misc, *-daemon: Implement s6-style readiness notification.

Adds two helper functions, and allows scripts to wait until services are
ready before proceeding with it's dependees by setting `ready=fd:<num>`,
where <num> is the file descriptor the daemon should write a new line to.

src/shared/misc.c

@@ -521,6 +521,46 @@ pid_t get_pid(const char *applet,const char *pidfile)
 	return pid;
 }
 
+struct ready ready_parse(const char *applet, const char *ready_string)
+{
+	struct ready ready = {0};
+	if (sscanf(ready_string, "fd:%d", &ready.fd) != 1)
+		eerrorx("%s: invalid ready '%s'.", applet, optarg);
+
+	ready.type = READY_FD;
+
+	if (pipe(ready.pipe) == -1)
+		eerrorx("%s: pipe failed: %s", applet, strerror(errno));
+
+	return ready;
+}
+
+bool ready_wait(const char *applet, struct ready ready)
+{
+	if (ready.type == READY_NONE)
+		return true;
+
+	close(ready.pipe[1]);
+	ready.fd = ready.pipe[0];
+
+	for (;;) {
+		char buf[BUFSIZ];
+		ssize_t bytes = read(ready.fd, buf, BUFSIZ);
+		if (bytes == -1) {
+			if (errno != EINTR) {
+				eerror("%s: read failed '%s'\n", applet, strerror(errno));
+				return false;
+			}
+			continue;
+		}
+
+		if (memchr(buf, '\n', bytes))
+			break;
+	}
+
+	return true;
+}
+
 #ifndef HAVE_CLOSE_RANGE
 static inline int close_range(int first RC_UNUSED,
 			      int last RC_UNUSED,

src/shared/misc.h

@@ -66,4 +66,17 @@ pid_t get_pid(const char *applet, const char *pidfile);
 
 void cloexec_fds_from(int);
 
+struct ready {
+	enum {
+		READY_NONE = 0,
+		READY_FD,
+	} type;
+
+	int pipe[2];
+	int fd;
+};
+
+struct ready ready_parse(const char *applet, const char *ready_string);
+bool ready_wait(const char *applet, struct ready ready);
+
 #endif

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;
+	struct ready ready;
+	int ret = EXIT_SUCCESS;
 
 	applet = basename_c(argv[0]);
 	atexit(cleanup);
@@ -618,6 +622,10 @@ int main(int argc, char **argv)
 			sscanf(optarg, "%d", &sched_prio);
 			break;
 
+		case LONGOPT_READY:
+			ready = ready_parse(applet, optarg);
+			break;
+
 		case_RC_COMMON_GETOPT
 		}
 
@@ -1100,6 +1108,11 @@ int main(int argc, char **argv)
 
 		cloexec_fds_from(3);
 
+		if (ready.type == READY_FD) {
+			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 +1197,11 @@ int main(int argc, char **argv)
 			start_wait = 0;
 	}
 
-	if (start_wait > 0) {
+	/* TODO: timeout on waiting readiness. */
+	if (ready.type != READY_NONE) {
+		if (!ready_wait(applet, ready))
+			ret = EXIT_FAILURE;
+	} else if (start_wait > 0) {
 		struct timespec ts;
 		bool alive = false;
 
@@ -1224,6 +1241,6 @@ int main(int argc, char **argv)
 		rc_service_daemon_set(svcname, exec,
 		    (const char *const *)margv, pidfile, true);
 
-	exit(EXIT_SUCCESS);
+	exit(ret);
 	/* NOTREACHED */
 }

src/supervise-daemon/supervise-daemon.c

@@ -82,6 +82,7 @@ enum {
   LONGOPT_SECBITS,
   LONGOPT_STDERR_LOGGER,
   LONGOPT_STDOUT_LOGGER,
+  LONGOPT_READY,
 };
 
 const char *applet = NULL;
@@ -116,6 +117,7 @@ const struct option longopts[] = {
 	{ "stdout-logger",1, NULL, LONGOPT_STDOUT_LOGGER},
 	{ "stderr-logger",1, NULL, LONGOPT_STDERR_LOGGER},
 	{ "reexec",       0, NULL, '3'},
+	{ "ready",        1, NULL, LONGOPT_READY},
 	longopts_COMMON
 };
 const char * const longopts_help[] = {
@@ -165,6 +167,7 @@ static int devnull_fd = -1;
 static int stdin_fd;
 static int stdout_fd;
 static int stderr_fd;
+static struct ready ready;
 static char *redirect_stderr = NULL;
 static char *redirect_stdout = NULL;
 static char *stderr_process = NULL;
@@ -588,6 +591,9 @@ RC_NORETURN static void child_process(char *exec, char **argv)
 
 	cloexec_fds_from(3);
 
+	if (ready.type == READY_FD)
+		dup2(ready.pipe[1], ready.fd);
+
 	cmdline = make_cmdline(argv);
 	syslog(LOG_INFO, "Child command line: %s", cmdline);
 	free(cmdline);
@@ -1068,6 +1074,10 @@ int main(int argc, char **argv)
 			stderr_process = optarg;
 			break;
 
+		case LONGOPT_READY:
+			ready = ready_parse(applet, optarg);
+			break;
+
 		case_RC_COMMON_GETOPT
 		}
 
@@ -1205,19 +1215,25 @@ int main(int argc, char **argv)
 		if (child_pid == -1)
 			eerrorx("%s: fork: %s", applet, strerror(errno));
 		if (child_pid != 0)
-			/* first parent process, do nothing. */
-			exit(EXIT_SUCCESS);
+			exit(ready_wait(applet, ready) ? EXIT_SUCCESS : EXIT_FAILURE);
+
 #ifdef TIOCNOTTY
 		tty_fd = open("/dev/tty", O_RDWR);
 #endif
 		devnull_fd = open("/dev/null", O_RDWR);
 		dup2(devnull_fd, STDIN_FILENO);
 		dup2(devnull_fd, STDOUT_FILENO);
 		dup2(devnull_fd, STDERR_FILENO);
+
+		if (ready.type == READY_FD)
+			close(ready.pipe[0]);
+
 		child_pid = fork();
 		if (child_pid == -1)
 			eerrorx("%s: fork: %s", applet, strerror(errno));
 		else if (child_pid != 0) {
+			if (ready.type == READY_FD)
+				close(ready.pipe[1]);
 			c = argv;
 			x = 0;
 			while (c && *c) {