westonpace commented on a change in pull request #9643:
URL: https://github.com/apache/arrow/pull/9643#discussion_r593123657
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File path: cpp/src/arrow/util/async_generator.h
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@@ -177,6 +364,126 @@ class TransformingGenerator {
std::shared_ptr<TransformingGeneratorState> state_;
};
+/// \brief Transforms an async generator using a transformer function
returning a new
+/// AsyncGenerator
+///
+/// The transform function here behaves exactly the same as the transform
function in
+/// MakeTransformedIterator and you can safely use the same transform function
to
+/// transform both synchronous and asynchronous streams.
+///
+/// This generator is not async-reentrant
+///
+/// This generator may queue up to 1 instance of T
+template <typename T, typename V>
+AsyncGenerator<V> MakeAsyncGenerator(AsyncGenerator<T> generator,
+ Transformer<T, V> transformer) {
+ return TransformingGenerator<T, V>(generator, transformer);
+}
+
+/// \see MakeSerialReadaheadGenerator
+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;
+ };
+
+ struct Callback {
+ Result<T> operator()(const Result<T>& maybe_next) {
+ if (!maybe_next.ok()) {
+ state_->finished.store(true);
+ return maybe_next;
+ }
+ const auto& next = *maybe_next;
+ if (IterationTraits<T>::IsEnd(next)) {
+ state_->finished = true;
+ return maybe_next;
+ }
+ auto last_available = state_->spaces_available.fetch_sub(1);
+ if (last_available > 1) {
Review comment:
There might be an off-by-1 here that I'll test out tomorrow. I agree
that `max_readahead == 1` should mean the first callback pumps. It's maybe as
simple as starting `spaces_available` with `max_readahead + 1` (in which case
`spaces_available` would be interpreted as the slots in the queue as well as
the last item we returned to the caller, in other words, all requests currently
in "flight"). This means `max_readahead = 1` would allow you to have 2
requests in flight at once but I think that is what I would expect.
However, for your second paragraph, spaces_available IS accessed
concurrently. It can be accessed by the caller (the end consuming the
generator) as well as the producer (the end marking the futures complete and
running the callbacks/continuations). I'm pretty sure it is correct that the
caller end `operator()()` does not decrement `spaces_available`.
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