UA-.NETStandard/Libraries/Opc.Ua.Client/Session/SessionReconnectHandler.cs

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 *
 * OPC Foundation MIT License 1.00
 *
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 * obtaining a copy of this software and associated documentation
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using System;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;
using Opc.Ua.Redaction;

namespace Opc.Ua.Client
{
    /// <summary>
    /// Attempts to reconnect to the server using the legacy reconnect
    /// flow. Operates against a raw <see cref="Session"/> (or a derived
    /// type) by driving its <c>ReconnectAsync</c> /
    /// <see cref="ISessionFactory.RecreateAsync(ISession, CancellationToken)"/>
    /// pipeline directly.
    /// </summary>
    /// <remarks>
    /// <para>
    /// New code should prefer <see cref="ManagedSession"/> (created via
    /// <see cref="ManagedSessionFactory"/> or <see cref="ManagedSessionBuilder"/>),
    /// which encapsulates connection lifecycle, reconnect, and server
    /// redundancy failover behind an <see cref="ISession"/> facade.
    /// </para>
    /// <para>
    /// <see cref="SessionReconnectHandler"/> is intentionally retained as
    /// a supported legacy entry point for callers that already manage
    /// <see cref="Session"/> instances directly. It does <b>not</b>
    /// support the <see cref="ManagedSession"/> facade — passing one to
    /// <see cref="BeginReconnect(ISession,int,EventHandler)"/> throws
    /// <see cref="NotSupportedException"/> because the managed session
    /// already runs its own reconnect state machine.
    /// </para>
    /// </remarks>
    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>
        [Flags]
        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>
        /// Obsolete constructor
        /// </summary>
        [Obsolete("Use SessionReconnectHandler(ITelemetryContext, bool, int) instead.")]
        public SessionReconnectHandler(bool reconnectAbort = false, int maxReconnectPeriod = -1)
            // Telemetry is required by the modern ctor; this obsolete bridge forwards null!
            // to preserve the parameterless reconnect-handler instantiation pattern.
            : this(null!, reconnectAbort, maxReconnectPeriod)
        {
        }

        /// <summary>
        /// Create a reconnect handler.
        /// </summary>
        /// <param name="telemetry">The telemetry context to use to create obvservability instruments</param>
        /// <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(
            ITelemetryContext telemetry,
            bool reconnectAbort = false,
            int maxReconnectPeriod = -1)
            : this(telemetry, reconnectAbort, maxReconnectPeriod, timeProvider: null)
        {
        }

        /// <summary>
        /// Create a reconnect handler.
        /// </summary>
        /// <param name="telemetry">The telemetry context to use to create obvservability instruments</param>
        /// <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>
        /// <param name="timeProvider">Optional time provider used for timers and elapsed-time
        /// calculations. Defaults to <see cref="TimeProvider.System"/>.</param>
        public SessionReconnectHandler(
            ITelemetryContext telemetry,
            bool reconnectAbort,
            int maxReconnectPeriod,
            TimeProvider? timeProvider)
        {
            m_timeProvider = timeProvider ?? TimeProvider.System;
            m_telemetry = telemetry;
            m_logger = telemetry.CreateLogger<SessionReconnectHandler>();
            m_reconnectAbort = reconnectAbort;
            m_reconnectTimer = m_timeProvider.CreateTimer(
                OnReconnectAsync,
                this,
                Timeout.InfiniteTimeSpan,
                Timeout.InfiniteTimeSpan);
            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));
        }

        /// <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)
                {
                    m_reconnectTimer?.Dispose();
                    m_reconnectTimer = null;
                    m_state = ReconnectState.Disposed;
                }
            }
        }

        /// <summary>
        /// Gets the session managed by the handler.
        /// </summary>
        /// <value>The session.</value>
        public ISession? Session { get; private set; }

        /// <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)
                {
                    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>
        /// <exception cref="ServiceResultException">if the handler is
        /// already disposed.</exception>
        /// <exception cref="NotSupportedException">if <paramref name="session"/>
        /// is not <see langword="null"/> and is not a <see cref="Session"/>
        /// (or a type derived from it). <see cref="ManagedSession"/> and
        /// other facade implementations of <see cref="ISession"/> drive
        /// their own reconnect state machine and must not be passed
        /// here.</exception>
        public ReconnectState BeginReconnect(
            ISession session,
            ReverseConnectManager? reverseConnectManager,
            int reconnectPeriod,
            EventHandler callback)
        {
            // The reconnect handler drives Session.ReconnectAsync /
            // ISessionFactory.RecreateAsync directly and assumes it owns
            // the session lifecycle. Facade sessions (e.g. ManagedSession)
            // already run their own reconnect / failover state machine,
            // so mixing them here would be incorrect. Allow null
            // (cancellation) and require Session-derived instances
            // otherwise.
            // (Use the fully qualified type to disambiguate from the
            // local Session property.)
            if (session is not null and
                not Client.Session)
            {
                throw new NotSupportedException(
                    "SessionReconnectHandler only supports the legacy " +
                    "Session class (or types derived from it). " +
                    "ManagedSession and other ISession facades manage " +
                    "their own reconnection — use ManagedSession / " +
                    "ManagedSessionFactory instead.");
            }

