MQTTnet/Source/MQTTnet.Server/Internal/MqttServerKeepAliveMonitor.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.

using MQTTnet.Diagnostics.Logger;
using MQTTnet.Internal;
using MQTTnet.Protocol;

namespace MQTTnet.Server.Internal
{
    public sealed class MqttServerKeepAliveMonitor
    {
        static readonly MqttServerClientDisconnectOptions DefaultDisconnectOptions = new MqttServerClientDisconnectOptions
        {
            ReasonCode = MqttDisconnectReasonCode.KeepAliveTimeout,
            UserProperties = null,
            ReasonString = null,
            ServerReference = null
        };

        readonly MqttNetSourceLogger _logger;
        readonly MqttServerOptions _options;
        readonly MqttClientSessionsManager _sessionsManager;

        public MqttServerKeepAliveMonitor(MqttServerOptions options, MqttClientSessionsManager sessionsManager, IMqttNetLogger logger)
        {
            _options = options ?? throw new ArgumentNullException(nameof(options));
            _sessionsManager = sessionsManager ?? throw new ArgumentNullException(nameof(sessionsManager));

            if (logger == null)
            {
                throw new ArgumentNullException(nameof(logger));
            }

            _logger = logger.WithSource(nameof(MqttServerKeepAliveMonitor));
        }

        public void Start(CancellationToken cancellationToken)
        {
            // The keep alive monitor spawns a real new thread (LongRunning) because it does not
            // support async/await. Async etc. is avoided here because the thread will usually check
            // the connections every few milliseconds and thus the context changes (due to async) are
            // only consuming resources. Also there is just 1 thread for the entire server which is fine at all!
            Task.Factory.StartNew(_ => DoWork(cancellationToken), cancellationToken, TaskCreationOptions.LongRunning).RunInBackground(_logger);
        }

        void DoWork(CancellationToken cancellationToken)
        {
            try
            {
                _logger.Info("Starting keep alive monitor");

                while (!cancellationToken.IsCancellationRequested)
                {
                    TryProcessClients();
                    Sleep(_options.KeepAliveOptions.MonitorInterval);
                }
            }
            catch (OperationCanceledException)
            {
            }
            catch (Exception exception)
            {
                _logger.Error(exception, "Unhandled exception while checking keep alive timeouts");
            }
            finally
            {
                _logger.Verbose("Stopped checking keep alive timeout");
            }
        }

        static void Sleep(TimeSpan timeout)
        {
            try
            {
                Thread.Sleep(timeout);
            }
            catch (ThreadAbortException)
            {
                // The ThreadAbortException is not actively catched in this project.
                // So we use a one which is similar and will be catched properly.
                throw new OperationCanceledException();
            }
        }

        void TryProcessClient(MqttConnectedClient connection, DateTime now)
        {
            try
            {
                if (!connection.IsRunning)
                {
                    // The connection is already dead or just created so there is no need to check it.
                    return;
                }

                if (connection.ConnectPacket.KeepAlivePeriod == 0)
                {
                    // The keep alive feature is not used by the current connection.
                    return;
                }

                // Values described here: [MQTT-3.1.2-24].
                // If the client sends 5 sec. the server will allow up to 7.5 seconds.
                // If the client sends 1 sec. the server will allow up to 1.5 seconds.
                var maxSecondsWithoutPacket = connection.ConnectPacket.KeepAlivePeriod * 1.5D;

                var secondsWithoutPackage = (now - connection.Statistics.LastPacketSentTimestamp).TotalSeconds;
                if (secondsWithoutPackage < maxSecondsWithoutPacket)
                {
                    // A packet was received before the timeout is affected.
                    return;
                }

                _logger.Warning("Client '{0}': Did not receive any packet or keep alive signal", connection.Id);

                // Execute the disconnection in background so that the keep alive monitor can continue
                // with checking other connections.
                // We do not need to wait for the task so no await is needed.
                // Also the internal state of the connection must be swapped to "Finalizing" because the
                // next iteration of the keep alive timer happens.
                _ = connection.StopAsync(DefaultDisconnectOptions);
            }
            catch (Exception exception)
            {
                _logger.Error(exception, "Client {0}: Unhandled exception while checking keep alive timeouts", connection.Id);
            }
        }

        void TryProcessClients()
        {
            var now = DateTime.UtcNow;
            foreach (var client in _sessionsManager.GetClients())
            {
                TryProcessClient(client, now);
            }
        }
    }
}