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/**
* Copyright (C) 2020-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <functional>
#include <memory>
#include "mongo/db/client.h"
#include "mongo/platform/atomic_word.h"
#include "mongo/platform/compiler.h"
#include "mongo/util/out_of_line_executor.h"
#include "mongo/util/producer_consumer_queue.h"
namespace mongo {
/**
* A client decoration that allows out-of-line, client-local execution of tasks.
* So long as the client thread exists, tasks can be scheduled to execute on the client thread.
* Other threads must always schedule tasks through an instance of `QueueHandle`.
* Once the decoration is destructed, `QueueHandle` runs tasks on the caller's thread, providing
* `ErrorCodes::CallbackCanceled` as the status.
*/
class ClientOutOfLineExecutor final : public OutOfLineExecutor {
public:
ClientOutOfLineExecutor() noexcept;
~ClientOutOfLineExecutor() noexcept;
static ClientOutOfLineExecutor* get(const Client*) noexcept;
using Task = OutOfLineExecutor::Task;
void shutdown();
void schedule(Task) override;
// Blocks until the executor is done running all scheduled tasks.
void consumeAllTasks() noexcept;
using QueueType = MultiProducerSingleConsumerQueue<Task>;
// Allows other threads to access the queue irrespective of the client's lifetime.
class QueueHandle final {
public:
QueueHandle() = default;
QueueHandle(const std::shared_ptr<QueueType>& queue) : _weakQueue(queue) {}
void schedule(Task&&);
private:
std::weak_ptr<QueueType> _weakQueue;
};
auto getHandle() noexcept {
// We always acquire a handle before scheduling tasks on the executor. The following ensures
// that acquiring a handle, thus exposing the executor to the outside world, always requires
// shutdown. A combination of load/store is preferred over using CAS due to performance
// considerations, and considering the fact that overwriting `_requireShutdown` is safe.
if (MONGO_unlikely(!_requireShutdown.load()))
_requireShutdown.store(true);
return QueueHandle(_taskQueue);
}
private:
// Shutdown is not required so long as the executor is not exposed to the outside world.
AtomicWord<bool> _requireShutdown{false};
class Impl;
std::unique_ptr<Impl> _impl;
std::shared_ptr<QueueType> _taskQueue;
// Provides the means to ensure `shutdown()` always precedes the destructor.
bool _isShutdown = false;
};
} // namespace mongo
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