Function DoTransfer

cpp/src/arrow/util/thread_pool.h:231–261

  template <typename T, typename FT = Future<T>, typename FTSync = typename FT::SyncType>
  Future<T> DoTransfer(Future<T> future, bool always_transfer = false) {
    auto transferred = Future<T>::Make();
    if (always_transfer) {
      CallbackOptions callback_options = CallbackOptions::Defaults();
      callback_options.should_schedule = ShouldSchedule::Always;
      callback_options.executor = this;
      auto sync_callback = [transferred](const FTSync& result) mutable {
        transferred.MarkFinished(result);
      };
      future.AddCallback(sync_callback, callback_options);
      return transferred;
    }

    // We could use AddCallback's ShouldSchedule::IfUnfinished but we can save a bit of
    // work by doing the test here.
    auto callback = [this, transferred](const FTSync& result) mutable {
      auto spawn_status =
          Spawn([transferred, result]() mutable { transferred.MarkFinished(result); });
      if (!spawn_status.ok()) {
        transferred.MarkFinished(spawn_status);
      }
    };
    auto callback_factory = [&callback]() { return callback; };
    if (future.TryAddCallback(callback_factory)) {
      return transferred;
    }
    // If the future is already finished and we aren't going to force spawn a thread
    // then we don't need to add another layer of callback and can return the original
    // future
    return future;
  }

  // Subclassing API
  virtual Status SpawnReal(TaskHints hints, FnOnce<void()> task, StopToken,