/**
 *    Copyright (C) 2014 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 <string>
#include <thread>

#include "mongo/base/disallow_copying.h"
#include "mongo/base/status.h"
#include "mongo/base/status_with.h"
#include "mongo/base/string_data.h"
#include "mongo/client/remote_command_runner.h"
#include "mongo/db/concurrency/lock_manager_defs.h"
#include "mongo/db/repl/task_runner.h"
#include "mongo/executor/task_executor.h"
#include "mongo/platform/compiler.h"
#include "mongo/platform/random.h"
#include "mongo/stdx/condition_variable.h"
#include "mongo/stdx/functional.h"
#include "mongo/stdx/list.h"
#include "mongo/stdx/mutex.h"
#include "mongo/util/concurrency/old_thread_pool.h"
#include "mongo/util/net/hostandport.h"
#include "mongo/util/time_support.h"

namespace mongo {

    class NamespaceString;
    class OperationContext;

namespace executor{
    class NetworkInterface;
} // namespace executor

namespace repl {

    class StorageInterface;

    /**
     * Implementation of the TaskExecutor interface for providing an event loop for driving state
     * machines in replication.
     *
     * Usage: Instantiate an executor, schedule a work item, call run().
     *
     * Implementation details:
     *
     * The executor is composed of several WorkQueues, which are queues of WorkItems.  WorkItems
     * describe units of work -- a callback and state needed to track its lifecycle.  The iterators
     * pointing to WorkItems are spliced between the WorkQueues, rather than copying WorkItems
     * themselves.  Further, those WorkQueue::iterators are never invalidated during the life of an
     * executor.  They may be recycled to represent new work items, but when that happens, a counter
     * on the WorkItem is incremented, to disambiguate.
     *
     * All work executed by the run() method of the executor is popped off the front of the
     * _readyQueue.  Remote commands blocked on the network can be found in the
     * _networkInProgressQueue.  Callbacks waiting for a timer to expire are in the _sleepersQueue.
     * When the network returns or the timer expires, items from these two queues are transferred to
     * the back of the _readyQueue.
     *
     * The _exclusiveLockInProgressQueue, which represents work items to execute while holding the
     * GlobalWrite lock, is exceptional.  WorkItems in that queue execute in unspecified order with
     * respect to work in the _readyQueue or other WorkItems in the _exclusiveLockInProgressQueue,
     * but they are executed in a single serial order with respect to those other WorkItems.  The
     * _terribleExLockSyncMutex is used to provide this serialization, until such time as the global
     * lock may be passed from one thread to another.
     */
    class ReplicationExecutor final : public executor::TaskExecutor {
        MONGO_DISALLOW_COPYING(ReplicationExecutor);
    public:

        /**
         * Constructs a new executor.
         *
         * Takes ownership of the passed NetworkInterface object.
         */
        ReplicationExecutor(executor::NetworkInterface* netInterface,
                            StorageInterface* storageInterface,
                            int64_t pnrgSeed);

        /**
         * Destroys an executor.
         */
        virtual ~ReplicationExecutor();

        std::string getDiagnosticString() override;
        Date_t now() override;
        void shutdown() override;
        void signalEvent(const EventHandle& event) override;
        StatusWith<EventHandle> makeEvent() override;
        StatusWith<CallbackHandle> onEvent(const EventHandle& event,
                                           const CallbackFn& work) override;
        void waitForEvent(const EventHandle& event) override;
        StatusWith<CallbackHandle> scheduleWork(const CallbackFn& work) override;
        StatusWith<CallbackHandle> scheduleWorkAt(Date_t when, const CallbackFn& work) override;
        StatusWith<CallbackHandle> scheduleRemoteCommand(
                const RemoteCommandRequest& request,
                const RemoteCommandCallbackFn& cb) override;
        void cancel(const CallbackHandle& cbHandle) override;
        void wait(const CallbackHandle& cbHandle) override;


        /**
         * Executes the run loop.  May be called up to one time.
         *
         * Returns after the executor has been shutdown and is safe to delete.
         */
        void run();

