// Copyright (c) 2004, 2019, Oracle and/or its affiliates. All rights reserved.
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License, version 2.0, as
// published by the Free Software Foundation.
//
// This program is also distributed with certain software (including
// but not limited to OpenSSL) that is licensed under separate terms,
// as designated in a particular file or component or in included license
// documentation.  The authors of MySQL hereby grant you an
// additional permission to link the program and your derivative works
// with the separately licensed software that they have included with
// MySQL.
//
// Without limiting anything contained in the foregoing, this file,
// which is part of MySQL Connector/NET, is also subject to the
// Universal FOSS Exception, version 1.0, a copy of which can be found at
// http://oss.oracle.com/licenses/universal-foss-exception.
//
// 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 General Public License, version 2.0, for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; if not, write to the Free Software Foundation, Inc.,
// 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Threading;

namespace MySql.Data.MySqlClient
{
    /// <summary>
    /// Summary description for MySqlPool.
    /// </summary>
    internal sealed class MySqlPool
    {
        private readonly List<Driver> _inUsePool;
        private readonly LinkedList<Driver> _idlePool;
        private readonly uint _minSize;
        private readonly uint _maxSize;
        private readonly AutoResetEvent _autoEvent;
        private int _available;

        private void EnqueueIdle(Driver driver)
        {
            driver.IdleSince = DateTime.Now;
            _idlePool.AddLast(driver);
        }
        public MySqlPool(MySqlConnectionStringBuilder settings)
        {
            _minSize = settings.MinimumPoolSize;
            _maxSize = settings.MaximumPoolSize;

            _available = (int)_maxSize;
            _autoEvent = new AutoResetEvent(false);

            if (_minSize > _maxSize)
                _minSize = _maxSize;
            this.Settings = settings;
            _inUsePool = new List<Driver>((int)_maxSize);
            _idlePool = new LinkedList<Driver>();

            // prepopulate the idle pool to minSize
            for (int i = 0; i < _minSize; i++)
                EnqueueIdle(CreateNewPooledConnection());

            ProcedureCache = new ProcedureCache((int)settings.ProcedureCacheSize);
        }

        #region Properties

        public MySqlConnectionStringBuilder Settings { get; set; }

        public ProcedureCache ProcedureCache { get; }

        /// <summary>
        /// It is assumed that this property will only be used from inside an active
        /// lock.
        /// </summary>
        private bool HasIdleConnections => _idlePool.Count > 0;

        private int NumConnections => _idlePool.Count + _inUsePool.Count;

        /// <summary>
        /// Indicates whether this pool is being cleared.
        /// </summary>
        public bool BeingCleared { get; private set; }

        internal Dictionary<string, string> ServerProperties { get; set; }

        #endregion

        /// <summary>
        /// It is assumed that this method is only called from inside an active lock.
        /// </summary>
        private Driver GetPooledConnection()
        {
            Driver driver = null;

            // if we don't have an idle connection but we have room for a new
            // one, then create it here.
            lock ((_idlePool as ICollection).SyncRoot)
            {
                if (HasIdleConnections)
                {
                    driver = _idlePool.Last.Value;
                    _idlePool.RemoveLast();
                }
            }

            // Obey the connection timeout
            if (driver != null)
            {
                try
                {
                    driver.ResetTimeout((int)Settings.ConnectionTimeout * 1000);
                }
                catch (Exception)
                {
                    driver.Close();
                    driver = null;
                }
            }

            if (driver != null)
            {
                // first check to see that the server is still alive
                if (!driver.Ping())
                {
                    driver.Close();
                    driver = null;
                }
                else if (Settings.ConnectionReset)
                {
                    // if the user asks us to ping/reset pooled connections
                    // do so now
                    driver.Reset();
                }
            }
            if (driver == null)
                driver = CreateNewPooledConnection();

            Debug.Assert(driver != null);
            lock ((_inUsePool as ICollection).SyncRoot)
            {
                _inUsePool.Add(driver);
            }
            return driver;
        }

        /// <summary>
        /// It is assumed that this method is only called from inside an active lock.
        /// </summary>
        private Driver CreateNewPooledConnection()
        {
            Debug.Assert((_maxSize - NumConnections) > 0, "Pool out of sync.");

            Driver driver = Driver.Create(Settings);
            driver.Pool = this;
            return driver;
        }

        public void ReleaseConnection(Driver driver)
        {
            lock ((_inUsePool as ICollection).SyncRoot)
            {
                if (_inUsePool.Contains(driver))
                    _inUsePool.Remove(driver);
            }

            if (driver.ConnectionLifetimeExpired() || BeingCleared)
            {
                driver.Close();
                Debug.Assert(!_idlePool.Contains(driver));
            }
            else
            {
                lock ((_idlePool as ICollection).SyncRoot)
                {
                    EnqueueIdle(driver);
                }
            }

            Interlocked.Increment(ref _available);
            _autoEvent.Set();
        }

        /// <summary>
        /// Removes a connection from the in use pool.  The only situations where this method 
        /// would be called are when a connection that is in use gets some type of fatal exception
        /// or when the connection is being returned to the pool and it's too old to be 
        /// returned.
        /// </summary>
        /// <param name="driver"></param>
        public void RemoveConnection(Driver driver)
        {
            lock ((_inUsePool as ICollection).SyncRoot)
            {
                if (_inUsePool.Contains(driver))
                {
                    _inUsePool.Remove(driver);
                    Interlocked.Increment(ref _available);
                    _autoEvent.Set();
                }
            }

            // if we are being cleared and we are out of connections then have
            // the manager destroy us.
            if (BeingCleared && NumConnections == 0)
                MySqlPoolManager.RemoveClearedPool(this);
        }

        private Driver TryToGetDriver()
        {
            int count = Interlocked.Decrement(ref _available);
            if (count < 0)
            {
                Interlocked.Increment(ref _available);
                return null;
            }
            try
            {
                Driver driver = GetPooledConnection();
                return driver;
            }
            catch (Exception ex)
            {
                MySqlTrace.LogError(-1, ex.Message);
                Interlocked.Increment(ref _available);
                throw;
            }
        }

        public Driver GetConnection()
        {
            int fullTimeOut = (int)Settings.ConnectionTimeout * 1000;
            int timeOut = fullTimeOut;

            DateTime start = DateTime.Now;

            while (timeOut > 0)
            {
                Driver driver = TryToGetDriver();
                if (driver != null) return driver;

                // We have no tickets right now, lets wait for one.
                if (!_autoEvent.WaitOne(timeOut, false)) break;

                timeOut = fullTimeOut - (int)DateTime.Now.Subtract(start).TotalMilliseconds;
            }
            throw new MySqlException(Resources.TimeoutGettingConnection);
        }

        /// <summary>
        /// Clears this pool of all idle connections and marks this pool and being cleared
        /// so all other connections are closed when they are returned.
        /// </summary>
        internal void Clear()
        {
            lock ((_idlePool as ICollection).SyncRoot)
            {
                // first, mark ourselves as being cleared
                BeingCleared = true;

                // then we remove all connections sitting in the idle pool
                while (_idlePool.Count > 0)
                {
                    Driver d = _idlePool.Last.Value;
                    d.Close();
                    _idlePool.RemoveLast();
                }

                // there is nothing left to do here.  Now we just wait for all
                // in use connections to be returned to the pool.  When they are
                // they will be closed.  When the last one is closed, the pool will
                // be destroyed.
            }
        }

        /// <summary>
        /// Remove expired drivers from the idle pool
        /// </summary>
        /// <returns></returns>
        /// <remarks>
        /// Closing driver is a potentially lengthy operation involving network
        /// IO. Therefore we do not close expired drivers while holding 
        /// idlePool.SyncRoot lock. We just remove the old drivers from the idle
        /// queue and return them to the caller. The caller will need to close 
        /// them (or let GC close them)
        /// </remarks>
        internal List<Driver> RemoveOldIdleConnections()
        {
            var connectionsToClose = new List<Driver>();
            var now = DateTime.Now;

            lock ((_idlePool as ICollection).SyncRoot)
            {
                while (_idlePool.Count > _minSize)
                {
                    var idleConnection = _idlePool.First.Value;

                    var expirationTime = idleConnection.IdleSince.Add(
                        new TimeSpan(0, 0, MySqlPoolManager.maxConnectionIdleTime));

                    if (expirationTime.CompareTo(now) < 0)
                    {
                        connectionsToClose.Add(idleConnection);
                        _idlePool.RemoveFirst();
                    }
                    else
                    {
                        break;
                    }
                }
            }
            return connectionsToClose;
        }
    }
}