Fix #369: allow runlevel change using initctl during bootstrap

This change opens up the runlevel change API from bootstrap.  The twist
is that the change is only queued, i.e., the call `initctl runlevel 9`
during bootstrap only changes the configured runlevel to go to after
bootstrap has completed.

Effectively, this change allows overriding the `runlevel` directive in
/etc/finit.conf without having to change the file.

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

doc/config.md

@@ -164,14 +164,19 @@ everything is OK.
     task [S] /lib/console-setup/console-setup.sh
     service [S12345] env:-/etc/default/rsyslog rsyslogd -n $RSYSLOGD_ARGS
 
-After runlevel S, Finit proceeds to runlevel 2.  (This can be changed in
+After runlevel S, Finit proceeds to runlevel 2.  This can be changed in
-`/etc/finit.conf` using the `runlevel N` directive.)  Before starting
+`/etc/finit.conf` using the `runlevel N` directive, or by a script
-the services in runlevel 2, Finit first stops everything that is not
+running in runlevel S that calls, e.g., `initctl runlevel 9`.  The
-allowed to run in 2, and then brings up networking.  Networking is
+latter is useful if startup scripts detect problems outside of Finit's
-expected to be available in all runlevels except: S, 1 (single user
+control, e.g., critical services/devices missing or hardware problems.
-level), 6, and 0.  Networking is enabled either by the `network script`
+
-directive, or if you have an `/etc/network/interfaces` file, Finit calls
+Before starting the services in runlevel 2, Finit first stops everything
-`ifup -a` -- at the very least the loopback interface is brought up.
+that is not allowed to run in 2, and then brings up networking.
+Networking is expected to be available in all runlevels except: S, 1
+(single user level), 6, and 0.  Networking is enabled either by the
+`network script` directive, or if you have an `/etc/network/interfaces`
+file, Finit calls `ifup -a` -- at the very least the loopback interface
+is brought up.
 
 > **Note:** when moving from runlevel S to 2, all run/task/services that
 > were constrained to runlevel S only are dropped from bookkeeping.  So

man/initctl.8

@@ -240,6 +240,11 @@ Show top-like listing based on cgroups.
 List installed plugins.
 .It Nm Ar runlevel Op Ar 0-9
 Show or set runlevel: 0 halt, 6 reboot.
+.Pp
+If called at boot (runlevel S) to set the runlevel, Finit only schedules
+the change, effectively overriding the configured runlevel from
+.Pa /etc/finit.conf .
+Useful to trigger a fail-safe mode, or similar.
 .It Nm Ar reboot
 Reboot system, default if
 .Cm reboot

src/api.c

@@ -367,7 +367,6 @@ static void api_cb(uev_t *w, void *arg, int events)
 		}
 
 		switch (rq.cmd) {
-		case INIT_CMD_RUNLVL:
 		case INIT_CMD_RELOAD:
 		case INIT_CMD_START_SVC:
 		case INIT_CMD_RESTART_SVC:
@@ -378,7 +377,7 @@ static void api_cb(uev_t *w, void *arg, int events)
 		case INIT_CMD_POWEROFF:
 		case INIT_CMD_SUSPEND:
 			if (IS_RESERVED_RUNLEVEL(runlevel)) {
-				warnx("Unsupported command in runlevel S and 6.");
+				warnx("Unsupported command in runlevel S and 6/0.");
 				goto leave;
 			}
 		default:
@@ -387,6 +386,14 @@ static void api_cb(uev_t *w, void *arg, int events)
 
 		switch (rq.cmd) {
 		case INIT_CMD_RUNLVL:
+			/* Allow changing cfglevel in runlevel S */
+			if (IS_RESERVED_RUNLEVEL(runlevel)) {
+				if (runlevel != INIT_LEVEL) {
+					warnx("Cannot abort runlevel 6/0.");
+					break;
+				}
+			}
+
 			switch (rq.runlevel) {
 			case 's':
 			case 'S':
@@ -400,7 +407,12 @@ static void api_cb(uev_t *w, void *arg, int events)
 					halt = SHUT_OFF;
 				if (lvl == 6)
 					halt = SHUT_REBOOT;
-				service_runlevel(lvl);
+
+				/* User requested change in next runlevel */
+				if (runlevel == INIT_LEVEL)
+					cfglevel = lvl;
+				else
+					service_runlevel(lvl);
 				break;
 
 			default: