westonpace commented on a change in pull request #9533:
URL: https://github.com/apache/arrow/pull/9533#discussion_r592263787
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File path: cpp/src/arrow/util/async_generator.h
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@@ -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:
So there are two ends to this operator, the consumer and the producer.
The consumer pulls items off the readahead queue while the producer is putting
items on the readahead queue. When the producer adds an item they check to see
if they filled up the queue. If they did then they stop (and it will need to
be restarted here). If they did not then they call State::Pump and start
reading the next item.
If the producer stopped and the consumer later grabs an item off the queue
then it needs to start the process up again and call State::Pump which is what
is happening here.
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