Fix #299: add support for service readiness notification

This patch adds service readiness notification to support daemons
employing systemd and s6 notification.  Complementing the native
Finit readiness support using PID files that exist already.

The two have slightly different ways of implementing readiness:

 - https://www.freedesktop.org/software/systemd/man/sd_notify.html
 - https://skarnet.org/software/s6/notifywhenup.html

Finit now provides both a NOTIFY_SOCKET environemnt variable, for
systemd, and a way to start s6 daemons with a descriptor argument.

For details on the syntax, see the `service` documentation.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/service.c

@@ -40,6 +40,7 @@
 #include <wordexp.h>
 
 #include "cgroup.h"
+#include "client.h"
 #include "conf.h"
 #include "cond.h"
 #include "finit.h"
@@ -589,14 +590,37 @@ static int service_start(svc_t *svc)
 	sigprocmask(SIG_BLOCK, &nmask, &omask);
 
 	pid = service_fork(svc);
-	if (pid == 0) {
+	if (pid < 0) {
+		result = -1;
+		goto fail;
+	}
+	if (pid > 1) {
+		svc->pid = pid;
+		svc->start_time = jiffies();
+	} else if (pid == 0) {
 		char *args[MAX_NUM_SVC_ARGS + 1];
 		int status;
 
+		if (!svc_is_tty(svc))
+			redirect(svc);
+
 		if (!svc_is_sysv(svc)) {
 			wordexp_t we = { 0 };
 			int rc;
 
+			if (svc->notify) {
+				if (client_command(INIT_CMD_NOTIFY_SOCKET)) {
+					err(1, "%s: failed creating notify socket", svc_ident(svc, NULL, 0));
+					client_disconnect();
+					svc->notify = 0; /* does not change parent, restarting may work. */
+				} else {
+					char val[20];
+
+					snprintf(val, sizeof(val), "%d", client_socket());
+					setenv("NOTIFY_SOCKET", val, 1);
+				}
+			}
+
 			if ((rc = wordexp(svc->cmd, &we, 0))) {
 				errx(1, "%s: failed wordexp(%s): %d", svc_ident(svc, NULL, 0), svc->cmd, rc);
 			nomem:
@@ -613,6 +637,19 @@ static int service_start(svc_t *svc)
 				if (len == 0)
 					break;
 
+				if (svc->notify) {
+					char *ptr = strstr(arg, "%n");
+
+					if (ptr) {
+						len = snprintf(str, sizeof(str), "%d", client_socket());
+						if (len > 0 && len <= 2) {
+							ptr[0] = ' ';
+							ptr[1] = ' ';
+							memcpy(ptr, str, len);
+						}
+					}
+				}
+
 				/*
 				 * Escape forbidden characters in wordexp()
 				 * but allowed in Finit run/task stanzas,
@@ -678,8 +715,6 @@ static int service_start(svc_t *svc)
 		if (pid < 1)
 			logit(LOG_ERR, "failed setsid(), pid %d: %s", pid, strerror(errno));
 
-		if (!svc_is_tty(svc))
-			redirect(svc);
 		sig_unblock();
 
 		if (svc_is_runtask(svc))
@@ -702,9 +737,6 @@ static int service_start(svc_t *svc)
 	if (!svc_is_sysv(svc))
 		logit(LOG_CONSOLE | LOG_NOTICE, "Starting %s[%d]", svc_ident(svc, NULL, 0), pid);
 
-	svc->pid = pid;
-	svc->start_time = jiffies();
-
 	switch (svc->type) {
 	case SVC_TYPE_RUN:
 		svc->status = complete(svc->cmd, pid);
@@ -728,6 +760,7 @@ static int service_start(svc_t *svc)
 		break;
 	}
 
+fail:
 	sigprocmask(SIG_SETMASK, &omask, NULL);
 	if (do_progress)
 		print_result(result);
@@ -779,6 +812,13 @@ static void service_cleanup(svc_t *svc)
 		logit(LOG_CRIT, "Failed removing service %s pidfile %s",
 		      svc_ident(svc, NULL, 0), fn);
 
+	/* Ensure we don't have any notify socket lingering */
+	if (svc->notify && svc->notify_watcher.fd > 0) {
+		uev_io_stop(&svc->notify_watcher);
+		close(svc->notify_watcher.fd);
+		svc->notify_watcher.fd = 0;
+	}
+
 	/* No longer running, update books. */
 	if (svc_is_tty(svc) && svc->pid > 1)
 		utmp_set_dead(svc->pid); /* Set DEAD_PROCESS UTMP entry */
@@ -1021,6 +1061,15 @@ static void parse_log(svc_t *svc, char *arg)
 	}
 }
 
