Fix #378: add run/task support for <!> to allow transition from bootstrap

This odd little feature makes it possible to declare run/tasks with a
condition that does not block Finit transitioning from bootstrap to the
next runlevel.

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

doc/conditions.md

@@ -27,6 +27,16 @@ specified separated by comma.  Multiple conditions are logically AND'ed
 during evaluation, i.e. all conditions must be satisfied in order for a
 service to run.
 
+A special syntax, using a leading `!` in run/task/service conditions,
+denote if a:
+
+ - service does not support `SIGHUP`
+ - run/task should not block runlevel changes (i.e., bootstrap)
+
+Finit guarantees by default that all run/tasks run (at least) once
+per runlevel.  For most tasks this is a good default, for example
+checking SSH host keys or loading keymap.  However, for conditions
+that are unlikely to happen it is not. (See example below.)
 
 ### Example
 
@@ -41,6 +51,19 @@ being created, i.e., the service's default readiness notification.
   asserted.  I.e., if the Zebra process above stops or restarts, netd
   will also stop or restart.
 
+Another example is `dropbear`, it does not support `SIGHUP`, but we can
+also see optional sourcing of arguments from an environment file:
+
+    service [2345789] <!> env:-/etc/default/dropbear dropbear -F -R $DROPBEAR_ARGS -- Dropbear SSH daemon
+
+Finally, the weird "block runlevel changes" example.  Here we see what
+happens when Finit receives `SIGPWR`, sent from a power daemon like
+[powstatd(8)][].  A condition is asserted and a user can set up their
+own task to act on it.  We do not want this task to block Finit from
+moving to the next runlevel after bootstrap, so we set `<!>`:
+
+    task [S0123456789] <!sys/pwr/fail> name:pwrfail initctl poweroff -- Power failure, shutting down
+
 
 Triggering
 ----------
@@ -260,3 +283,5 @@ Therefore, any plugin that supplies Finit with conditions must ensure
 that their state is updated after each reconfiguration.  This can be
 done by binding to the `HOOK_SVC_RECONF` hook.  For an example of how
 to do this, see `plugins/pidfile.c`.
+
+[powstatd(8)]: https://manpages.ubuntu.com/manpages/trusty/en/man8/powstatd.8.html

src/conf.c

@@ -656,11 +656,22 @@ void conf_parse_cond(svc_t *svc, char *cond)
 		return;
 	}
 
-	/* First character must be '!' if SIGHUP is not supported. */
+	/*
+	 * First character must be '!' if:
+	 *   - service:  SIGHUP is not supported
+	 *   - run/task: Do not block bootstrap
+	 */
 	ptr = cond;
 	if (ptr[i] == '!') {
-		svc->sighup = 0;
 		ptr++;
+
+		if (svc_is_runtask(svc)) {
+			/* see service_runtask_clean() */
+			svc->sighup = 1;
+			svc->once = 1;
+		} else {
+			svc->sighup = 0;
+		}
 	}
 
 	while (ptr[i] != '>' && ptr[i] != 0)

src/service.c

@@ -1414,7 +1414,11 @@ static void parse_cmdline_args(svc_t *svc, char *cmd, char **args)
  * before they are started.  Or restarted, or even SIGHUP'ed, when the
  * gateway changes or interfaces come and go.  The special case when a
  * service is declared with <!> means it does not support SIGHUP but
- * must be STOP/START'ed at system reconfiguration.
+ * must be STOP/START'ed at system reconfiguration.  For run/task a <!>
+ * means Finit can relax its promise to run at least once per runlevel.
+ * I.e., for run/task conditions that would otherwise block bootstrap:
+ *
+ *     task [S0123456789] <!sys/pwr/fail> name:pwrfail initctl poweroff -- Power failure, shutting down
  *
  * Conditions can for example be: pid/NAME:ID for process dependencies,
  * net/<IFNAME>/up or net/<IFNAME>/exists.  The condition handling is
@@ -2500,7 +2504,7 @@ restart:
 
 		case COND_ON:
 			if (svc_is_changed(svc)) {
-				if (svc_nohup(svc))
+				if (svc_nohup(svc) || !svc_is_daemon(svc))
 					service_stop(svc);
 				else {
 					/*
@@ -2601,7 +2605,12 @@ void service_runtask_clean(void)
 		if (!svc_is_runtask(svc))
 			continue;
 
-		svc->once = 0;
+		/* run/task declared with <!> */
+		if (svc->sighup)
+			svc->once = 1;
+		else
+			svc->once = 0;
+
 		if (svc->state == SVC_DONE_STATE)
 			svc_set_state(svc, SVC_HALTED_STATE);
 	}