Method MarkFinalError

cpp/src/arrow/util/async_generator.h:1207–1219

This is called outside the mutex but it is only ever called once and Future<>::AddCallback is thread-safe

    // This is called outside the mutex but it is only ever called
    // once and Future<>::AddCallback is thread-safe
    void MarkFinalError(const Status& err, Future<T> maybe_sink) {
      if (maybe_sink.is_valid()) {
        // Someone is waiting for this error so lets mark it complete when
        // all the work is done
        all_finished.AddCallback([maybe_sink, err](const Status& status) mutable {
          maybe_sink.MarkFinished(err);
        });
      } else {
        // No one is waiting for this error right now so it will be delivered
        // next.
        final_error = err;
      }
    }

    bool IsCompleteUnlocked(const util::Mutex::Guard& guard) {
      return outstanding_requests == 0 &&