            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)
                    {
                        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)
                {
                    Session = session;
                    m_baseReconnectPeriod = reconnectPeriod;
                    m_reconnectFailed = false;
                    m_cancelReconnect = false;
                    m_callback = callback;
                    m_reverseConnectManager = reverseConnectManager;
                    m_reconnectTimer.Change(
                        TimeSpan.FromMilliseconds(JitteredReconnectPeriod(reconnectPeriod)),
                        Timeout.InfiniteTimeSpan);
                    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(
                        TimeSpan.FromMilliseconds(JitteredReconnectPeriod(reconnectPeriod)),
                        Timeout.InfiniteTimeSpan);
                    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 *
                UnsecureRandom.Shared.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);
            }

            return Math.Max(reconnectPeriod, MinReconnectPeriod);
        }

        /// <summary>
        /// Called when the reconnect timer expires.
        /// </summary>
        private async void OnReconnectAsync(object? state)
        {
            long reconnectStartTimestamp = m_timeProvider.GetTimestamp();
            try
            {
                // check for exit.
                ISession? session = Session;
                lock (m_lock)
                {
                    if (m_reconnectTimer == null || 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_reconnectAbort &&
                    session.Connected &&
                    !session.KeepAliveStopped)
                {
                    keepaliveRecovered = true;
                    // breaking change, the callback must only assign the new
                    // session if the property is != null
                    m_logger.LogInformation(
                        "Reconnect {SessionId} aborted, KeepAlive recovered.",
                        session.SessionId);
                    Session = session = null;
                }
                else
                {
                    m_logger.LogInformation("Reconnect {SessionId}.", session.SessionId);
                }

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

                    // notify the caller.
                    m_callback?.Invoke(this, EventArgs.Empty);

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

            // schedule the next reconnect.
            lock (m_lock)
            {
                if (m_state != ReconnectState.Disposed)
                {
                    if (m_cancelReconnect)
                    {
                        EnterReadyState();
                    }
                    else
                    {
                        int elapsed = (int)m_timeProvider
                            .GetElapsedTime(reconnectStartTimestamp).TotalMilliseconds;
                        m_logger.LogInformation(
                            "Reconnect period is {ReconnectPeriod} ms, {Elapsed} ms elapsed in reconnect.",
                            m_reconnectPeriod,
                            elapsed);
                        int adjustedReconnectPeriod = CheckedReconnectPeriod(
                            m_reconnectPeriod - elapsed);
                        adjustedReconnectPeriod = JitteredReconnectPeriod(adjustedReconnectPeriod);
                        m_reconnectTimer?.Change(TimeSpan.FromMilliseconds(adjustedReconnectPeriod), Timeout.InfiniteTimeSpan);
                        m_logger.LogInformation(
                            "Next adjusted reconnect scheduled in {ReconnectPeriod} ms.",
                            adjustedReconnectPeriod);
                        m_reconnectPeriod = CheckedReconnectPeriod(m_reconnectPeriod, true);
                        m_state = ReconnectState.Triggered;
                    }
                }
            }
        }

        /// <summary>
        /// Reconnects to the server.
        /// </summary>
        /// <exception cref="ServiceResultException"></exception>
        private async Task<bool> DoReconnectAsync()
        {
            // helper to override operation timeout
            ITransportChannel? transportChannel = null;
            ISession current = Session ??
                throw ServiceResultException.Unexpected("Session is null");

            // try a reconnect.
            if (!m_reconnectFailed)
            {
                try
                {
                    if (m_reverseConnectManager != null)
                    {
                        string? endpointUrl = current.Endpoint.EndpointUrl
                            ?? throw ServiceResultException.Unexpected(
                                "Endpoint Url is null for reverse connect session RECONNECT.");
                        ITransportWaitingConnection connection = await m_reverseConnectManager
                            .WaitForConnectionAsync(
                                new Uri(endpointUrl),
                                current.Endpoint.Server.ApplicationUri)
                            .ConfigureAwait(false);

                        await current.ReconnectAsync(connection).ConfigureAwait(false);
                    }
                    else
                    {
                        await current.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)
                    {
                        m_logger.LogWarning("Reconnect failed. Reason={Reason}.", 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 ||
                            sre.StatusCode == StatusCodes.BadNoCommunication ||
                            sre.StatusCode == StatusCodes.BadConnectionClosed)
                        {
                            // check if reactivating is still an option.
                            int timeout =
                                Convert.ToInt32(current.SessionTimeout) -
                                (int)m_timeProvider.GetElapsedTime(
                                    current.LastKeepAliveTimestamp).TotalMilliseconds;
                            if (timeout > 0)
                            {
                                m_logger.LogInformation(
                                    "Retry to reactivate, est. session timeout in {Timeout} ms.",
                                    timeout);
                                return false;
                            }
                        }

