pitrou commented on a change in pull request #9095: URL: https://github.com/apache/arrow/pull/9095#discussion_r565981888
########## File path: cpp/src/arrow/util/async_iterator.h ########## @@ -0,0 +1,295 @@ +// Licensed to the Apache Software Foundation (ASF) under one +// or more contributor license agreements. See the NOTICE file +// distributed with this work for additional information +// regarding copyright ownership. The ASF licenses this file +// to you under the Apache License, Version 2.0 (the +// "License"); you may not use this file except in compliance +// with the License. You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, +// software distributed under the License is distributed on an +// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +// KIND, either express or implied. See the License for the +// specific language governing permissions and limitations +// under the License. + +#pragma once +#include <queue> + +#include "arrow/util/functional.h" +#include "arrow/util/future.h" +#include "arrow/util/iterator.h" +#include "arrow/util/optional.h" +#include "arrow/util/thread_pool.h" + +namespace arrow { + +template <typename T> +using AsyncGenerator = std::function<Future<T>()>; + +/// Iterates through a generator of futures, visiting the result of each one and +/// returning a future that completes when all have been visited +template <typename T> +Future<> VisitAsyncGenerator(AsyncGenerator<T> generator, + std::function<Status(T)> visitor) { + auto loop_body = [generator, visitor] { + auto next = generator(); + return next.Then([visitor](const T& result) -> Result<ControlFlow<detail::Empty>> { + if (result == IterationTraits<T>::End()) { + return Break(detail::Empty()); + } else { + auto visited = visitor(result); + if (visited.ok()) { + return Continue(); + } else { + return visited; + } + } + }); + }; + return Loop(loop_body); +} + +template <typename T> +Future<std::vector<T>> CollectAsyncGenerator(AsyncGenerator<T> generator) { + auto vec = std::make_shared<std::vector<T>>(); + auto loop_body = [generator, vec] { + auto next = generator(); + return next.Then([vec](const T& result) -> Result<ControlFlow<std::vector<T>>> { + if (result == IterationTraits<T>::End()) { + return Break(*vec); + } else { + vec->push_back(result); + return Continue(); + } + }); + }; + return Loop(loop_body); +} + +template <typename T, typename V> +class TransformingGenerator { + public: + explicit TransformingGenerator(AsyncGenerator<T> generator, + Transformer<T, V> transformer) + : finished_(), last_value_(), generator_(generator), transformer_(transformer) {} + + // See comment on TransformingIterator::Pump + Result<util::optional<V>> Pump() { + if (!finished_ && last_value_.has_value()) { + ARROW_ASSIGN_OR_RAISE(TransformFlow<V> next, transformer_(*last_value_)); + if (next.ReadyForNext()) { + if (*last_value_ == IterationTraits<T>::End()) { + finished_ = true; + } + last_value_.reset(); + } + if (next.Finished()) { + finished_ = true; + } + if (next.HasValue()) { + return next.Value(); + } + } + if (finished_) { + return IterationTraits<V>::End(); + } + return util::nullopt; + } + + Future<V> operator()() { + while (true) { + auto maybe_next_result = Pump(); + if (!maybe_next_result.ok()) { + return Future<V>::MakeFinished(maybe_next_result.status()); + } + auto maybe_next = maybe_next_result.ValueUnsafe(); + if (maybe_next.has_value()) { + return Future<V>::MakeFinished(*maybe_next); + } + + auto next_fut = generator_(); + // If finished already, process results immediately inside the loop to avoid stack + // overflow + if (next_fut.is_finished()) { + auto next_result = next_fut.result(); + if (next_result.ok()) { + last_value_ = *next_result; + } else { + return Future<V>::MakeFinished(next_result.status()); + } + // Otherwise, if not finished immediately, add callback to process results + } else { + return next_fut.Then([this](const Result<T>& next_result) { + if (next_result.ok()) { + last_value_ = *next_result; + return (*this)(); + } else { + return Future<V>::MakeFinished(next_result.status()); + } + }); + } + } + } + + protected: + bool finished_; + util::optional<T> last_value_; + AsyncGenerator<T> generator_; + Transformer<T, V> transformer_; +}; + +template <typename T> +static std::function<void(const Result<T>&)> MakeCallback( + std::shared_ptr<bool> finished) { + return [finished](const Result<T>& next_result) { + if (!next_result.ok()) { + *finished = true; + } else { + auto next = *next_result; + *finished = (next == IterationTraits<T>::End()); + } + }; +} + +template <typename T> +AsyncGenerator<T> AddReadahead(AsyncGenerator<T> source_generator, int max_readahead) { + // Using a shared_ptr instead of a lambda capture here because it's possible that + // the inner mark_finished_if_done outlives the outer lambda + auto finished = std::make_shared<bool>(false); + auto mark_finished_if_done = [finished](const Result<T>& next_result) { + if (!next_result.ok()) { + *finished = true; + } else { + auto next = *next_result; + *finished = (next == IterationTraits<T>::End()); + } + }; + + std::queue<Future<T>> readahead_queue; + return [=]() mutable -> Future<T> { + if (readahead_queue.empty()) { + // This is the first request, let's pump the underlying queue + for (int i = 0; i < max_readahead; i++) { + auto next = source_generator(); + next.AddCallback(mark_finished_if_done); + readahead_queue.push(std::move(next)); + } + } + // Pop one and add one + auto result = readahead_queue.front(); + readahead_queue.pop(); + if (*finished) { + readahead_queue.push(Future<T>::MakeFinished(IterationTraits<T>::End())); + } else { + auto back_of_queue = source_generator(); + back_of_queue.AddCallback(mark_finished_if_done); Review comment: Ok, thank you. ---------------------------------------------------------------- This is an automated message from the Apache Git Service. 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