Improve session handling.

Source/MQTTnet/Client/MqttClient.cs

@@ -66,24 +66,26 @@ namespace MQTTnet.Client
                 _packetIdentifierProvider.Reset();
                 _packetDispatcher.Reset();
 
-                _cancellationTokenSource = new CancellationTokenSource();
+                var cancellationTokenSource = new CancellationTokenSource();
+                _cancellationTokenSource = cancellationTokenSource;
+
                 _disconnectGate = 0;
                 _adapter = _adapterFactory.CreateClientAdapter(options, _logger);
 
                 _logger.Verbose($"Trying to connect with server ({Options.ChannelOptions}).");
-                await _adapter.ConnectAsync(Options.CommunicationTimeout, _cancellationTokenSource.Token).ConfigureAwait(false);
+                await _adapter.ConnectAsync(Options.CommunicationTimeout, cancellationTokenSource.Token).ConfigureAwait(false);
                 _logger.Verbose("Connection with server established.");
 
-                StartReceivingPackets(_cancellationTokenSource.Token);
+                StartReceivingPackets(cancellationTokenSource.Token);
 
-                var connectResult = await AuthenticateAsync(options.WillMessage, _cancellationTokenSource.Token).ConfigureAwait(false);
+                var connectResult = await AuthenticateAsync(options.WillMessage, cancellationTokenSource.Token).ConfigureAwait(false);
                 _logger.Verbose("MQTT connection with server established.");
 
                 _sendTracker.Restart();
 
                 if (Options.KeepAlivePeriod != TimeSpan.Zero)
                 {
-                    StartSendingKeepAliveMessages(_cancellationTokenSource.Token);
+                    StartSendingKeepAliveMessages(cancellationTokenSource.Token);
                 }
 
                 IsConnected = true;
@@ -112,7 +114,7 @@ namespace MQTTnet.Client
             {
                 _cleanDisconnectInitiated = true;
 
-                if (IsConnected && !_cancellationTokenSource.IsCancellationRequested)
+                if (IsConnected && _cancellationTokenSource?.IsCancellationRequested == false)
                 {
                     var disconnectPacket = CreateDisconnectPacket(options);
                     await SendAsync(disconnectPacket, _cancellationTokenSource.Token).ConfigureAwait(false);

Source/MQTTnet/Client/MqttClientExtensions.cs

@@ -1,5 +1,4 @@
 using System;
-using System.Linq;
 using System.Threading.Tasks;
 using MQTTnet.Client.Subscribing;
 using MQTTnet.Client.Unsubscribing;
@@ -21,7 +20,7 @@ namespace MQTTnet.Client
             if (client == null) throw new ArgumentNullException(nameof(client));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            return client.SubscribeAsync(topicFilters.ToList());
+            return client.SubscribeAsync(topicFilters);
         }
 
         public static Task<MqttClientSubscribeResult> SubscribeAsync(this IMqttClient client, string topic, MqttQualityOfServiceLevel qualityOfServiceLevel)
@@ -45,7 +44,7 @@ namespace MQTTnet.Client
             if (client == null) throw new ArgumentNullException(nameof(client));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            return client.UnsubscribeAsync(topicFilters.ToList());
+            return client.UnsubscribeAsync(topicFilters);
         }
     }
 }

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -93,7 +93,7 @@ namespace MQTTnet.Implementations
             using (cancellationToken.Register(() => _socket.Dispose()))
             {
                 await _stream.WriteAsync(buffer, offset, count, cancellationToken).ConfigureAwait(false);
-                await _stream.FlushAsync(cancellationToken);
+                //await _stream.FlushAsync(cancellationToken);
             }
         }
 

Source/MQTTnet/Server/IMqttClientSessionStatus.cs

@@ -18,7 +18,11 @@ namespace MQTTnet.Server
 
         TimeSpan LastNonKeepAlivePacketReceived { get; }
 
-        int PendingApplicationMessagesCount { get; }
+        long PendingApplicationMessagesCount { get; }
+
+        long ReceivedApplicationMessagesCount { get; }
+
+        long SentApplicationMessagesCount { get; }
 
         Task DisconnectAsync();
 

Source/MQTTnet/Server/IMqttServer.cs

@@ -21,8 +21,8 @@ namespace MQTTnet.Server
         IList<MqttApplicationMessage> GetRetainedMessages();
         Task ClearRetainedMessagesAsync();
 
