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421b1b9494
A set of futures that can be polled. Polling the set (`poll` function) returns the value of one of the futures which became available or `std::nullopt` if the given logical durationd passes (according to the given timer), whichever event happens first. The current implementation assumes sequential polling. New futures can be added to the set with `add`. All futures can be removed from the set with `release`.
106 lines
3.5 KiB
C++
106 lines
3.5 KiB
C++
/*
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* Copyright (C) 2021 ScyllaDB
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*/
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/*
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* This file is part of Scylla.
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*
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* Scylla is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License as published by
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* the Free Software Foundation, either version 3 of the License, or
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* (at your option) any later version.
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*
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* Scylla is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with Scylla. If not, see <http://www.gnu.org/licenses/>.
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*/
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#pragma once
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#include <seastar/core/coroutine.hh>
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#include <seastar/core/weak_ptr.hh>
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#include <seastar/core/condition-variable.hh>
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using namespace seastar;
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// A set of futures that can be polled to obtain the result of some ready future in the set.
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//
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// Note: the set must be empty on destruction. Call `release` to ensure emptiness.
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template <typename T>
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class future_set {
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struct cond_var_container : public seastar::weakly_referencable<cond_var_container> {
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seastar::condition_variable v;
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};
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std::vector<future<T>> _futures;
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cond_var_container _container;
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public:
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// Polling the set returns the value of one of the futures which became available
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// or `std::nullopt` if the logical duration `d` passes (according to `timer`),
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// whichever event happens first.
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//
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// Cannot be called in parallel.
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// TODO: we could probably lift this restriction by using `broadcast()` instead of `signal()`. Think about it.
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future<std::optional<T>> poll(logical_timer& timer, raft::logical_clock::duration d) {
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auto timeout = timer.now() + d;
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auto wake_condition = [this, &timer, timeout] {
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return std::any_of(_futures.begin(), _futures.end(), std::mem_fn(&future<T>::available)) || timer.now() >= timeout;
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};
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if (timer.now() < timeout) { // i.e. d > 0
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// Wake ourselves up when the timeout passes (if we're still waiting at that point).
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// If nothing else wakes us, this will.
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timer.schedule(timeout, [ptr = _container.weak_from_this()] {
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if (ptr) {
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ptr->v.signal();
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}
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});
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co_await _container.v.wait(wake_condition);
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}
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assert(wake_condition());
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for (auto& f : _futures) {
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if (f.available()) {
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std::swap(f, _futures.back());
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auto ff = std::move(_futures.back());
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_futures.pop_back();
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co_return std::move(ff).get();
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}
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}
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// No future was available, so `wake_condition()` implies:
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assert(timer.now() >= timeout);
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co_return std::nullopt;
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}
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void add(future<T> f) {
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_futures.push_back(std::move(f).finally([ptr = _container.weak_from_this()] {
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if (ptr) {
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ptr->v.signal();
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}
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}));
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}
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// Removes all futures from the set and returns them (even if they are not ready yet).
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// The user must ensure that there are no futures in the set when it's destroyed; this is a good way to do so.
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std::vector<future<T>> release() {
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return std::exchange(_futures, {});
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}
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bool empty() const {
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return _futures.empty();
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}
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~future_set() {
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assert(_futures.empty());
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}
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};
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