Calling setsid in pre_exec isn't exactly correct, you'd do this to daemonize a process in order to prevent it from getting signals from the process which spawned you exited or its terminal disconnected.

If you read exit(3) you'll see that you also need a controlling terminal for the kernel to send SIGHUP to the processes in your foreground process group.

In python it'd look roughly like:

  master, slave = os.openpty()

  # ensure we can call setsid successfully

  try:

    pid = os.fork()

    if pid > 0:

      os.close(master)

      os.close(slave)

      sys.exit(0)

  finally:

    sys.exit(1)

  os.setsid()
  # we are now session leader and process group leader

  fcntl.ioctl(master, termios.TIOCSCTTY, 0)
  # we now have a controlling pty - closing master will send us a SIGHUP

  os.close(slave)

  # go start spawning subprocesses - if _this_ process is killed, they will receive SIGHUPs. Unless they take themselves out of your session or the foreground process group.