Method Next

cpp/src/arrow/util/thread_pool.h:346–377

      }

      Result<T> Next() {
        executor->Unpause();
        // This call may lead to tasks being scheduled in the serial executor
        Future<T> next_fut = generator();
        next_fut.AddCallback([this](const Result<T>& res) {
          // If we're done iterating we should drain the rest of the tasks in the executor
          if (!res.ok() || IsIterationEnd(*res)) {
            executor->Finish();
            return;
          }
          // Otherwise we will break out immediately, leaving the remaining tasks for
          // the next call.
          executor->Pause();
        });
#ifdef ARROW_ENABLE_THREADING
        // future must run on this thread
        // Borrow this thread and run tasks until the future is finished
        executor->RunLoop();
#else
        next_fut.Wait();
#endif
        if (!next_fut.is_finished()) {
          // Not clear this is possible since RunLoop wouldn't generally exit
          // unless we paused/finished which would imply next_fut has been
          // finished.
          return Status::Invalid(
              "Serial executor terminated before next result computed");
        }
        // At this point we may still have tasks in the executor, that is ok.
        // We will run those tasks the next time through.
        return next_fut.result();
      }

      std::unique_ptr<SerialExecutor> executor;
      std::function<Future<T>()> generator;