SessionReconnectHandler.cs

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 *
 * OPC Foundation MIT License 1.00
 * 
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using System;
using System.Threading;
using System.Threading.Tasks;
using Opc.Ua.Redaction;

namespace Opc.Ua.Client
{
    /// <summary>
    /// Attempts to reconnect to the server.
    /// </summary>
    public class SessionReconnectHandler : IDisposable
    {
        /// <summary>
        /// The minimum reconnect period in ms.
        /// </summary>
        public const int MinReconnectPeriod = 500;

        /// <summary>
        /// The maximum reconnect period in ms.
        /// </summary>
        public const int MaxReconnectPeriod = 30000;

        /// <summary>
        /// The default reconnect period in ms.
        /// </summary>
        public const int DefaultReconnectPeriod = 1000;

        /// <summary>
        /// The default reconnect operation timeout in ms.
        /// </summary>
        public const int MinReconnectOperationTimeout = 5000;

        /// <summary>
        /// The internal state of the reconnect handler.
        /// </summary>
        public enum ReconnectState
        {
            /// <summary>
            /// The reconnect handler is ready to start the reconnect timer.
            /// </summary>
            Ready = 0,

            /// <summary>
            /// The reconnect timer is triggered and waiting to reconnect.
            /// </summary>
            Triggered = 1,

            /// <summary>
            /// The reconnection is in progress.
            /// </summary>
            Reconnecting = 2,

            /// <summary>
            /// The reconnect handler is disposed and can not be used for further reconnect attempts.
            /// </summary>
            Disposed = 4
        };

        /// <summary>
        /// Create a reconnect handler.
        /// </summary>
        /// <param name="reconnectAbort">Set to <c>true</c> to allow reconnect abort if keep alive recovered.</param>
        /// <param name="maxReconnectPeriod">
        ///     The upper limit for the reconnect period after exponential backoff.
        ///     -1 (default) indicates that no exponential backoff should be used.
        /// </param>
        public SessionReconnectHandler(bool reconnectAbort = false, int maxReconnectPeriod = -1)
        {
            m_reconnectAbort = reconnectAbort;
            m_reconnectTimer = new Timer(OnReconnectAsync, this, Timeout.Infinite, Timeout.Infinite);
            m_state = ReconnectState.Ready;
            m_cancelReconnect = false;
            m_updateFromServer = false;
            m_baseReconnectPeriod = DefaultReconnectPeriod;
            m_maxReconnectPeriod = maxReconnectPeriod < 0 ? -1 :
                Math.Max(MinReconnectPeriod, Math.Min(maxReconnectPeriod, MaxReconnectPeriod));
            m_random = new Random();
        }

        #region IDisposable Members
        /// <summary>
        /// Frees any unmanaged resources.
        /// </summary>
        public void Dispose()
        {
            Dispose(true);
            GC.SuppressFinalize(this);
        }

        /// <summary>
        /// An overrideable version of the Dispose.
        /// </summary>
        protected virtual void Dispose(bool disposing)
        {
            if (disposing)
            {
                lock (m_lock)
                {
                    if (m_reconnectTimer != null)
                    {
                        m_reconnectTimer.Dispose();
                        m_reconnectTimer = null;
                    }
                    m_state = ReconnectState.Disposed;
                }
            }
        }
        #endregion

        #region Public Methods
        /// <summary>
        /// Gets the session managed by the handler.
        /// </summary>
        /// <value>The session.</value>
        public ISession Session => m_session;

        /// <summary>
        /// The internal state of the reconnect handler.
        /// </summary>
        public ReconnectState State
        {
            get
            {
                lock (m_lock)
                {
                    if (m_reconnectTimer == null)
                    {
                        return ReconnectState.Disposed;
                    }
                    return m_state;
                }
            }
        }

        /// <summary>
        /// Cancel a reconnect in progress.
        /// </summary>
        public void CancelReconnect()
        {
            lock (m_lock)
            {
                if (m_reconnectTimer == null)
                {
                    return;
                }

                if (m_state == ReconnectState.Triggered)
                {
                    m_session = null;
                    EnterReadyState();
                    return;
                }

                m_cancelReconnect = true;
            }
        }

        /// <summary>
        /// Begins the reconnect process.
        /// </summary>
        public ReconnectState BeginReconnect(ISession session, int reconnectPeriod, EventHandler callback)
        {
            return BeginReconnect(session, null, reconnectPeriod, callback);
        }

