when_any.hh

/*
 * This file is open source software, licensed to you under the terms
 * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
 * distributed with this work for additional information regarding copyright
 * ownership.  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.
 */
 
/*
 * author: Niek J Bouman
 * reviewers: Avi Kivity, Benny Halevy
 * November 2021
 */ 
 
#pragma once
 
#ifndef SEASTAR_MODULE
#include <iterator>
#include <cstddef>
#include <type_traits>
#include <vector>
#include <tuple>
#include <utility>
#include <seastar/core/future.hh>
#include <seastar/core/shared_ptr.hh>
#include <seastar/util/modules.hh>
#endif
 
namespace seastar {
 
template <class Sequence>
struct when_any_result {
    std::size_t index;
    Sequence futures;
};
 
namespace internal {
class waiter {
    bool _done = false;
    promise<std::size_t> _promise;
 
public:
    void done(std::size_t index) {
        if (!_done) {
            _done = true;
            _promise.set_value(index);
        }
    }
    auto get_future() { return _promise.get_future(); }
};
 
} // namespace internal
 
SEASTAR_MODULE_EXPORT
template <class FutureIterator>
requires requires (FutureIterator i) { { *i++ }; requires is_future<std::remove_reference_t<decltype(*i)>>::value; }
auto when_any(FutureIterator begin, FutureIterator end) noexcept
  -> future<when_any_result<std::vector<std::decay_t<typename std::iterator_traits<FutureIterator>::value_type>>>>
{
    using ReturnType = when_any_result<std::vector<typename std::iterator_traits<FutureIterator>::value_type>>;
    if (begin == end) {
        return make_ready_future<ReturnType>();
    }
    ReturnType result;
    result.futures.reserve(std::distance(begin, end));
    auto waiter_obj = make_lw_shared<internal::waiter>();
    std::size_t index{0};
    for (auto it = begin; it != end; ++it) {
        if (it->available()) {
            result.futures.push_back(std::move(*it));
            waiter_obj->done(index);
        } else {
            result.futures.push_back(it->finally([waiter_obj, index] {
                waiter_obj->done(index);
            }));
        }
        index++;
    }
    return waiter_obj->get_future().then(
        [result = std::move(result)](std::size_t index) mutable {
            result.index = index;
            return std::move(result);
        }
    );
}
 
namespace internal {
 
template <class... Futures, std::size_t... I>
future<when_any_result<std::tuple<Futures...>>>
when_any_impl(std::index_sequence<I...>, Futures&&... futs) noexcept
{
    auto waiter_obj = make_lw_shared<waiter>();
    auto attach_notifier = [&](auto&& fut, size_t index) {
        if (fut.available()) {
            waiter_obj->done(index);
            return std::move(fut);
        }
        else {
            return fut.finally([waiter_obj, index] { waiter_obj->done(index); });
        }
    };
 
    auto result =
        when_any_result<std::tuple<Futures...>>{0, std::make_tuple(attach_notifier(std::forward<Futures>(futs), I)...)};
    return waiter_obj->get_future().then([result = std::move(result)](std::size_t index) mutable {
        result.index = index;
        return std::move(result);
    });
}
 
} // namespace internal
 
SEASTAR_MODULE_EXPORT
template <class... FutOrFuncs>
auto when_any(FutOrFuncs&&... fut_or_funcs) noexcept
{
    return internal::when_any_impl(std::make_index_sequence<sizeof...(FutOrFuncs)>{},
                                   futurize_invoke_if_func(std::forward<FutOrFuncs>(fut_or_funcs))...);
}
 
} // namespace seastar