weston-launch: reset tty properly

On weston-launch exit we see errors such as:
  failed to restore keyboard mode: Invalid argument
  failed to set KD_TEXT mode on tty: Invalid argument

This has been resolved by making sure the tty file descriptor
does not get closed. However, the ioctrl's KDSKBMODE/KDSETMODE
and VT_SETMODE still fail with -EIO:
  failed to restore keyboard mode: Input/output error
  failed to set KD_TEXT mode on tty: Input/output error

It turns out the reason for this lies in some very particular
behavior of the kernel, the separation of weston-launch/weston
and the fact that we restore the tty only after the weston
process quits: When the controlling process for a TTY exits,
all open file descriptors for that TTY are put in a hung-up
state! For more details see this systemd-logind issue:
https://github.com/systemd/systemd/issues/989

We can work around by reopening the particular TTY. This allows
to properly restore the TTY settings such that a successive VT
switch will show text terminals fine again.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>

libweston/weston-launch.c

@@ -440,6 +440,7 @@ quit(struct weston_launch *wl, int status)
 {
 	struct vt_mode mode = { 0 };
 	int err;
+	int oldtty;
 
 	close(wl->signalfd);
 	close(wl->sock[0]);
@@ -452,6 +453,19 @@ quit(struct weston_launch *wl, int status)
 		pam_end(wl->ph, err);
 	}
 
+	/*
+	 * Get a fresh handle to the tty as the previous one is in
+	 * hang-up state since weston (the controlling process for
+	 * the tty) exit at this point. Reopen before closing the
+	 * file descriptor to avoid a potential race condition.
+	 *
+	 * A similar fix exists in logind, see:
+	 * https://github.com/systemd/systemd/pull/990
+	 */
+	oldtty = wl->tty;
+	wl->tty = open_tty_by_number(wl->ttynr);
+	close(oldtty);
+
 	if (ioctl(wl->tty, KDSKBMUTE, 0) &&
 	    ioctl(wl->tty, KDSKBMODE, wl->kb_mode))
 		fprintf(stderr, "failed to restore keyboard mode: %s\n",