service: clean stale pidfile after unclean daemon exit

With `pid:!/path` Finit does not manage the file -- the daemon
creates it on start and removes it on graceful exit.  If the daemon
dies before cleanup (SIGKILL, OOM, segfault, exit during startup)
the file lingers and can block the next instance from starting,
e.g. dbus-daemon refuses with EEXIST and the restart loop fails.

Remove the file when it still names the just-reaped PID and that
PID is no longer alive (the liveness check guards against reuse).
Called from service_cleanup(), and from service_monitor()'s
forking+starting branch where cleanup was previously skipped.

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

src/service.c

@@ -1120,6 +1120,35 @@ static void service_notify_stop(svc_t *svc)
 	}
 }
 
+/*
+ * Drop a daemon-owned (pid:!) pidfile if it still names the just-reaped
+ * PID and that PID is gone.  The liveness check guards against reuse.
+ */
+static void service_clean_pidfile(svc_t *svc, pid_t reaped)
+{
+	pid_t pid;
+	char *fn;
+
+	if (reaped <= 1)
+		return;
+
+	fn = pid_file(svc);
+	if (!fn)
+		return;
+
+	pid = pid_file_read(fn);
+	if (pid != reaped || pid_alive(pid))
+		return;
+
+	if (remove(fn) && errno != ENOENT) {
+		logit(LOG_CRIT, "Failed removing stale service %s pidfile %s",
+		      svc_ident(svc, NULL, 0), fn);
+		return;
+	}
+
+	dbg("Removed stale service %s pidfile %s", svc_ident(svc, NULL, 0), fn);
+}
+
 /*
  * Clean up any lingering state from dead/killed services
  */
@@ -1137,6 +1166,8 @@ static void service_cleanup(svc_t *svc)
 		if (remove(fn) && errno != ENOENT)
 			logit(LOG_CRIT, "Failed removing service %s pidfile %s",
 			      svc_ident(svc, NULL, 0), fn);
+	} else if (svc->pidfile[0] == '!') {
+		service_clean_pidfile(svc, svc->pid);
 	}
 
 	/*
@@ -2405,7 +2436,10 @@ void service_monitor(pid_t lost, int status)
 	if (svc_is_forking(svc)) {
 		/* Likely start script exiting */
 		if (svc_is_starting(svc)) {
-			svc->pid = 0;	/* Expect no more activity from this one */
+			/* Daemon died before clearing 'starting'; drop any stale pidfile. */
+			service_clean_pidfile(svc, lost);
+			svc->oldpid = lost;	/* So service_retry() logs the real PID */
+			svc->pid = 0;		/* Expect no more activity from this one */
 			goto cont;
 		}