TUN_DYING
if ((tp->tun_flags & (TUN_OPEN | TUN_DYING)) != 0) {
tp->tun_flags |= TUN_DYING;
if ((tp->tun_flags & TUN_DYING) != 0) {
MPASS((tp->tun_flags & (TUN_DYING | TUN_TRANSIENT)) != TUN_DYING);
tp->tun_flags |= TUN_DYING;
tp->tun_flags &= ~TUN_DYING;