        /**
         * Schedules DB "work" to be run by the executor..
         *
         * Takes no locks for caller - global, database or collection.
         *
         * The "work" will run exclusively with other DB work items. All DB work items
         * are run the in order they are scheduled.
         *
         * The "work" may run concurrently with other non-DB work items,
         * but there are no ordering guarantees provided with respect to
         * any other work item.
         *
         * Returns a handle for waiting on or canceling the callback, or
         * ErrorCodes::ShutdownInProgress.
         *
         * May be called by client threads or callbacks running in the executor.
         */
        StatusWith<CallbackHandle> scheduleDBWork(const CallbackFn& work);

        /**
         * Schedules DB "work" to be run by the executor while holding the collection lock.
         *
         * Takes collection lock in specified mode (and slightly more permissive lock for the
         * database lock) but not the global exclusive lock.
         *
         * The "work" will run exclusively with other DB work items. All DB work items
         * are run the in order they are scheduled.
         *
         * The "work" may run concurrently with other non-DB work items,
         * but there are no ordering guarantees provided with respect to
         * any other work item.
         *
         * Returns a handle for waiting on or canceling the callback, or
         * ErrorCodes::ShutdownInProgress.
         *
         * May be called by client threads or callbacks running in the executor.
         */
        StatusWith<CallbackHandle> scheduleDBWork(const CallbackFn& work,
                                                  const NamespaceString& nss,
                                                  LockMode mode);

        /**
         * Schedules "work" to be run by the executor while holding the global exclusive lock.
         *
         * Takes collection lock in specified mode (and slightly more permissive lock for the
         * database lock) but not the global exclusive lock.
         *
         * The "work" will run exclusively, as though it were executed by the main
         * run loop, but there are no ordering guarantees provided with respect to
         * any other work item.
         *
         * Returns a handle for waiting on or canceling the callback, or
         * ErrorCodes::ShutdownInProgress.
         *
         * May be called by client threads or callbacks running in the executor.
         */
        StatusWith<CallbackHandle> scheduleWorkWithGlobalExclusiveLock(
                const CallbackFn& work);

        /**
         * Returns an int64_t generated by the prng with a max value of "limit".
         */
        int64_t nextRandomInt64(int64_t limit);

    private:
        class Callback;
        class Event;
        struct WorkItem;
        friend class Callback;
        friend class Event;


        /**
         * A linked list of WorkItem objects.
         *
         * WorkItems get moved among lists by splicing iterators of work lists together,
         * not by copying underlying WorkItem objects.
         */
        typedef stdx::list<WorkItem> WorkQueue;

        /**
         * A linked list of EventHandles.
         */
        typedef stdx::list<EventHandle> EventList;

        /**
         * Returns diagnostic info
         */
        std::string _getDiagnosticString_inlock() const;

        /**
         * Implementation of makeEvent() for use when _mutex is already held.
         */
        StatusWith<EventHandle> makeEvent_inlock();

        /**
         * Implementation of signalEvent() for use when _mutex is already held.
         */
        void signalEvent_inlock(const EventHandle&);

        /**
         * Gets a single piece of work to execute.
         *
         * If the "callback" member of the returned WorkItem is falsey, that is a signal
         * to the run loop to wait for shutdown.
         */
        std::pair<WorkItem, CallbackHandle> getWork();

        /**
         * Marks as runnable any sleepers whose ready date has passed as of "now".
         * Returns the date when the next sleeper will be ready, or Date_t(~0ULL) if there are no
         * remaining sleepers.
         */
        Date_t scheduleReadySleepers_inlock(Date_t now);

        /**
         * Enqueues "callback" into "queue".
         */
        StatusWith<CallbackHandle> enqueueWork_inlock(WorkQueue* queue, const CallbackFn& callback);

        /**
         * Notifies interested parties that shutdown has completed, if it has.
         */
        void maybeNotifyShutdownComplete_inlock();

        /**
         * Completes the shutdown process.  Called by run().
         */
        void finishShutdown();

        void _finishRemoteCommand(
                const RemoteCommandRequest& request,
                const StatusWith<RemoteCommandResponse>& response,
                const CallbackHandle& cbHandle,
                const uint64_t expectedHandleGeneration,
                const RemoteCommandCallbackFn& cb);

