westonpace commented on a change in pull request #9533:
URL: https://github.com/apache/arrow/pull/9533#discussion_r592565500
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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});
Review comment:
I'm technically not mutating anything the queue accesses so it should be
safe. It is possible for the reader to pull many times between the `Write`
call and `*next_slot = ...` but it is not possible for it to read the item
being written.
We don't have to worry about the case where `Pump` is called by the reader
becuase the reader is never pulled in a sync-reentrant manner (no async
generators are) so in that case it isn't possible.
However, while a callback is being filled the reader could be polled many
times. Let's call the callback being filled the callback for future X. It
will be creating future X+1 (not X+N because this is serial readahead) and
adding it to the queue.
The reader cannot poll for future X+1 until it finishes processing all
callbacks for future X (this is why it is important to use future.Then here and
not future.AddCallback). This is because this generator is never pulled
async-reentrantly. So the reader cannot poll for future X+1 until this
callback is finished completely. So it would be impossible to access the item
written to the queue until we have had a chance to populate it.
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