-        Task SubscribeAsync(string clientId, IList<TopicFilter> topicFilters);
-        Task UnsubscribeAsync(string clientId, IList<string> topicFilters);
+        Task SubscribeAsync(string clientId, IEnumerable<TopicFilter> topicFilters);
+        Task UnsubscribeAsync(string clientId, IEnumerable<string> topicFilters);
 
         Task StartAsync(IMqttServerOptions options);
         Task StopAsync();

Source/MQTTnet/Server/MqttClientSession.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
@@ -18,7 +19,7 @@ namespace MQTTnet.Server
         private readonly MqttRetainedMessagesManager _retainedMessagesManager;
         private readonly MqttServerEventDispatcher _eventDispatcher;
         private readonly MqttClientKeepAliveMonitor _keepAliveMonitor;
-        private readonly MqttClientPendingPacketsQueue _pendingPacketsQueue;
+        private readonly MqttClientSessionPendingMessagesQueue _pendingMessagesQueue;
         private readonly MqttClientSubscriptionsManager _subscriptionsManager;
         private readonly MqttClientSessionsManager _sessionsManager;
 
@@ -31,6 +32,9 @@ namespace MQTTnet.Server
         private Task _workerTask;
         private IMqttChannelAdapter _channelAdapter;
 
+        private long _receivedMessagesCount;
+        private bool _isCleanSession = true;
+
         public MqttClientSession(
             string clientId,
             IMqttServerOptions options,
@@ -52,7 +56,7 @@ namespace MQTTnet.Server
 
             _keepAliveMonitor = new MqttClientKeepAliveMonitor(this, _logger);
             _subscriptionsManager = new MqttClientSubscriptionsManager(clientId, _options, eventDispatcher);
-            _pendingPacketsQueue = new MqttClientPendingPacketsQueue(_options, this, _logger);
+            _pendingMessagesQueue = new MqttClientSessionPendingMessagesQueue(_options, this, _logger);
         }
 
         public string ClientId { get; }
@@ -63,28 +67,38 @@ namespace MQTTnet.Server
             status.IsConnected = _cancellationTokenSource != null;
             status.Endpoint = _channelAdapter?.Endpoint;
             status.ProtocolVersion = _channelAdapter?.PacketFormatterAdapter?.ProtocolVersion;
-            status.PendingApplicationMessagesCount = _pendingPacketsQueue.Count;
+            status.PendingApplicationMessagesCount = _pendingMessagesQueue.Count;
+            status.ReceivedApplicationMessagesCount = _pendingMessagesQueue.SentMessagesCount;
+            status.SentApplicationMessagesCount = Interlocked.Read(ref _receivedMessagesCount);
             status.LastPacketReceived = _keepAliveMonitor.LastPacketReceived;
             status.LastNonKeepAlivePacketReceived = _keepAliveMonitor.LastNonKeepAlivePacketReceived;
         }
 
-        public Task StopAsync(MqttClientDisconnectType type)
+        public async Task StopAsync(MqttClientDisconnectType type)
         {
-            return StopAsync(type, false);
+            StopInternal(type);
+
+            var task = _workerTask;
+            if (task != null && !task.IsCompleted)
+            {
+                await task.ConfigureAwait(false);
+            }
         }
 
-        public async Task SubscribeAsync(IList<TopicFilter> topicFilters)
+        public async Task SubscribeAsync(IEnumerable<TopicFilter> topicFilters)
         {
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
+            var topicFiltersCollection = topicFilters.ToList();
+
             var packet = new MqttSubscribePacket();
-            packet.TopicFilters.AddRange(topicFilters);
+            packet.TopicFilters.AddRange(topicFiltersCollection);
 
             await _subscriptionsManager.SubscribeAsync(packet).ConfigureAwait(false);
-            await EnqueueSubscribedRetainedMessagesAsync(topicFilters).ConfigureAwait(false);
+            await EnqueueSubscribedRetainedMessagesAsync(topicFiltersCollection).ConfigureAwait(false);
         }
 
-        public Task UnsubscribeAsync(IList<string> topicFilters)
+        public Task UnsubscribeAsync(IEnumerable<string> topicFilters)
         {
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
@@ -98,12 +112,12 @@ namespace MQTTnet.Server
 
         public void ClearPendingApplicationMessages()
         {
-            _pendingPacketsQueue.Clear();
+            _pendingMessagesQueue.Clear();
         }
 
         public void Dispose()
         {
-            _pendingPacketsQueue?.Dispose();
+            _pendingMessagesQueue?.Dispose();
 
