  Http::AsyncClient::Callbacks* popPendingCallback() {
    if (callbacks_.empty()) {
      // Can't use ASSERT_* as this is not a test function
      throw std::underflow_error("empty deque");
    }

    auto callbacks = callbacks_.front();
    callbacks_.pop_front();
    return callbacks;
  }
  Http::AsyncClient::Callbacks* popPendingCallback() {
    if (callbacks_.empty()) {
       
      throw std::underflow_error("empty deque");
    }

    auto callbacks = callbacks_.front();
    callbacks_.pop_front();
    return callbacks;
  }