+static int parse_notify(char *arg)
+{
+	if (!strcmp(arg, "systemd"))
+		return 1;
+	if (!strcmp(arg, "s6"))
+		return 2;
+	return 0;		/* unsupported/disabled */
+}
+
 static void parse_env(svc_t *svc, char *env)
 {
 	if (!env)
@@ -1267,6 +1316,7 @@ int service_register(int type, char *cfg, struct rlimit rlimit[], char *file)
 	char *id = NULL, *env = NULL, *cgroup = NULL;
 	char *pre_script = NULL, *post_script = NULL;
 	char *ready_script = NULL;
+	char *notify = NULL;
 	struct tty tty = { 0 };
 	char *dev = NULL;
 	int respawn = 0;
@@ -1332,6 +1382,8 @@ int service_register(int type, char *cfg, struct rlimit rlimit[], char *file)
 			pid = cmd;
 		else if (MATCH_CMD(cmd, "name:", arg))
 			name = cmd;
+		else if (MATCH_CMD(cmd, "notify:", arg))
+			notify = arg;
 		else if (MATCH_CMD(cmd, "type:forking", arg))
 			forking = 1;
 		else if (MATCH_CMD(cmd, "manual:yes", arg))
@@ -1523,6 +1575,17 @@ int service_register(int type, char *cfg, struct rlimit rlimit[], char *file)
 		parse_script("ready", ready_script, svc->ready_script, sizeof(svc->ready_script));
 	if (log)
 		parse_log(svc, log);
+	if (notify) {
+		int type = parse_notify(notify);
+
+		if (type <= 0)
+			goto disable_notify;
+		svc->notify = type;
+	} else if (svc->notify) {
+	disable_notify:
+		/* XXX: close existing socket */
+		svc->notify = 0;
+	}
 	if (desc)
 		strlcpy(svc->desc, desc, sizeof(svc->desc));
 	else if (type == SVC_TYPE_TTY)
@@ -2008,6 +2071,12 @@ restart:
 			char condstr[MAX_COND_LEN];
 
 			dbg("%s: stopped, cleaning up timers and conditions ...", svc_ident(svc, NULL, 0));
+			if (svc->notify && svc->notify_watcher.fd > 0) {
+				uev_io_stop(&svc->notify_watcher);
+				close(svc->notify_watcher.fd);
+				svc->notify_watcher.fd = 0;
+			}
+
 			service_timeout_cancel(svc);
 			cond_clear(mkcond(svc, condstr, sizeof(condstr)));
 
@@ -2144,6 +2213,11 @@ restart:
 
 				svc_mark_clean(svc);
 			}
+			if (svc->notify == 2 && svc->notify_watcher.fd == 0) {
+				char buf[120];
+				snprintf(buf, sizeof(buf), "service/%s/ready", svc_ident(svc, NULL, 0));
+				cond_set(buf);
+			}
 			break;
 		}
 		break;
@@ -2274,6 +2348,43 @@ int service_completed(void)
 	return 1;
 }
 
+/*
+ * Called when a service sends readiness notification, or when
+ * the service closes its end of the IPC connection.
+ */
+void service_notify_cb(uev_t *w, void *arg, int events)
+{
+	svc_t *svc = (svc_t *)arg;
+	char buf[120];
+	ssize_t len;
+
+	if (UEV_ERROR == events) {
+		warn("Spurious problem with %s notify callback, restarting.", svc_ident(svc, NULL, 0));
+		uev_io_start(w);
+		return;
+	}
+
+	len = read(w->fd, buf, sizeof(buf) - 1);
+	if (len == -1) {
+		warn("Failed reading notification from %s", svc_ident(svc, NULL, 0));
+		return;
+	}
+
+	buf[len] = 0;
+
+	/* systemd and s6, respectively.  The latter then closes the socket */
+	if (!strcmp(buf, "READY=1\n") || !strcmp(buf, "\n")) {
+		snprintf(buf, sizeof(buf), "service/%s/ready", svc_ident(svc, NULL, 0));
+		cond_set(buf);
+		/* s6 applications close their socket after notification */
+		if (svc->notify == 2) {
+			uev_io_stop(w);
+			close(w->fd);
+			w->fd = 0;
+		}
+	}
+}
+
 /*
  * Every five¹ minutes we sweep over all services, skipping crashed or
  * otherwise no longer running ones.  Decrement non-zero crash counters