             _cancellationTokenSource?.Cancel();
             _cancellationTokenSource?.Dispose();
@@ -161,7 +175,7 @@ namespace MQTTnet.Server
                 publishPacket.QualityOfServiceLevel = context.ApplicationMessage.QualityOfServiceLevel;
             }
 
-            _pendingPacketsQueue.Enqueue(publishPacket);
+            _pendingMessagesQueue.Enqueue(publishPacket);
         }
 
         private async Task RunInternalAsync(MqttConnectPacket connectPacket, IMqttChannelAdapter channelAdapter)
@@ -170,26 +184,41 @@ namespace MQTTnet.Server
 
             try
             {
-                channelAdapter.ReadingPacketStarted += OnAdapterReadingPacketStarted;
-                channelAdapter.ReadingPacketCompleted += OnAdapterReadingPacketCompleted;
+                _logger.Info("Client '{0}': Connected.", ClientId);
+                _eventDispatcher.OnClientConnected(ClientId);
+
+                _channelAdapter = channelAdapter;
+
+                _channelAdapter.ReadingPacketStarted += OnAdapterReadingPacketStarted;
+                _channelAdapter.ReadingPacketCompleted += OnAdapterReadingPacketCompleted;
 
-                _cancellationTokenSource = new CancellationTokenSource();
+                var cancellationTokenSource = new CancellationTokenSource();
+                _cancellationTokenSource = cancellationTokenSource;
 
                 _wasCleanDisconnect = false;
                 _willMessage = connectPacket.WillMessage;
 
-                _pendingPacketsQueue.Start(channelAdapter, _cancellationTokenSource.Token);
-                _keepAliveMonitor.Start(connectPacket.KeepAlivePeriod, _cancellationTokenSource.Token);
+                _pendingMessagesQueue.Start(channelAdapter, cancellationTokenSource.Token);
+                _keepAliveMonitor.Start(connectPacket.KeepAlivePeriod, cancellationTokenSource.Token);
 
-                _channelAdapter = channelAdapter;
+                await channelAdapter.SendPacketAsync(
+                    new MqttConnAckPacket
+                    {
+                        ReturnCode = MqttConnectReturnCode.ConnectionAccepted,
+                        ReasonCode = MqttConnectReasonCode.Success,
+                        IsSessionPresent = _isCleanSession
+                    },
+                    cancellationTokenSource.Token).ConfigureAwait(false);
 
-                while (!_cancellationTokenSource.IsCancellationRequested)
+                _isCleanSession = false;
+
+                while (!cancellationTokenSource.IsCancellationRequested)
                 {
-                    var packet = await channelAdapter.ReceivePacketAsync(TimeSpan.Zero, _cancellationTokenSource.Token).ConfigureAwait(false);
+                    var packet = await channelAdapter.ReceivePacketAsync(TimeSpan.Zero, cancellationTokenSource.Token).ConfigureAwait(false);
                     if (packet != null)
                     {
                         _keepAliveMonitor.PacketReceived(packet);
-                        await ProcessReceivedPacketAsync(channelAdapter, packet, _cancellationTokenSource.Token).ConfigureAwait(false);
+                        await ProcessReceivedPacketAsync(channelAdapter, packet, cancellationTokenSource.Token).ConfigureAwait(false);
                     }
                 }
             }
@@ -203,6 +232,9 @@ namespace MQTTnet.Server
                     if (exception is MqttCommunicationClosedGracefullyException)
                     {
                         _logger.Verbose("Client '{0}': Connection closed gracefully.", ClientId);
+
+                        StopInternal(MqttClientDisconnectType.Clean);
+                        return;
                     }
                     else
                     {
@@ -214,69 +246,50 @@ namespace MQTTnet.Server
                     _logger.Error(exception, "Client '{0}': Unhandled exception while receiving client packets.", ClientId);
                 }
 
-                await StopAsync(MqttClientDisconnectType.NotClean, true).ConfigureAwait(false);
+                StopInternal(MqttClientDisconnectType.NotClean);
             }
             finally
             {
-                channelAdapter.ReadingPacketStarted -= OnAdapterReadingPacketStarted;
-                channelAdapter.ReadingPacketCompleted -= OnAdapterReadingPacketCompleted;
-
-                _cancellationTokenSource?.Cancel(false);
-                _cancellationTokenSource?.Dispose();
-                _cancellationTokenSource = null;
-
-                _workerTask = null;
-            }
-        }
-
-        private async Task StopAsync(MqttClientDisconnectType type, bool isInsideSession)
-        {
-            try
-            {
-                var cts = _cancellationTokenSource;
-                if (cts == null || cts.IsCancellationRequested)
-                {
-                    return;
-                }
-
-                _wasCleanDisconnect = type == MqttClientDisconnectType.Clean;
-
-                _cancellationTokenSource?.Cancel(false);
-
                 if (_willMessage != null && !_wasCleanDisconnect)
                 {
                     _sessionsManager.EnqueueApplicationMessage(this, _willMessage);
                 }
 
                 _willMessage = null;
-
-                if (!isInsideSession)
-                {
-                    if (_workerTask != null)
-                    {
-                        await _workerTask.ConfigureAwait(false);
-                    }
-                }
                 
-                await Task.FromResult(0);
-            }
-            finally
-            {
+                _channelAdapter.ReadingPacketStarted -= OnAdapterReadingPacketStarted;
+                _channelAdapter.ReadingPacketCompleted -= OnAdapterReadingPacketCompleted;
+                _channelAdapter = null;
+                
                 _logger.Info("Client '{0}': Session stopped.", ClientId);
                 _eventDispatcher.OnClientDisconnected(ClientId, _wasCleanDisconnect);
+
+                _workerTask = null;
             }
         }
+        
+        private void StopInternal(MqttClientDisconnectType type)
+        {
+            var cts = _cancellationTokenSource;
+            if (cts == null || cts.IsCancellationRequested)
+            {
+                return;
+            }
+
+            _wasCleanDisconnect = type == MqttClientDisconnectType.Clean;
+            _cancellationTokenSource?.Cancel(false);
+        }
 
-        private Task ProcessReceivedPacketAsync(IMqttChannelAdapter adapter, MqttBasePacket packet, CancellationToken cancellationToken)
+        private Task ProcessReceivedPacketAsync(IMqttChannelAdapter channelAdapter, MqttBasePacket packet, CancellationToken cancellationToken)
         {
             if (packet is MqttPublishPacket publishPacket)
             {
-                return HandleIncomingPublishPacketAsync(adapter, publishPacket, cancellationToken);
+                return HandleIncomingPublishPacketAsync(channelAdapter, publishPacket, cancellationToken);
             }
 
             if (packet is MqttPingReqPacket)
             {
-                return adapter.SendPacketAsync(new MqttPingRespPacket(), cancellationToken);
+                return channelAdapter.SendPacketAsync(new MqttPingRespPacket(), cancellationToken);
             }
 
             if (packet is MqttPubRelPacket pubRelPacket)
@@ -287,7 +300,7 @@ namespace MQTTnet.Server
                     ReasonCode = MqttPubCompReasonCode.Success
                 };
 
-                return adapter.SendPacketAsync(responsePacket, cancellationToken);
+                return channelAdapter.SendPacketAsync(responsePacket, cancellationToken);
             }
 
             if (packet is MqttPubRecPacket pubRecPacket)
@@ -298,7 +311,7 @@ namespace MQTTnet.Server
                     ReasonCode = MqttPubRelReasonCode.Success
                 };
 
-                return adapter.SendPacketAsync(responsePacket, cancellationToken);
+                return channelAdapter.SendPacketAsync(responsePacket, cancellationToken);
             }
 
             if (packet is MqttPubAckPacket || packet is MqttPubCompPacket)
@@ -308,27 +321,24 @@ namespace MQTTnet.Server
 
             if (packet is MqttSubscribePacket subscribePacket)
             {
-                return HandleIncomingSubscribePacketAsync(adapter, subscribePacket, cancellationToken);
+                return HandleIncomingSubscribePacketAsync(channelAdapter, subscribePacket, cancellationToken);
             }
 
             if (packet is MqttUnsubscribePacket unsubscribePacket)
             {
-                return HandleIncomingUnsubscribePacketAsync(adapter, unsubscribePacket, cancellationToken);
+                return HandleIncomingUnsubscribePacketAsync(channelAdapter, unsubscribePacket, cancellationToken);
             }
 
             if (packet is MqttDisconnectPacket)
             {
-                return StopAsync(MqttClientDisconnectType.Clean, true);
-            }
-
-            if (packet is MqttConnectPacket)
-            {
-                return StopAsync(MqttClientDisconnectType.NotClean, true);
+                StopInternal(MqttClientDisconnectType.Clean);
+                return Task.FromResult(0);
             }
 
