/**
 *    Copyright (C) 2015 MongoDB Inc.
 *
 *    This program is free software: you can redistribute it and/or  modify
 *    it under the terms of the GNU Affero General Public License, version 3,
 *    as published by the Free Software Foundation.
 *
 *    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
 *    GNU Affero General Public License for more details.
 *
 *    You should have received a copy of the GNU Affero General Public License
 *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 *    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 GNU Affero General 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 <list>

#include "mongo/base/disallow_copying.h"
#include "mongo/stdx/condition_variable.h"
#include "mongo/stdx/functional.h"
#include "mongo/stdx/mutex.h"

namespace mongo {

    class OperationContext;
    class Status;
    class OldThreadPool;

namespace repl {

    class TaskRunner {
        MONGO_DISALLOW_COPYING(TaskRunner);
    public:

        /**
         * Represents next steps of task runner.
         */
        enum class NextAction {
            kInvalid=0,
            kDisposeOperationContext=1,
            kKeepOperationContext=2,
            kCancel=3,
        };

        using CreateOperationContextFn = stdx::function<OperationContext*()>;
        using Task = stdx::function<NextAction (OperationContext*, const Status&)>;

        /**
         * Creates a Task returning kCancel. This is useful in shutting down the task runner after
         * running a series of tasks.
         *
         * Without a cancellation task, the client would need to coordinate the completion of the
         * last task with calling cancel() on the task runner.
         */
        static Task makeCancelTask();

        TaskRunner(OldThreadPool* threadPool,
                   const CreateOperationContextFn& createOperationContext);

        virtual ~TaskRunner();

        /**
         * Returns diagnostic information.
         */
        std::string getDiagnosticString() const;

        /**
         * Returns true if there are any scheduled or actively running tasks.
         */
        bool isActive() const;

        /**
         * Schedules a task to be run by the task runner. Tasks are run in the same order that they
         * are scheduled.
         *
         * This transitions the task runner to an active state.
         *
         * The task runner creates an operation context using '_createOperationContext'
         * prior to running a scheduled task. Depending on the NextAction returned from the
         * task, operation contexts may be shared between consecutive tasks invoked by the task
         * runner.
         *
         * On completion, each task is expected to return a NextAction to the task runner.
         *
         *     If the task returns kDisposeOperationContext, the task runner destroys the operation
         *     context. The next task to be invoked will receive a new operation context.
         *
         *     If the task returns kKeepOperationContext, the task runner will retain the operation
         *     context to pass to the next task in the queue.
         *
         *     If the task returns kCancel, the task runner will destroy the operation context and
         *     cancel the remaining tasks (each task will be invoked with a status containing the
         *     code ErrorCodes::CallbackCanceled). After all the tasks have been canceled, the task
         *     runner will become inactive.
         *
         *     If the task returns kInvalid, this NextAction will be handled in the same way as
         *     kDisposeOperationContext.
         *
         * If the status passed to the task is not OK, the task should not proceed and return
         * immediately. This is usually the case when the task runner is canceled. Accessing the
         * operation context in the task will result in undefined behavior.
         */
        void schedule(const Task& task);

        /**
         * If there is a task that is already running, allows the task to run to completion.
         * Cancels all scheduled tasks that have not been run. Canceled tasks will still be
         * invoked with a status containing the code ErrorCodes::CallbackCanceled.
         * After all active tasks have completed and unscheduled tasks have been canceled, the
         * task runner will go into an inactive state.
         *
         * It is a no-op to call cancel() before scheduling any tasks.
         */
        void cancel();

    private:

        /**
         * Runs tasks in a loop.
         * Loop exits when any of the tasks returns a non-kContinue next action.
         */
        void _runTasks();
        void _finishRunTasks_inlock();

        /**
         * Waits for next scheduled task to be added to queue.
         * Returns null task when task runner is stopped.
         */
        Task _waitForNextTask();

        OldThreadPool* _threadPool;
        CreateOperationContextFn _createOperationContext;

        // Protects member data of this TaskRunner.
        mutable stdx::mutex _mutex;

        stdx::condition_variable _condition;

        // _active is true when there are scheduled tasks in the task queue or
        // when a task is being run by the task runner.
        bool _active;

        bool _cancelRequested;

        // FIFO queue of scheduled tasks
        std::list<Task> _tasks;
    };

} // namespace repl
} // namespace mongo