criu-ns: capture controlling tty

When we are restoring in new pidns we specifically do setsid() from
criu-ns init so that sids of restored tasks are non-zero in this pidns
and on next dump CRIU would not have problems with zero sids, see [1].

But after this CRIU tries to inherit and setup a tty for the restored
process, and it fails to set it's process group via TIOCSPGRP to be a
foreground group for it's tty, because tty already is a controlling tty
for other session (which we had before setsid).

So to make it restore we need to reset tty to be a controlling tty of
criu-ns init via TIOCSCTTY before calling criu.

Else when restoring first time via criu-ns (from criu-ns dump) we get:

Error (criu/tty.c:689): tty: Failed to set group 40816 on 0: Inappropriate ioctl for device

checkpoint-restore#232 [1]

v2: add why and what comment in code, set controlling tty only for
--shell-job and fail if stdin is not a tty.

Fixes: checkpoint-restore#1893
Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

scripts/criu-ns

@@ -4,6 +4,8 @@ import ctypes.util
 import errno
 import sys
 import os
+import fcntl
+import termios
 
 # <sched.h> constants for unshare
 CLONE_NEWNS = 0x00020000
@@ -124,6 +126,16 @@ def wrap_restore():
     criu_pid = os.fork()
     if criu_pid == 0:
         os.setsid()
+        # Set stdin tty to be a controlling tty of our new session, this is
+        # required by --shell-job option, as for it CRIU would try to set a
+        # process group of restored root task to be a foreground group on the
+        # terminal.
+        if '--shell-job' in restore_args or '-j' in restore_args:
+            if os.isatty(sys.stdin.fileno()):
+                fcntl.ioctl(sys.stdin.fileno(), termios.TIOCSCTTY, 1)
+            else:
+                raise OSError(errno.EINVAL, 'The stdin is not a tty for a --shell-job')
+
         _mount_new_proc()
         run_criu(restore_args)