        /// <summary>
        /// Begins the reconnect process using a reverse connection.
        /// </summary>
        public ReconnectState BeginReconnect(ISession session, ReverseConnectManager reverseConnectManager, int reconnectPeriod, EventHandler callback)
        {
            lock (m_lock)
            {
                if (m_reconnectTimer == null)
                {
                    throw new ServiceResultException(StatusCodes.BadInvalidState);
                }

                // cancel reconnect requested, if possible
                if (session == null)
                {
                    if (m_state == ReconnectState.Triggered)
                    {
                        m_session = null;
                        EnterReadyState();
                        return m_state;
                    }
                    // reconnect already in progress, schedule cancel
                    m_cancelReconnect = true;
                    return m_state;
                }

                // set reconnect period within boundaries
                reconnectPeriod = CheckedReconnectPeriod(reconnectPeriod);

                // ignore subsequent trigger requests
                if (m_state == ReconnectState.Ready)
                {
                    m_session = session;
                    m_baseReconnectPeriod = reconnectPeriod;
                    m_reconnectFailed = false;
                    m_cancelReconnect = false;
                    m_callback = callback;
                    m_reverseConnectManager = reverseConnectManager;
                    m_reconnectTimer.Change(JitteredReconnectPeriod(reconnectPeriod), Timeout.Infinite);
                    m_reconnectPeriod = CheckedReconnectPeriod(reconnectPeriod, true);
                    m_state = ReconnectState.Triggered;
                    return m_state;
                }

                // if triggered, reset timer only if requested reconnect period is shorter
                if (m_state == ReconnectState.Triggered && reconnectPeriod < m_baseReconnectPeriod)
                {
                    m_baseReconnectPeriod = reconnectPeriod;
                    m_reconnectTimer.Change(JitteredReconnectPeriod(reconnectPeriod), Timeout.Infinite);
                    m_reconnectPeriod = CheckedReconnectPeriod(reconnectPeriod, true);
                }

                return m_state;
            }
        }

        /// <summary>
        /// Returns the reconnect period with a random jitter.
        /// </summary>
        public virtual int JitteredReconnectPeriod(int reconnectPeriod)
        {
            // The factors result in a jitter of 10%.
            const int JitterResolution = 1000;
            const int JitterFactor = 10;
            int jitter = (reconnectPeriod * m_random.Next(-JitterResolution, JitterResolution)) /
                (JitterResolution * JitterFactor);
            return reconnectPeriod + jitter;
        }

        /// <summary>
        /// Returns the reconnect period within the min and max boundaries.
        /// </summary>
        public virtual int CheckedReconnectPeriod(int reconnectPeriod, bool exponentialBackoff = false)
        {
            // exponential backoff is controlled by m_maxReconnectPeriod
            if (m_maxReconnectPeriod > MinReconnectPeriod)
            {
                if (exponentialBackoff)
                {
                    reconnectPeriod *= 2;
                }
                return Math.Min(Math.Max(reconnectPeriod, MinReconnectPeriod), m_maxReconnectPeriod);
            }
            else
            {
                return Math.Max(reconnectPeriod, MinReconnectPeriod);
            }
        }
        #endregion

        #region Private Methods
        /// <summary>
        /// Called when the reconnect timer expires.
        /// </summary>
        private async void OnReconnectAsync(object state)
        {
            DateTime reconnectStart = DateTime.UtcNow;
            try
            {
                // check for exit.
                lock (m_lock)
                {
                    if (m_reconnectTimer == null || m_session == null)
                    {
                        return;
                    }
                    if (m_state != ReconnectState.Triggered)
                    {
                        return;
                    }
                    // enter reconnecting state
                    m_state = ReconnectState.Reconnecting;
                }

                bool keepaliveRecovered = false;

