/**
 *    Copyright (C) 2018-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 <deque>

#include "mongo/base/status.h"
#include "mongo/db/service_context.h"
#include "mongo/platform/atomic_word.h"
#include "mongo/platform/mutex.h"
#include "mongo/stdx/condition_variable.h"
#include "mongo/transport/service_executor.h"
#include "mongo/util/hierarchical_acquisition.h"

namespace mongo {
namespace transport {

/** Transitional for differential benchmarking of ServiceExecutorSynchronous refactor */
#define TRANSITIONAL_SERVICE_EXECUTOR_SYNCHRONOUS_HAS_RESERVE 0

/**
 * Creates a fresh worker thread for each top-level scheduled task. Any tasks
 * scheduled during the execution of that top-level task as it runs on such a
 * worker thread are pushed to the queue of that worker thread.
 *
 * Thus, the top-level task is expected to represent a chain of operations, each
 * of which schedules its successor before returning. The entire chain of
 * operations, and nothing else, executes on the same worker thread.
 */
class ServiceExecutorSynchronous final : public ServiceExecutor {
public:
    /** Returns the ServiceExecutorSynchronous decoration on `ctx`. */
    static ServiceExecutorSynchronous* get(ServiceContext* ctx);

    explicit ServiceExecutorSynchronous(ServiceContext*);

    ~ServiceExecutorSynchronous();

    Status start() override;
    Status shutdown(Milliseconds timeout) override;

    std::unique_ptr<TaskRunner> makeTaskRunner() override;

    size_t getRunningThreads() const override;

    void appendStats(BSONObjBuilder* bob) const override;

private:
    class SharedState;

    /**
     * The behavior of `schedule` depends on whether the calling thread is a
     * worker thread spawned by a previous `schedule` call.
     *
     * If a nonworker thread schedules a task, a worker thread is spawned, and
     * the task is transferred to the new worker thread's queue.
     *
     * If a worker thread schedules a task, the task is pushed to the back of its
     * queue. The worker thread exits when the queue becomes empty.
     */
    void _schedule(Task task);

    void _runOnDataAvailable(const std::shared_ptr<Session>& session, Task onCompletionCallback);


    std::shared_ptr<SharedState> _sharedState;
};

}  // namespace transport
}  // namespace mongo