/**
 *    Copyright (C) 2013 10gen 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 <boost/thread/thread.hpp>
#include <boost/thread/condition.hpp>

#include "mongo/base/disallow_copying.h"
#include "mongo/platform/cstdint.h"
#include "mongo/platform/unordered_map.h"
#include "mongo/util/concurrency/mutex.h"
#include "mongo/util/time_support.h"

namespace mongo {

    /**
     * DeadlineMonitor
     *
     * Monitors tasks which are required to complete before a deadline.  When
     * a deadline is started on a _Task*, either the deadline must be stopped,
     * or _Task::kill() will be called when the deadline arrives.
     *
     * Each instance of a DeadlineMonitor spawns a thread which waits for one of the
     * following conditions:
     *    - a task is added to the monitor
     *    - a task is removed from the monitor
     *    - the nearest deadline has arrived
     *
     * Ownership:
     * The _Task* must not be freed until the deadline has elapsed or stopDeadline()
     * has been called.
     *
     * NOTE: Each instance of this class spawns a new thread.  It is intended to be a stop-gap
     *       solution for simple deadline monitoring until a more robust solution can be
     *       implemented.
     *
     * NOTE: timing is based on wallclock time, which may not be precise.
     */
    template <typename _Task>
    class DeadlineMonitor {
    MONGO_DISALLOW_COPYING(DeadlineMonitor);
    public:
        DeadlineMonitor() {
            // NOTE(schwerin): Because _monitorThread takes a pointer to "this", all of the fields
            // of this instance must be initialized before the thread is created.  As a result, we
            // should not create the thread in the initializer list.  Creating it there leaves us
            // vulnerable to errors introduced by rearranging the order of fields in the class.
            _monitorThread =
                boost::thread(&mongo::DeadlineMonitor<_Task>::deadlineMonitorThread, this);
        }

        ~DeadlineMonitor() {
            {
                // ensure the monitor thread has been stopped before destruction
                boost::lock_guard<boost::mutex> lk(_deadlineMutex);
                _inShutdown = true;
                _newDeadlineAvailable.notify_one();
            }
            _monitorThread.join();
        }

        /**
         * Start monitoring a task for deadline lapse.  User must call stopDeadline() before
         * deleting the task.  Note that stopDeadline() cannot be called from within the
         * kill() method.
         * @param   task        the task to kill()
         * @param   timeoutMs   number of milliseconds before the deadline expires
         */
        void startDeadline(_Task* const task, uint64_t timeoutMs) {
            const auto deadline = Date_t::now() + Milliseconds(timeoutMs);
            boost::lock_guard<boost::mutex> lk(_deadlineMutex);

            _tasks[task] = deadline;

            if (deadline < _nearestDeadlineWallclock) {
                _nearestDeadlineWallclock = deadline;
                _newDeadlineAvailable.notify_one();
            }
        }

        /**
         * Stop monitoring a task.  Can be called multiple times, before or after a
         * deadline has expired (as long as the task remains allocated).
         * @return true  if the task was found and erased
         */
        bool stopDeadline(_Task* const task) {
            boost::lock_guard<boost::mutex> lk(_deadlineMutex);
            return _tasks.erase(task);
        }

    private:
        /**
         * Main deadline monitor loop.  Waits on a condition variable until a task
         * is started, stopped, or the nearest deadline arrives.  If a deadline arrives,
         * _Task::kill() is invoked.
         */
        void deadlineMonitorThread() {
            boost::unique_lock<boost::mutex> lk(_deadlineMutex);
            while (!_inShutdown) {

                // get the next interval to wait
                const Date_t now = Date_t::now();

                // wait for a task to be added or a deadline to expire
                if (_nearestDeadlineWallclock > now) {
                    if (_nearestDeadlineWallclock == Date_t::max()) {
                        _newDeadlineAvailable.wait(lk);
                    }
                    else {
                        _newDeadlineAvailable.wait_until(
                                lk, _nearestDeadlineWallclock.toSystemTimePoint());
                    }
                    continue;
                }

                // set the next interval to wait for deadline completion
                _nearestDeadlineWallclock = Date_t::max();
                typename TaskDeadlineMap::iterator i = _tasks.begin();
                while (i != _tasks.end()) {
                    if (i->second < now) {
                        // deadline expired
                        i->first->kill();
                        _tasks.erase(i++);
                    }
                    else {
                        if (i->second < _nearestDeadlineWallclock) {
                            // nearest deadline seen so far
                            _nearestDeadlineWallclock = i->second;
                        }
                        ++i;
                    }
                }
            }
        }

        typedef unordered_map<_Task*, Date_t> TaskDeadlineMap;
        TaskDeadlineMap _tasks; // map of running tasks with deadlines
        boost::mutex _deadlineMutex; // protects all non-const members, except _monitorThread
        boost::condition_variable _newDeadlineAvailable; // Signaled for timeout, start and stop
        boost::thread _monitorThread; // the deadline monitor thread
        Date_t _nearestDeadlineWallclock = Date_t::max(); // absolute time of the nearest deadline
        bool _inShutdown = false;
    };

} // namespace mongo