                // preserve legacy behavior if reconnectAbort is not set
                if (m_session != null && m_reconnectAbort &&
                    m_session.Connected && !m_session.KeepAliveStopped)
                {
                    keepaliveRecovered = true;
                    // breaking change, the callback must only assign the new
                    // session if the property is != null
                    m_session = null;
                    Utils.LogInfo("Reconnect {0} aborted, KeepAlive recovered.", m_session?.SessionId);
                }
                else
                {
                    Utils.LogInfo("Reconnect {0}.", m_session?.SessionId);
                }

                // do the reconnect or recover state.
                if (keepaliveRecovered ||
                    await DoReconnectAsync().ConfigureAwait(false))
                {
                    lock (m_lock)
                    {
                        EnterReadyState();
                    }

                    // notify the caller.
                    m_callback(this, null);

                    return;
                }
            }
            catch (Exception exception)
            {
                Utils.LogError("Unexpected error during reconnect: {0}", Redact.Create(exception));
            }

            // schedule the next reconnect.
            lock (m_lock)
            {
                if (m_state != ReconnectState.Disposed)
                {
                    if (m_cancelReconnect)
                    {
                        EnterReadyState();
                    }
                    else
                    {
                        int elapsed = (int)DateTime.UtcNow.Subtract(reconnectStart).TotalMilliseconds;
                        Utils.LogInfo("Reconnect period is {0} ms, {1} ms elapsed in reconnect.", m_reconnectPeriod, elapsed);
                        int adjustedReconnectPeriod = CheckedReconnectPeriod(m_reconnectPeriod - elapsed);
                        adjustedReconnectPeriod = JitteredReconnectPeriod(adjustedReconnectPeriod);
                        m_reconnectTimer.Change(adjustedReconnectPeriod, Timeout.Infinite);
                        Utils.LogInfo("Next adjusted reconnect scheduled in {0} ms.", adjustedReconnectPeriod);
                        m_reconnectPeriod = CheckedReconnectPeriod(m_reconnectPeriod, true);
                        m_state = ReconnectState.Triggered;
                    }
                }
            }
        }

        /// <summary>
        /// Reconnects to the server.
        /// </summary>
        private async Task<bool> DoReconnectAsync()
        {
            // helper to override operation timeout
            int operationTimeout = m_session.OperationTimeout;
            int reconnectOperationTimeout = Math.Max(m_reconnectPeriod, MinReconnectOperationTimeout);

            // try a reconnect.
            if (!m_reconnectFailed)
            {
                try
                {
                    m_session.OperationTimeout = reconnectOperationTimeout;
                    if (m_reverseConnectManager != null)
                    {
                        var connection = await m_reverseConnectManager.WaitForConnection(
                                new Uri(m_session.Endpoint.EndpointUrl),
                                m_session.Endpoint.Server.ApplicationUri
                            ).ConfigureAwait(false);

                        await m_session.ReconnectAsync(connection).ConfigureAwait(false);
                    }
                    else
                    {
                        await m_session.ReconnectAsync().ConfigureAwait(false);
                    }

                    // monitored items should start updating on their own.
                    return true;
                }
                catch (Exception exception)
                {
                    // recreate the session if it has been closed.
                    if (exception is ServiceResultException sre)
                    {
                        Utils.LogWarning("Reconnect failed. Reason={0}.", sre.Result);

                        // check if the server endpoint could not be reached.
                        if (sre.StatusCode == StatusCodes.BadTcpInternalError ||
                            sre.StatusCode == StatusCodes.BadCommunicationError ||
                            sre.StatusCode == StatusCodes.BadNotConnected ||
                            sre.StatusCode == StatusCodes.BadRequestTimeout ||
                            sre.StatusCode == StatusCodes.BadTimeout)
                        {
                            // check if reactivating is still an option.
                            TimeSpan timeout = m_session.LastKeepAliveTime.AddMilliseconds(m_session.SessionTimeout) - DateTime.UtcNow;
                            if (timeout.TotalMilliseconds > 0)
                            {
                                Utils.LogInfo("Retry to reactivate, est. session timeout in {0} ms.", timeout.TotalMilliseconds);
                                return false;
                            }
                        }

