start-stop-daemon: add s6-style readiness notification.

sh/start-stop-daemon.sh

@@ -56,6 +56,7 @@ ssd_start()
 		${pidfile:+--pidfile} $pidfile \
 		${command_user+--user} $command_user \
 		${umask+--umask} $umask \
+		${ready+--ready} $ready \
 		$_background $start_stop_daemon_args \
 		-- $command_args $command_args_background
 	if eend $? "Failed to start ${name:-$RC_SVCNAME}"; then

src/start-stop-daemon/start-stop-daemon.c

@@ -89,6 +89,7 @@ enum {
   LONGOPT_SCHEDULER,
   LONGOPT_SCHEDULER_PRIO,
   LONGOPT_SECBITS,
+  LONGOPT_READY,
 };
 
 const char *applet = NULL;
@@ -130,6 +131,7 @@ const struct option longopts[] = {
 	{ "progress",     0, NULL, 'P'},
 	{ "scheduler",    1, NULL, LONGOPT_SCHEDULER},
 	{ "scheduler-priority",    1, NULL, LONGOPT_SCHEDULER_PRIO},
+	{ "ready",        1, NULL, LONGOPT_READY},
 	longopts_COMMON
 };
 const char * const longopts_help[] = {
@@ -353,6 +355,8 @@ int main(int argc, char **argv)
 	int pipefd[2];
 	char readbuf[1];
 	ssize_t ss;
+	int ready_fd = -1;
+	int ready_pipe[2];
 
 	applet = basename_c(argv[0]);
 	atexit(cleanup);
@@ -618,6 +622,12 @@ int main(int argc, char **argv)
 			sscanf(optarg, "%d", &sched_prio);
 			break;
 
+		case LONGOPT_READY:
+			if (sscanf(optarg, "fd:%d", &ready_fd) != 1)
+				eerrorx("%s: invalid ready '%s'.", applet, optarg);
+			pipe(ready_pipe);
+			break;
+
 		case_RC_COMMON_GETOPT
 		}
 
@@ -1100,6 +1110,11 @@ int main(int argc, char **argv)
 
 		cloexec_fds_from(3);
 
+		if (ready_fd != -1) {
+			close(ready_pipe[0]);
+			dup2(ready_pipe[1], ready_fd);
+		}
+
 		if (scheduler != NULL) {
 			int scheduler_index;
 			struct sched_param sched =  {.sched_priority = sched_prio};
@@ -1184,7 +1199,21 @@ int main(int argc, char **argv)
 			start_wait = 0;
 	}
 
-	if (start_wait > 0) {
+	/* TODO: timeout on waiting readiness. */
+	if (ready_fd != -1) {
+		close(ready_pipe[1]);
+		for (;;) {
+			char buf[BUFSIZ];
+			int bytes = read(ready_pipe[0], buf, BUFSIZ);
+			if (bytes == -1 && errno != EINTR) {
+				eerror("%s: read failed '%s'\n", applet, strerror(errno));
+				exit(EXIT_FAILURE);
+			}
+
+			if (memchr(buf, '\n', bytes))
+				break;
+		}
+	} else if (start_wait > 0) {
 		struct timespec ts;
 		bool alive = false;