-            _logger.Warning(null, "Client '{0}': Received not supported packet ({1}). Closing connection.", ClientId, packet);
+            _logger.Warning(null, "Client '{0}': Received invalid packet ({1}). Closing connection.", ClientId, packet);
 
-            return StopAsync(MqttClientDisconnectType.NotClean, true);
+            StopInternal(MqttClientDisconnectType.NotClean);
+            return Task.FromResult(0);
         }
 
         private async Task EnqueueSubscribedRetainedMessagesAsync(ICollection<TopicFilter> topicFilters)
@@ -347,7 +357,8 @@ namespace MQTTnet.Server
 
             if (subscribeResult.CloseConnection)
             {
-                await StopAsync(MqttClientDisconnectType.NotClean, true).ConfigureAwait(false);
+                StopInternal(MqttClientDisconnectType.NotClean);
+                return;
             }
 
             await EnqueueSubscribedRetainedMessagesAsync(subscribePacket.TopicFilters).ConfigureAwait(false);
@@ -361,12 +372,13 @@ namespace MQTTnet.Server
 
         private Task HandleIncomingPublishPacketAsync(IMqttChannelAdapter adapter, MqttPublishPacket publishPacket, CancellationToken cancellationToken)
         {
+            Interlocked.Increment(ref _receivedMessagesCount);
+
             switch (publishPacket.QualityOfServiceLevel)
             {
                 case MqttQualityOfServiceLevel.AtMostOnce:
                     {
-                        HandleIncomingPublishPacketWithQoS0(publishPacket);
-                        return Task.FromResult(0);
+                        return HandleIncomingPublishPacketWithQoS0Async(publishPacket);
                     }
                 case MqttQualityOfServiceLevel.AtLeastOnce:
                     {
@@ -383,11 +395,13 @@ namespace MQTTnet.Server
             }
         }
 
-        private void HandleIncomingPublishPacketWithQoS0(MqttPublishPacket publishPacket)
+        private Task HandleIncomingPublishPacketWithQoS0Async(MqttPublishPacket publishPacket)
         {
             _sessionsManager.EnqueueApplicationMessage(
                 this,
                 _channelAdapter.PacketFormatterAdapter.DataConverter.CreateApplicationMessage(publishPacket));
+
+            return Task.FromResult(0);
         }
 
         private Task HandleIncomingPublishPacketWithQoS1Async(

Source/MQTTnet/Server/MqttClientPendingPacketsQueue.cs → Source/MQTTnet/Server/MqttClientSessionPendingMessagesQueue.cs

@@ -11,22 +11,24 @@ using MQTTnet.Protocol;
 
 namespace MQTTnet.Server
 {
-    public class MqttClientPendingPacketsQueue : IDisposable
+    public class MqttClientSessionPendingMessagesQueue : IDisposable
     {
         private readonly Queue<MqttBasePacket> _queue = new Queue<MqttBasePacket>();
-        private readonly AsyncAutoResetEvent _queueAutoResetEvent = new AsyncAutoResetEvent();
+        private readonly AsyncAutoResetEvent _queueLock = new AsyncAutoResetEvent();
 
         private readonly IMqttServerOptions _options;
         private readonly MqttClientSession _clientSession;
         private readonly IMqttNetChildLogger _logger;
 
-        public MqttClientPendingPacketsQueue(IMqttServerOptions options, MqttClientSession clientSession, IMqttNetChildLogger logger)
+        private long _sentPacketsCount;
+
+        public MqttClientSessionPendingMessagesQueue(IMqttServerOptions options, MqttClientSession clientSession, IMqttNetChildLogger logger)
         {
             if (logger == null) throw new ArgumentNullException(nameof(logger));
             _options = options ?? throw new ArgumentNullException(nameof(options));
             _clientSession = clientSession ?? throw new ArgumentNullException(nameof(clientSession));
 
