Method operator()

cpp/src/arrow/util/async_generator_test.cc:108–115

class ReentrantChecker {
 public:
  Future<T> operator()() {
    if (state_->generated_unfinished_future.load()) {
      state_->valid.store(false);
    }
    state_->generated_unfinished_future.store(true);
    auto result = state_->source();
    return result.Then(Callback{state_});
  }

  bool valid() { return state_->valid.load(); }