westonpace commented on a change in pull request #9533:
URL: https://github.com/apache/arrow/pull/9533#discussion_r592255544
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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
+ ARROW_RETURN_NOT_OK(state_->Pump(state_));
+ }
+ return next;
+ }
+
+ private:
+ struct State {
+ State(AsyncGenerator<T> source_, int max_readahead)
+ : first(true),
+ source(std::move(source_)),
+ finished(false),
+ spaces_available(max_readahead),
+ readahead_queue(max_readahead) {}
+
+ Status Pump(const std::shared_ptr<State>& self) {
+ // Can't do readahead_queue.write(source().Then(Callback{self})) because
then the
+ // callback might run immediately and add itself to the queue before
this gets added
+ // to the queue messing up the order
+ auto next_slot = std::make_shared<Future<T>>();
+ auto written = readahead_queue.Write(next_slot);
+ if (!written) {
+ return Status::UnknownError("Could not write to readahead_queue");
+ }
+ *next_slot = source().Then(Callback{self});
+ return Status::OK();
+ }
+
+ // Only accessed by the consumer end
+ bool first;
+ // Accessed by both threads
+ AsyncGenerator<T> source;
+ std::atomic<bool> finished;
+ std::atomic<uint32_t> spaces_available;
+ util::SpscQueue<std::shared_ptr<Future<T>>> readahead_queue;
Review comment:
We have to set the value of the future after it is added to the queue.
We want the value to be the value returned by `source().Then()` but we can't
run `Then()` until after we add it to the queue. Otherwise, if `Then` triggers
synchronously it adds itself to the queue and we get items out of order.
So we use a queue of shared pointers so we can add a pointer to the queue
and set the value of that pointer later. That's also why it's called
`next_slot` instead of `next` because we are adding a slot for the future and
then we go and get the future.
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