Method OneTaskDone

cpp/src/arrow/util/task_group.cc:177–202

  }

  void OneTaskDone() {
    // Can be called unlocked thanks to atomics
    auto nremaining = nremaining_.fetch_sub(1, std::memory_order_release) - 1;
    DCHECK_GE(nremaining, 0);
    if (nremaining == 0) {
      // Take the lock so that ~ThreadedTaskGroup cannot destroy cv
      // before cv.notify_one() has returned
      std::unique_lock<std::mutex> lock(mutex_);
      cv_.notify_one();
      if (completion_future_.has_value()) {
        // MarkFinished could be slow.  We don't want to call it while we are holding
        // the lock.
        auto& future = *completion_future_;
        const auto finished = completion_future_->is_finished();
        const auto& status = status_;
        // This will be redundant if the user calls Finish and not FinishAsync
        if (!finished && !finished_) {
          finished_ = true;
          lock.unlock();
          future.MarkFinished(status);
        } else {
          lock.unlock();
        }
      }
    }
  }

  // These members are usable unlocked
  Executor* executor_;