-            _logger = logger.CreateChildLogger(nameof(MqttClientPendingPacketsQueue));
+            _logger = logger.CreateChildLogger(nameof(MqttClientSessionPendingMessagesQueue));
         }
 
         public int Count
@@ -40,6 +42,8 @@ namespace MQTTnet.Server
             }
         }
 
+        public long SentMessagesCount => Interlocked.Read(ref _sentPacketsCount);
+
         public void Start(IMqttChannelAdapter adapter, CancellationToken cancellationToken)
         {
             if (adapter == null) throw new ArgumentNullException(nameof(adapter));
@@ -52,7 +56,7 @@ namespace MQTTnet.Server
             Task.Run(() => SendQueuedPacketsAsync(adapter, cancellationToken), cancellationToken);
         }
 
-        public void Enqueue(MqttBasePacket packet)
+        public void Enqueue(MqttPublishPacket packet)
         {
             if (packet == null) throw new ArgumentNullException(nameof(packet));
 
@@ -70,11 +74,11 @@ namespace MQTTnet.Server
                         _queue.Dequeue();
                     }
                 }
-                
+
                 _queue.Enqueue(packet);
             }
 
-            _queueAutoResetEvent.Set();
+            _queueLock.Set();
 
             _logger.Verbose("Enqueued packet (ClientId: {0}).", _clientSession.ClientId);
         }
@@ -114,6 +118,11 @@ namespace MQTTnet.Server
             MqttBasePacket packet = null;
             try
             {
+                if (cancellationToken.IsCancellationRequested)
+                {
+                    return;
+                }
+
                 lock (_queue)
                 {
                     if (_queue.Count > 0)
@@ -124,18 +133,15 @@ namespace MQTTnet.Server
 
                 if (packet == null)
                 {
-                    await _queueAutoResetEvent.WaitOneAsync(cancellationToken).ConfigureAwait(false);
-                    return;
-                }
-
-                if (cancellationToken.IsCancellationRequested)
-                {
+                    await _queueLock.WaitOneAsync(cancellationToken).ConfigureAwait(false);
                     return;
                 }
 
                 await adapter.SendPacketAsync(packet, cancellationToken).ConfigureAwait(false);
 
                 _logger.Verbose("Enqueued packet sent (ClientId: {0}).", _clientSession.ClientId);
+
+                Interlocked.Increment(ref _sentPacketsCount);
             }
             catch (Exception exception)
             {

Source/MQTTnet/Server/MqttClientSessionStatus.cs

@@ -21,7 +21,9 @@ namespace MQTTnet.Server
         public MqttProtocolVersion? ProtocolVersion { get; set; }
         public TimeSpan LastPacketReceived { get; set; }
         public TimeSpan LastNonKeepAlivePacketReceived { get; set; }
-        public int PendingApplicationMessagesCount { get; set; }
+        public long PendingApplicationMessagesCount { get; set; }
+        public long ReceivedApplicationMessagesCount { get; set; }
+        public long SentApplicationMessagesCount { get; set; }
 
         public Task DisconnectAsync()
         {

Source/MQTTnet/Server/MqttClientSessionsManager.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
@@ -49,27 +50,31 @@ namespace MQTTnet.Server
 
         public async Task StopAsync()
         {
-            using (await _sessionsLock.WaitAsync(_cancellationToken).ConfigureAwait(false))
+            List<MqttClientSession> sessions;
+            using (await _sessionsLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
             {
-                foreach (var session in _sessions)
-                {
-                    await session.Value.StopAsync(MqttClientDisconnectType.NotClean).ConfigureAwait(false);
-                }
+                sessions = _sessions.Values.ToList();
+            }
 
-                _sessions.Clear();
+            foreach (var session in sessions)
+            {
+                await session.StopAsync(MqttClientDisconnectType.Clean).ConfigureAwait(false);
             }
         }
 
         public Task HandleConnectionAsync(IMqttChannelAdapter clientAdapter)
         {
-            return Task.Run(() => HandleConnectionAsync(clientAdapter, _cancellationToken), _cancellationToken);
+            return HandleConnectionAsync(clientAdapter, _cancellationToken);
+
+            // TODO: Check if Task.Run is required.
+            //return Task.Run(() => HandleConnectionAsync(clientAdapter, _cancellationToken), _cancellationToken);
         }
 
         public async Task<IList<IMqttClientSessionStatus>> GetClientStatusAsync()
         {
             var result = new List<IMqttClientSessionStatus>();
 
