Finit bringup: add optional fallback shell

This patch adds an `--enable-fallback-shell` option to confiugre.
When enabled Finit will start `/bin/sh` on the console, but only
if there are no active TTYs running in the current runlevel.

With this it will be easier to do board bringup with Finit, but it also
enables using the fallback shell in a rescue mode (runlevel) where no
TTYs are allowed to run.

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

tty.c

@@ -23,6 +23,8 @@
  */
 
 #include <signal.h>
+#include <sys/ioctl.h>
+#include <sys/types.h>
 #include <sys/wait.h>
 #include <lite/lite.h>
 
@@ -34,6 +36,7 @@
 #include "utmp-api.h"
 
 LIST_HEAD(, tty_node) tty_list = LIST_HEAD_INITIALIZER();
+static pid_t fallback = 0;
 
 
 /* tty [!1-9,S] <DEV> [BAUD[,BAUD,...]] [TERM] */
@@ -208,13 +211,54 @@ int tty_enabled(finit_tty_t *tty, int runlevel)
 	return 0;
 }
 
-/* TTY monitor, called by service_monitor() */
+/*
+ * Fallback shell if no TTYs are active
+ */
+int tty_fallback(pid_t lost)
+{
+#ifdef FALLBACK_SHELL
+	if (lost == 1) {
+		if (fallback) {
+			_d("Stopping fallback shell");
+			kill(fallback, SIGKILL);
+			fallback = 0;
+		}
+
+		return 0;
+	}
+
+	if (fallback != lost || tty_num_active()) {
+		_d("Not starting fallback (%d) shell, lost pid %d", fallback, lost);
+		return 0;
+	}
+
+	_d("Starting fallback shell");
+	fallback = fork();
+	if (fallback)
+		return 1;
+
+	/*
+	 * Become session leader and set controlling TTY
+	 * to enable Ctrl-C and job control in shell.
+	 */
+	setsid();
+	ioctl(STDIN_FILENO, TIOCSCTTY, 1);
+
+	_exit(execl(_PATH_BSHELL, _PATH_BSHELL, NULL));
+#endif /* FALLBACK_SHELL */
+
+	return 0;
+}
+
+/*
+ * TTY monitor, called by service_monitor()
+ */
 int tty_respawn(pid_t pid)
 {
 	tty_node_t *entry = tty_find_by_pid(pid);
 
 	if (!entry)
-		return 0;
+		return tty_fallback(pid);
 
 	/* Set DEAD_PROCESS UTMP entry */
 	utmp_set_dead(pid);
@@ -241,6 +285,9 @@ void tty_runlevel(int runlevel)
 		else
 			tty_start(&entry->data);
 	}
+
+	/* Start fallback shell if enabled && no TTYs */
+	tty_fallback(tty_num_active() > 0 ? 1 : 0);
 }
 
 /**