuring
 	that a *before-sleep* command has finished before the system goes to sleep.