        /**
         * Executes the callback referenced by "cbHandle", and moves the underlying
         * WorkQueue::iterator from "workQueue" into the _freeQueue.
         *
         * "txn" is a pointer to the OperationContext.
         *
         * "status" is the callback status from the task runner. Only possible values are
         * Status::OK and ErrorCodes::CallbackCanceled (when task runner is canceled).
         *
         * If "terribleExLockSyncMutex" is not null, serializes execution of "cbHandle" with the
         * execution of other callbacks.
         */
        void _doOperation(OperationContext* txn,
                          const Status& taskRunnerStatus,
                          const CallbackHandle& cbHandle,
                          WorkQueue* workQueue,
                          stdx::mutex* terribleExLockSyncMutex);

        /**
         * Wrapper around TaskExecutor::getCallbackFromHandle that return an Event* instead of
         * a generic EventState*.
         */
        Event* _getEventFromHandle(const EventHandle& eventHandle);

        /**
         * Wrapper around TaskExecutor::getCallbackFromHandle that return an Event* instead of
         * a generic EventState*.
         */
        Callback* _getCallbackFromHandle(const CallbackHandle& callbackHandle);

        // PRNG; seeded at class construction time.
        PseudoRandom _random;

        std::unique_ptr<executor::NetworkInterface> _networkInterface;
        std::unique_ptr<StorageInterface> _storageInterface;
        stdx::mutex _mutex;
        stdx::mutex _terribleExLockSyncMutex;
        stdx::condition_variable _noMoreWaitingThreads;
        WorkQueue _freeQueue;
        WorkQueue _readyQueue;
        WorkQueue _dbWorkInProgressQueue;
        WorkQueue _exclusiveLockInProgressQueue;
        WorkQueue _networkInProgressQueue;
        WorkQueue _sleepersQueue;
        EventList _unsignaledEvents;
        int64_t _totalEventWaiters;
        bool _inShutdown;
        OldThreadPool _dblockWorkers;
        TaskRunner _dblockTaskRunner;
        TaskRunner _dblockExclusiveLockTaskRunner;
        uint64_t _nextId;
    };

    class ReplicationExecutor::Callback : public executor::TaskExecutor::CallbackState {
        friend class ReplicationExecutor;

    public:

        Callback(ReplicationExecutor* executor,
                 const CallbackFn callbackFn,
                 const WorkQueue::iterator& iter,
                 const EventHandle& finishedEvent);
        virtual ~Callback();

        void cancel() override;
        void waitForCompletion() override;

    private:

        ReplicationExecutor* _executor;

        // All members other than _executor are protected by the executor's _mutex.
        CallbackFn _callbackFn;
        bool _isCanceled;
        WorkQueue::iterator _iter;
        EventHandle _finishedEvent;
    };

    typedef ReplicationExecutor::ResponseStatus ResponseStatus;

    /**
     * Description of a scheduled but not-yet-run work item.
     *
     * Once created, WorkItem objects remain in scope until the executor is destroyed.
     * However, over their lifetime, they may represent many different work items.  This
     * divorces the lifetime of CallbackHandles from the lifetime of WorkItem objects, but
     * requires a unique generation identifier in CallbackHandles and WorkItem objects.
     *
     * WorkItem is copyable so that it may be stored in a list.  However, in practice they
     * should only be copied by getWork() and when allocating new entries into a WorkQueue (not
     * when moving entries between work lists).
     */
    struct ReplicationExecutor::WorkItem {
        WorkItem();
        uint64_t generation;
        CallbackHandle callback;
        EventHandle finishedEvent;
        Date_t readyDate;
        bool isNetworkOperation;
    };

    /**
     * Description of an event.
     *
     * Like WorkItem, above, but for events.  On signaling, the executor removes the event from the
     * "unsignaled" EventList and schedules all work items in the _waiters list.
     */
    class ReplicationExecutor::Event : public executor::TaskExecutor::EventState {
        friend class ReplicationExecutor;

    public:

        Event(ReplicationExecutor* executor, const EventList::iterator& iter);
        virtual ~Event();

        void signal() override;
        void waitUntilSignaled() override;
        bool isSignaled() override;

    private:

        // Note that the caller is responsible for removing any references to any EventHandles
        // pointing to this event.
        void _signal_inlock();

        ReplicationExecutor* _executor;

        // All members other than _executor are protected by the executor's _mutex.
        bool _isSignaled;
        stdx::condition_variable _isSignaledCondition;
        EventList::iterator _iter;
        WorkQueue _waiters;
    };

}  // namespace repl
}  // namespace mongo