-            using (await _sessionsLock.WaitAsync(_cancellationToken).ConfigureAwait(false))
+            using (await _sessionsLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
             {
                 foreach (var session in _sessions.Values)
                 {
@@ -90,42 +95,47 @@ namespace MQTTnet.Server
             _messageQueue.Add(new MqttEnqueuedApplicationMessage(senderClientSession, applicationMessage), _cancellationToken);
         }
 
-        public Task SubscribeAsync(string clientId, IList<TopicFilter> topicFilters)
+        public async Task SubscribeAsync(string clientId, IEnumerable<TopicFilter> topicFilters)
         {
             if (clientId == null) throw new ArgumentNullException(nameof(clientId));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            lock (_sessions)
+            using (await _sessionsLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
             {
                 if (!_sessions.TryGetValue(clientId, out var session))
                 {
                     throw new InvalidOperationException($"Client session '{clientId}' is unknown.");
                 }
 
-                return session.SubscribeAsync(topicFilters);
+                await session.SubscribeAsync(topicFilters).ConfigureAwait(false);
             }
         }
 
-        public Task UnsubscribeAsync(string clientId, IList<string> topicFilters)
+        public async Task UnsubscribeAsync(string clientId, IEnumerable<string> topicFilters)
         {
             if (clientId == null) throw new ArgumentNullException(nameof(clientId));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            lock (_sessions)
+            using (await _sessionsLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
             {
                 if (!_sessions.TryGetValue(clientId, out var session))
                 {
                     throw new InvalidOperationException($"Client session '{clientId}' is unknown.");
                 }
 
-                return session.UnsubscribeAsync(topicFilters);
+                await session.UnsubscribeAsync(topicFilters).ConfigureAwait(false);
             }
         }
 
         public async Task DeleteSessionAsync(string clientId)
         {
-            using (await _sessionsLock.WaitAsync(_cancellationToken).ConfigureAwait(false))
+            using (await _sessionsLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
             {
+                if (_sessions.TryGetValue(clientId, out var session))
+                {
+                    session.Dispose();
+                }
+
                 _sessions.Remove(clientId);
             }
 
@@ -187,7 +197,7 @@ namespace MQTTnet.Server
                     await _retainedMessagesManager.HandleMessageAsync(sender?.ClientId, applicationMessage).ConfigureAwait(false);
                 }
 
-                using (await _sessionsLock.WaitAsync(_cancellationToken).ConfigureAwait(false))
+                using (await _sessionsLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
                 {
                     foreach (var clientSession in _sessions.Values)
                     {
@@ -207,49 +217,37 @@ namespace MQTTnet.Server
             }
         }
 
-        private async Task HandleConnectionAsync(IMqttChannelAdapter clientAdapter, CancellationToken cancellationToken)
+        private async Task HandleConnectionAsync(IMqttChannelAdapter channelAdapter, CancellationToken cancellationToken)
         {
             var clientId = string.Empty;
-            
+
             try
             {
-                var firstPacket = await clientAdapter.ReceivePacketAsync(_options.DefaultCommunicationTimeout, cancellationToken).ConfigureAwait(false);
+                var firstPacket = await channelAdapter.ReceivePacketAsync(_options.DefaultCommunicationTimeout, cancellationToken).ConfigureAwait(false);
                 if (!(firstPacket is MqttConnectPacket connectPacket))
                 {
-                    _logger.Warning(null, "The first packet from client '{0}' was no 'CONNECT' packet [MQTT-3.1.0-1].", clientAdapter.Endpoint);
+                    _logger.Warning(null, "The first packet from client '{0}' was no 'CONNECT' packet [MQTT-3.1.0-1].", channelAdapter.Endpoint);
                     return;
                 }
 
                 clientId = connectPacket.ClientId;
 
