Replace experimental emergency shell with actual rescue mode

Most major init systems have a way to start a rescue shell.  This is
usally started when the string `rescue` is read from the kernel's
command line.

The traditional Finit way of handling this has been to skip certain
phases of the standard boot, skip /etc/finit.conf et al, and go for a
/lib/finit/rescue.conf instead.  This pratice has been imroved over
the last few commits with a sulogin.  However, there was no way to
resume boot, like most other init systems offer.

This patch implements a very simple rescue mode, utilizing the new
sulogin, replacing the old (and experimental) emergency shell.  If
Ctrl-D is pressed at the maintenance login prompt the boot is now
resumed.  The same goes for exit/Ctrl-D from the maintenace shell.

The preferred sulogin is the bundled /libexec/finit/sulogin, but
if that isn't installed, the first one in $PATH is used.  If no
sulogin at all is found, Finit proceeds as before this patch, to
skip certains steps and go for a /lib/finit/rescue.conf.  Hence,
al steps can be controlled by an administrator.

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

configure.ac

@@ -48,11 +48,6 @@ PKG_CHECK_MODULES([uev],  [libuev >= 2.2.0])
 PKG_CHECK_MODULES([lite], [libite >= 2.2.0])
 
 # Check for configured Finit features
-AC_ARG_ENABLE(emergency_shell,
-	AS_HELP_STRING([--enable-emergency-shell], [Experimental emergency fallback at boot.
-	                Use only for debugging very early boot problems with Finit]),,[
-	enable_emergency_shell=no])
-
 AC_ARG_ENABLE(auto_reload,
         AS_HELP_STRING([--enable-auto-reload], [Automatic reload on any .conf change]),,[
 	enable_auto_reload=no])
@@ -127,9 +122,6 @@ AC_ARG_WITH(watchdog,
 ### Enable features ###########################################################################
 
 # Create config.h from selected features and fallback defaults
-AS_IF([test "x$enable_emergency_shell" = "xyes"], [
-        AC_DEFINE(EMERGENCY_SHELL,  1, [Fork off init and let PID 1 wait and start /bin/sh if init crashes.])])
-
 AS_IF([test "x$enable_auto_reload" = "xyes"], [
         AC_DEFINE(AUTO_RELOAD, 1, [Finit automatically does `initctl reload` on any .conf change])])
 
@@ -253,7 +245,6 @@ Optional features:
   Built-in watchdogd....: $with_watchdog $watchdog
   Built-in logrotate....: $enable_logrotate
   Scripting tool logit..: $enable_logit
-  Emergency shell.......: $enable_emergency_shell
   Redirect output.......: $enable_redirect
   Default hostname......: $hostname
   Default runlevel......: $runlevel

src/finit.c

@@ -235,48 +235,6 @@ static void fs_init(void)
 	}
 }
 
-/*
- * If everything goes south we can use this to give the operator an
- * emergency shell to debug the problem -- Finit should not crash!
- *
- * Note: Only use this for debugging a new Finit setup, don't use
- *       this in production since it gives a root shell to anyone
- *       if Finit crashes.
- *
- * This emergency shell steps in to prevent "Aieee, PID 1 crashed"
- * messages from the kernel, which usually results in a reboot, so
- * that the operator instead can debug the problem.
- */
-static void emergency_shell(void)
-{
-#ifdef EMERGENCY_SHELL
-	pid_t pid;
-
-	pid = fork();
-	if (pid) {
-		while (1) {
-			pid_t id;
-
-			/* Reap 'em (prevents Zombies) */
-			id = waitpid(-1, NULL, WNOHANG);
-			if (id == pid)
-				break;
-		}
-
-		fprintf(stderr, "\n=> Embarrassingly, Finit has crashed.  Check /dev/kmsg for details.\n");
-		fprintf(stderr,   "=> To debug, add 'debug' to the kernel command line.\n\n");
-
-		/*
-		 * Become session leader and set controlling TTY
-		 * to enable Ctrl-C and job control in shell.
-		 */
-		setsid();
-		ioctl(STDIN_FILENO, TIOCSCTTY, 1);
-
-		execl(_PATH_BSHELL, _PATH_BSHELL, NULL);
-	}
-#endif /* EMERGENCY_SHELL */
-}
 
 /*
  * Handle bootstrap transition to configured runlevel, start TTYs
@@ -505,13 +463,24 @@ int main(int argc, char *argv[])
 	if (chdir("/"))
 		_pe("Failed cd /");
 
-	/* Allow progress, if enabled */
-	enable_progress(1);
-
 	/*
 	 * In case of emergency.
 	 */
-	emergency_shell();
+	if (rescue) {
+		char *sulogin[] = {
+			_PATH_SULOGIN,
+			"sulogin",
+		};
+		size_t i;
+
+		for (i = 0; i < NELEMS(sulogin); i++) {
+			if (systemf(sulogin[i]))
+				continue;
+
+			rescue = 0;
+			break;
+		}
+	}
 
 	/*
 	 * Load plugins early, the first hook is in banner(), so we
@@ -522,6 +491,7 @@ int main(int argc, char *argv[])
 	/*
 	 * Hello world.
 	 */
+	enable_progress(1);	/* Allow progress, if enabled */
 	banner();
 
 	/*