Function MakeAutoStartingGenerator

cpp/src/arrow/util/async_generator.h:721–738

/// This generator buffers one item (the first result) until it is delivered.
template <typename T>
AsyncGenerator<T> MakeAutoStartingGenerator(AsyncGenerator<T> generator) {
  struct AutostartGenerator {
    Future<T> operator()() {
      if (first_future->is_valid()) {
        Future<T> result = *first_future;
        *first_future = Future<T>();
        return result;
      }
      return source();
    }

    std::shared_ptr<Future<T>> first_future;
    AsyncGenerator<T> source;
  };

  std::shared_ptr<Future<T>> first_future = std::make_shared<Future<T>>(generator());
  return AutostartGenerator{std::move(first_future), std::move(generator)};
}

/// \see MakeReadaheadGenerator
template <typename T>