                        // check if the security configuration may have changed
                        if (sre.StatusCode == StatusCodes.BadSecurityChecksFailed ||
                            sre.StatusCode == StatusCodes.BadCertificateInvalid)
                        {
                            m_updateFromServer = true;
                            m_logger.LogInformation(
                                "Reconnect failed due to security check. Request endpoint update from server. {Message}",
                                sre.Message);
                        }
                        // recreate session immediately, use existing channel
                        else if (sre.StatusCode == StatusCodes.BadSessionIdInvalid)
                        {
                            transportChannel = Session.NullableTransportChannel;
#pragma warning disable CS0618 // legacy reconnect flow still uses Detach to keep channel
                            Session.DetachChannel();
#pragma warning restore CS0618
                        }
                        else
                        {
                            // wait for next scheduled reconnect if connection failed,
                            // next attempt is to recreate session
                            m_reconnectFailed = true;
                            return false;
                        }
                    }
                    else
                    {
                        m_logger.LogError(exception, "Reconnect failed.");
                    }

                    m_reconnectFailed = true;
                }
            }

            // re-create the session.
            try
            {
                ISession session;
                if (m_reverseConnectManager != null)
                {
                    ITransportWaitingConnection? connection;
                    do
                    {
                        EndpointDescription? endpointDescription =
                            current.Endpoint ?? transportChannel?.EndpointDescription;
                        string? endpointUrl = endpointDescription?.EndpointUrl
                            ?? throw ServiceResultException.Unexpected(
                                "Endpoint Url is null for reverse connect session RECREATE.");
                        connection = await m_reverseConnectManager
                            .WaitForConnectionAsync(
                                new Uri(endpointUrl),
                                endpointDescription?.Server?.ApplicationUri)
                            .ConfigureAwait(false);

                        if (m_updateFromServer)
                        {
                            await UpdateEndpointFromServerAsync(
                                current.ConfiguredEndpoint,
                                connection).ConfigureAwait(false);
                            m_updateFromServer = false;
                            connection = null;
                        }
                    } while (connection == null);

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

        /// <summary>
        /// Updates the configured endpoint from the server discovery endpoint.
        /// Falls back to the best available endpoint if the original security
        /// configuration is no longer supported by the server.
        /// </summary>
        /// <param name="endpoint">The configured endpoint to update.</param>
        /// <param name="connection">The optional transport connection for reverse connect.</param>
        protected virtual async Task UpdateEndpointFromServerAsync(
            ConfiguredEndpoint endpoint,
            ITransportWaitingConnection? connection = null)
        {
            // EndpointUrl and SecurityPolicyUri are nullable on ConfiguredEndpoint but every
            // session entering reconnect was created with both values populated, so the
            // bangs reflect that lifecycle invariant. The null! arguments in the catch block
            // are intentional sentinels for "no security policy" passed to the modern API
            // which retains a non-nullable parameter for backward compatibility.
            try
            {
                if (connection != null)
                {
                    await endpoint
                        .UpdateFromServerAsync(
                            endpoint.EndpointUrl!,
                            connection,
                            endpoint.Description.SecurityMode,
                            endpoint.Description.SecurityPolicyUri!,
                            m_telemetry)
                        .ConfigureAwait(false);
                }
                else
                {
                    await endpoint
                        .UpdateFromServerAsync(
                            endpoint.EndpointUrl!,
                            endpoint.Description.SecurityMode,
                            endpoint.Description.SecurityPolicyUri!,
                            m_telemetry)
                        .ConfigureAwait(false);
                }
            }
            catch (ServiceResultException sre) when (
                sre.StatusCode == StatusCodes.BadSecurityPolicyRejected ||
                sre.StatusCode == StatusCodes.BadSecurityModeRejected)
            {
                // The original endpoint security configuration is no longer available on the server.
                // Fall back to the best available endpoint without security constraints.
                m_logger.LogWarning(
                    "Original endpoint security configuration not available on server, falling back to best available endpoint. {Message}",
                    sre.Message);
                if (connection != null)
                {
                    await endpoint
                        .UpdateFromServerAsync(
                            endpoint.EndpointUrl!,
                            connection,
                            MessageSecurityMode.Invalid,
                            null!,
                            m_telemetry)
                        .ConfigureAwait(false);
                }
                else
                {
                    await endpoint
                        .UpdateFromServerAsync(
                            endpoint.EndpointUrl!,
                            MessageSecurityMode.Invalid,
                            null!,
                            m_telemetry)
                        .ConfigureAwait(false);
                }
            }
        }

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

        private readonly ITelemetryContext m_telemetry;
        private readonly Lock m_lock = new();
        private ReconnectState m_state;
        private bool m_reconnectFailed;
        private readonly ILogger m_logger;
        private readonly bool m_reconnectAbort;
        private bool m_cancelReconnect;
        private bool m_updateFromServer;
        private int m_reconnectPeriod;
        private int m_baseReconnectPeriod;
        private readonly int m_maxReconnectPeriod;
        private ITimer? m_reconnectTimer;
        private readonly TimeProvider m_timeProvider;
        private EventHandler? m_callback;
        private ReverseConnectManager? m_reverseConnectManager;
    }
}