pitrou commented on a change in pull request #9533:
URL: https://github.com/apache/arrow/pull/9533#discussion_r592416358
##########
File path: cpp/src/arrow/util/async_generator.h
##########
@@ -177,6 +179,94 @@ class TransformingGenerator {
std::shared_ptr<TransformingGeneratorState> state_;
};
+template <typename T>
+class SerialReadaheadGenerator {
+ public:
+ SerialReadaheadGenerator(AsyncGenerator<T> source_generator, int
max_readahead)
+ : state_(std::make_shared<State>(std::move(source_generator),
max_readahead)) {}
+
+ Future<T> operator()() {
+ if (state_->first) {
+ // Lazy generator, need to wait for the first ask to prime the pump
+ state_->first = false;
+ auto next = state_->source();
+ return next.Then(Callback{state_});
+ }
+
+ // This generator is not async-reentrant. We won't be called until the
last
+ // future finished so we know there is something in the queue
+ auto finished = state_->finished.load();
+ if (finished && state_->readahead_queue.IsEmpty()) {
+ return Future<T>::MakeFinished(IterationTraits<T>::End());
+ }
+
+ auto next_ptr = state_->readahead_queue.FrontPtr();
+ auto next = std::move(**next_ptr);
+ state_->readahead_queue.PopFront();
+
+ auto last_available = state_->spaces_available.fetch_add(1);
+ if (last_available == 0 && !finished) {
+ // Reader idled out, we need to restart it
Review comment:
Makes sense indeed. Thanks for the explanation!
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