Method NotifyAll

cpp/src/arrow/acero/test_nodes.cc:237–243

 private:
  void NotifyAll() {
    if (current_waiter_.is_valid()) {
      Future<> to_unlock = current_waiter_;
      current_waiter_ = {};
      to_unlock.MarkFinished();
    }
  }

  Future<> current_waiter_;
  int num_released_batches_ = 0;