                        // check if the security configuration may have changed
                        if (sre.StatusCode == StatusCodes.BadSecurityChecksFailed ||
                            sre.StatusCode == StatusCodes.BadCertificateInvalid)
                        {
                            m_updateFromServer = true;
                            Utils.LogInfo("Reconnect failed due to security check. Request endpoint update from server. {0}", sre.Message);
                        }
                        // wait for next scheduled reconnect if connection failed,
                        // otherwise recreate session immediately
                        else if (sre.StatusCode != StatusCodes.BadSessionIdInvalid)
                        {
                            // next attempt is to recreate session
                            m_reconnectFailed = true;
                            return false;
                        }
                    }
                    else
                    {
                        Utils.LogError(exception, "Reconnect failed.");
                    }

                    m_reconnectFailed = true;
                }
                finally
                {
                    m_session.OperationTimeout = operationTimeout;
                }
            }

            // re-create the session.
            try
            {
                ISession session;
                m_session.OperationTimeout = reconnectOperationTimeout;
                if (m_reverseConnectManager != null)
                {
                    ITransportWaitingConnection connection;
                    do
                    {
                        connection = await m_reverseConnectManager.WaitForConnection(
                                new Uri(m_session.Endpoint.EndpointUrl),
                                m_session.Endpoint.Server.ApplicationUri
                            ).ConfigureAwait(false);

                        if (m_updateFromServer)
                        {
                            var endpoint = m_session.ConfiguredEndpoint;
                            await endpoint.UpdateFromServerAsync(
                                endpoint.EndpointUrl, connection,
                                endpoint.Description.SecurityMode,
                                endpoint.Description.SecurityPolicyUri).ConfigureAwait(false);
                            m_updateFromServer = false;
                            connection = null;
                        }
                    } while (connection == null);

                    session = await m_session.SessionFactory.RecreateAsync(m_session, connection).ConfigureAwait(false);
                }
                else
                {
                    if (m_updateFromServer)
                    {
                        var endpoint = m_session.ConfiguredEndpoint;
                        await endpoint.UpdateFromServerAsync(
                            endpoint.EndpointUrl,
                            endpoint.Description.SecurityMode,
                            endpoint.Description.SecurityPolicyUri).ConfigureAwait(false);
                        m_updateFromServer = false;
                    }

                    session = await m_session.SessionFactory.RecreateAsync(m_session).ConfigureAwait(false);
                }
                // note: the template session is not connected at this point
                //       and must be disposed by the owner
                m_session = session;
                return true;
            }
            catch (ServiceResultException sre)
            {
                if (sre.InnerResult?.StatusCode == StatusCodes.BadSecurityChecksFailed ||
                    sre.InnerResult?.StatusCode == StatusCodes.BadCertificateInvalid)
                {
                    // schedule endpoint update and retry
                    m_updateFromServer = true;
                    if (m_maxReconnectPeriod > MinReconnectPeriod &&
                        m_reconnectPeriod >= m_maxReconnectPeriod)
                    {
                        m_reconnectPeriod = m_baseReconnectPeriod;
                    }
                    Utils.LogError("Could not reconnect due to failed security check. Request endpoint update from server. {0}", Redact.Create(sre));
                }
                else
                {
                    Utils.LogError("Could not reconnect the Session. {0}", Redact.Create(sre));
                }
                return false;
            }
            catch (Exception exception)
            {
                Utils.LogError("Could not reconnect the Session. {0}", Redact.Create(exception));
                return false;
            }
            finally
            {
                m_session.OperationTimeout = operationTimeout;
            }
        }

        /// <summary>
        /// Reset the timer and enter ready state. 
        /// </summary>
        private void EnterReadyState()
        {
            m_reconnectTimer.Change(Timeout.Infinite, Timeout.Infinite);
            m_state = ReconnectState.Ready;
            m_cancelReconnect = false;
            m_updateFromServer = false;
        }
        #endregion

        #region Private Fields
        private readonly object m_lock = new object();
        private ISession m_session;
        private ReconnectState m_state;
        private Random m_random;
        private bool m_reconnectFailed;
        private bool m_reconnectAbort;
        private bool m_cancelReconnect;
        private bool m_updateFromServer;
        private int m_reconnectPeriod;
        private int m_baseReconnectPeriod;
        private int m_maxReconnectPeriod;
        private Timer m_reconnectTimer;
        private EventHandler m_callback;
        private ReverseConnectManager m_reverseConnectManager;
        #endregion
    }
}