How to quit when V4L2 poll

@ndufresne I found below code will be called when gst_v4l2_object_unlock(). I don't know why below function can interrupt gst_poll_wait()? I checked coda v4l2 driver. The write function is NULL.

static gboolean
wake_event (GstPoll * set)
{
  ssize_t num_written;
  while ((num_written = write (set->control_write_fd.fd, "W", 1)) != 1) {
    if (num_written == -1 && errno != EAGAIN && errno != EINTR) {
      g_critical ("%p: failed to wake event: %s", set, strerror (errno));
      return FALSE;
    }
  }
  return TRUE;
}