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/**
 *    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 <boost/thread/mutex.hpp>
#include <map>

#include "mongo/base/disallow_copying.h"
#include "mongo/client/dbclientinterface.h"
#include "mongo/platform/unordered_map.h"
#include "mongo/stdx/list.h"
#include "mongo/util/net/hostandport.h"
#include "mongo/util/time_support.h"

namespace mongo {

    /**
     * Represents a pool of connections to a MongoDB server. The pool is synchronized internally
     * and its methods can be called from multiple threads, however once a connection is obtained
     * it should only be used from one thread in accordance with the rules of DBClientInterface.
     */
    class ConnectionPool {
        MONGO_DISALLOW_COPYING(ConnectionPool);
    public:
        /**
         * Information about a connection in the pool.
         */
        struct ConnectionInfo {
            ConnectionInfo() : conn(NULL) {}
            ConnectionInfo(DBClientConnection* theConn, Date_t date)
                : conn(theConn),
                  creationDate(date) {}

            // A connection in the pool.
            DBClientConnection* const conn;

            // The date at which the connection was created.
            Date_t creationDate;
        };

        typedef stdx::list<ConnectionInfo> ConnectionList;
        typedef unordered_map<HostAndPort, ConnectionList> HostConnectionMap;
        typedef std::map<HostAndPort, Date_t> HostLastUsedMap;


        /**
         * RAII class for connections from the pool.  To use the connection pool, instantiate one of
         * these with a pointer to the pool, the identity of the target node and the timeout for
         * network operations, use it like a pointer to a connection, and then call done() on
         * successful completion.  Failure to call done() will lead to the connection being reaped
         * when the holder goes out of scope.
         */
        class ConnectionPtr {
            MONGO_DISALLOW_COPYING(ConnectionPtr);
        public:
            /**
             * Constructs a ConnectionPtr referring to a connection to "target" drawn from "pool",
             * with the network timeout set to "timeout".
             *
             * Throws DBExceptions if the connection cannot be established.
             */
            ConnectionPtr(ConnectionPool* pool,
                          const HostAndPort& target,
                          Date_t now,
                          Milliseconds timeout);

            /**
             * Destructor reaps the connection if it wasn't already returned to the pool by calling
             * done().
             */
            ~ConnectionPtr();

            /**
             * Obtains the underlying connection which can be used for making calls to the server.
             */
            DBClientConnection* get() const { return _connInfo->conn; }

            /**
             * Releases the connection back to the pool from which it was drawn.
             */
            void done(Date_t now);

        private:
            ConnectionPool* _pool;
            const ConnectionList::iterator _connInfo;
        };


        /**
         * Instantiates a new connection pool with the specified tags to be applied to the
         * messaging ports that it opens.
         *
         * @param messagingPortTags tags to be applied to the messaging ports for each of the
         *      connections. If no tags are required, use 0.
         */
        ConnectionPool(int messagingPortTags);
        ~ConnectionPool();

        /**
         * Acquires a connection to "target" with the given "timeout", or throws a DBException.
         * Intended for use by ConnectionPtr.
         */
        ConnectionList::iterator acquireConnection(const HostAndPort& target,
                                                   Date_t now,
                                                   Milliseconds timeout);

        /**
         * Releases a connection back into the pool.
         * Intended for use by ConnectionPtr.
         * Call this for connections that can safely be reused.
         */
        void releaseConnection(ConnectionList::iterator iter, Date_t now);

        /**
         * Destroys a connection previously acquired from the pool.
         * Intended for use by ConnectionPtr.
         * Call this for connections that cannot be reused.
         */
        void destroyConnection(ConnectionList::iterator);

        /**
         * Closes all connections currently in use, to ensure that the network threads
         * terminate promptly during shutdown.
         */
        void closeAllInUseConnections();

        /**
         * Reaps all connections in the pool that are too old as of "now".
         */
        void cleanUpOlderThan(Date_t now);

    private:
        /**
         * Returns true if the given connection is young enough to keep in the pool.
         */
        bool _shouldKeepConnection(Date_t now, const ConnectionInfo& connInfo) const;

        /**
         * Apply cleanup policy to any host(s) not active in the last kCleanupInterval milliseconds.
         */
        void _cleanUpStaleHosts_inlock(Date_t now);

        /**
         * Implementation of cleanUpOlderThan which assumes that _mutex is already held.
         */
        void _cleanUpOlderThan_inlock(Date_t now);

        /**
         * Reaps connections in "hostConns" that are too old or have been in the pool too long as of
         * "now".  Expects _mutex to be held.
         */
        void _cleanUpOlderThan_inlock(Date_t now, ConnectionList* hostConns);

        /**
         * Destroys the connection associated with "iter" and removes "iter" fron connList.
         */
        static void _destroyConnection_inlock(ConnectionList* connList,
                                              ConnectionList::iterator iter);


        // Flags to apply to the opened connections
        const int _messagingPortTags;

        // Mutex guarding members of the connection pool
        boost::mutex _mutex;

        // Map from HostAndPort to idle connections.
        HostConnectionMap _connections;

        // List of non-idle connections.
        ConnectionList _inUseConnections;

        // Map of HostAndPorts to when they were last used.
        HostLastUsedMap _lastUsedHosts;
        
        // Time representing when the connections were last cleaned.
        Date_t _lastCleanUpTime;
    };

} // namespace mongo