void Filter::UpstreamCallbacks::drain(Drainer& drainer) {
  ASSERT(drainer_ == nullptr); // This should only get set once.
  drainer_ = &drainer;
  parent_ = nullptr;
}
void Filter::UpstreamCallbacks::drain(Drainer& drainer) {
  ASSERT(drainer_ == nullptr);  
  drainer_ = &drainer;
  parent_ = nullptr;
}