Add support for optional reboot delay

Slightly different take on issue #334 making it optional, possible to
enable per system.

    reboot-delay <0-60>           # default: 0 (disabled)

When enabled (non-zero), runs after filesystems have been unmounted,
the root filesystems has been remounted read-only, and sync(2) has
been called, twice.

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

doc/config.md

@@ -850,6 +850,27 @@ Log rotation is controlled using the global `log` setting.
     service log:prio:user.warn,tag:ntpd /sbin/ntpd pool.ntp.org -- NTP daemon
 
 
+### Misc Settings
+
+**Syntax:** `reboot-delay <0-60>`
+
+Optional delay at reboot (or shutdown or halt) to allow kernel
+filesystem threads to complete after calling `sync(2)` before
+rebooting.  This applies primarily to filesystems that do not
+have a reboot notifier implemented.  At the point of writing,
+the only known filesystems affected are: ubifs, jffs2.
+
+*Default:* 0 (disabled)
+
+When enabled (non-zeoro), this delay runs after file systems have been
+unmounted and the root filesystem has been remounted read-only, and
+sync(2) has been called, twice.
+
+> "On Linux, sync is only guaranteed to schedule the dirty blocks for
+> writing; it can actually take a short time before all the blocks are
+> finally written.
+
+
 Rescue Mode
 -----------
 

src/conf.c

@@ -764,6 +764,11 @@ static int parse_static(char *line, int is_rcsd)
 		return 0;
 	}
 
+	if (MATCH_CMD(line, "reboot-delay ", x)) {
+		syncsec = strtonum(strip_line(x), 0, 60, NULL);
+		return 0;
+	}
+
 	/*
 	 * Periodic check and instability index leveler, seconds
 	 */

src/finit.c

@@ -70,6 +70,7 @@ int   rescue    = 0;		/* rescue mode from kernel cmdline */
 int   single    = 0;		/* single user mode from kernel cmdline */
 int   bootstrap = 1;		/* set while bootrapping (for TTYs) */
 int   kerndebug = 0;		/* set if /proc/sys/kernel/printk > 7 */
+int   syncsec   = 0;		/* reboot delay */
 char *finit_conf= NULL;
 char *finit_rcsd= NULL;
 char *fstab     = NULL;

src/finit.h

@@ -118,6 +118,7 @@ extern int    rescue;
 extern int    single;
 extern int    bootstrap;
 extern int    kerndebug;
+extern int    syncsec;
 extern char  *fstab;
 extern char  *sdown;
 extern char  *network;

src/sig.c

@@ -382,6 +382,20 @@ void do_shutdown(shutop_t op)
 	print(0, "Calling hook/svc/down scripts ...");
 	plugin_run_hooks(HOOK_SYS_DN);
 
+	/*
+	 * Optional reboot-delay, needed on systems running ubifs, jffs,
+	 * or similar.  From fileutils-4.0, sync(8); "On Linux, sync is
+	 * only guaranteed to schedule the dirty blocks for writing; it
+	 * can actually take a short time before all the blocks are
+	 * finally written." -- https://linux.die.net/man/8/sync
+	 */
+	sync();
+	if (syncsec > 0) {
+		print(-1, "Reboot delay, waiting for filesystem sync, %d sec", syncsec);
+		do_sleep(syncsec);
+		print(0, NULL);
+	}
+
 	/* Reboot via watchdog or kernel, or shutdown? */
 	if (op == SHUT_REBOOT) {
 		print(0, "Rebooting ...");