If the lock file already exists, then fail silently. This will become

Maybe "skip all maintenance steps silently"?

+	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
+		/*
+		 * Another maintenance command is running.
+		 *
+		 * If --auto was provided, then it is likely due to a
+		 * recursive process stack. Do not report an error in
+		 * that case.
+		 */
+		if (!opts->auto_flag && !opts->quiet)
+			error(_("lock file '%s' exists, skipping maintenance"),
+			      lock_path);
+		free(lock_path);
+		return 0;
+	}

As it is, this doesn't seem very silent. :-) If we do want a message to
be printed maybe make it warning instead of error.

Other than that, the idea of having a lock file and the implementation
in this patch look good to me.