-                var connectReturnCode = await ValidateConnectionAsync(connectPacket, clientAdapter).ConfigureAwait(false);
+                var connectReturnCode = await ValidateConnectionAsync(connectPacket, channelAdapter).ConfigureAwait(false);
                 if (connectReturnCode != MqttConnectReturnCode.ConnectionAccepted)
                 {
-                    await clientAdapter.SendPacketAsync(
+                    await channelAdapter.SendPacketAsync(
                         new MqttConnAckPacket
                         {
-                            ReturnCode = connectReturnCode
+                            ReturnCode = connectReturnCode,
+                            ReasonCode = MqttConnectReasonCode.NotAuthorized
                         },
                         cancellationToken).ConfigureAwait(false);
 
                     return;
                 }
 
-                var result = await PrepareClientSessionAsync(connectPacket).ConfigureAwait(false);
-
-                await clientAdapter.SendPacketAsync(
-                    new MqttConnAckPacket
-                    {
-                        ReturnCode = connectReturnCode,
-                        ReasonCode = MqttConnectReasonCode.Success,
-                        IsSessionPresent = result.IsExistingSession
-                    },
-                    cancellationToken).ConfigureAwait(false);
-
-                _logger.Info("Client '{0}': Connected.", clientId);
-                _eventDispatcher.OnClientConnected(clientId);
-
-                await result.Session.RunAsync(connectPacket, clientAdapter).ConfigureAwait(false);
+                var session = await PrepareClientSessionAsync(connectPacket).ConfigureAwait(false);
+                await session.RunAsync(connectPacket, channelAdapter).ConfigureAwait(false);
             }
             catch (OperationCanceledException)
             {
@@ -260,12 +258,15 @@ namespace MQTTnet.Server
             }
             finally
             {
-                await clientAdapter.DisconnectAsync(_options.DefaultCommunicationTimeout, cancellationToken).ConfigureAwait(false);
-                clientAdapter.Dispose();
+                await TryCleanupChannelAsync(channelAdapter).ConfigureAwait(false);
 
                 if (!_options.EnablePersistentSessions)
                 {
-                    await DeleteSessionAsync(clientId).ConfigureAwait(false);
+                    // TODO: Check if the session will be used later.
+                    // Consider reference counter or "Recycle" property
+                    // Or add timer (will be required for MQTTv5 (session life time) "IsActiveProperty".
+                    //öö
+                    //await DeleteSessionAsync(clientId).ConfigureAwait(false);
                 }
             }
         }
@@ -288,18 +289,17 @@ namespace MQTTnet.Server
             return context.ReturnCode;
         }
 
-        private async Task<PrepareClientSessionResult> PrepareClientSessionAsync(MqttConnectPacket connectPacket)
+        private async Task<MqttClientSession> PrepareClientSessionAsync(MqttConnectPacket connectPacket)
         {
-            using (await _sessionsLock.WaitAsync(_cancellationToken).ConfigureAwait(false))
+            using (await _sessionsLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
             {
                 var isSessionPresent = _sessions.TryGetValue(connectPacket.ClientId, out var clientSession);
                 if (isSessionPresent)
                 {
-                    await clientSession.StopAsync(MqttClientDisconnectType.Clean).ConfigureAwait(false);
-
                     if (connectPacket.CleanSession)
                     {
-                        _sessions.Remove(connectPacket.ClientId);
+                        await clientSession.StopAsync(MqttClientDisconnectType.Clean).ConfigureAwait(false);
+
                         clientSession.Dispose();
                         clientSession = null;
 
@@ -307,22 +307,21 @@ namespace MQTTnet.Server
                     }
                     else
                     {
+                        await clientSession.StopAsync(MqttClientDisconnectType.Clean).ConfigureAwait(false);
+
                         _logger.Verbose("Reusing existing session of client '{0}'.", connectPacket.ClientId);
                     }
                 }
 
-                var isExistingSession = true;
                 if (clientSession == null)
                 {
-                    isExistingSession = false;
-
                     clientSession = new MqttClientSession(connectPacket.ClientId, _options, this, _retainedMessagesManager, _eventDispatcher, _logger);
                     _sessions[connectPacket.ClientId] = clientSession;
 
                     _logger.Verbose("Created a new session for client '{0}'.", connectPacket.ClientId);
                 }
 
-                return new PrepareClientSessionResult { IsExistingSession = isExistingSession, Session = clientSession };
+                return clientSession;
             }
         }
 
@@ -338,5 +337,21 @@ namespace MQTTnet.Server
             await interceptor.InterceptApplicationMessagePublishAsync(interceptorContext).ConfigureAwait(false);
             return interceptorContext;
         }
+
+        private async Task TryCleanupChannelAsync(IMqttChannelAdapter channelAdapter)
+        {
+            try
+            {
+                await channelAdapter.DisconnectAsync(_options.DefaultCommunicationTimeout, CancellationToken.None).ConfigureAwait(false);
+            }
+            catch (Exception exception)
+            {
+                _logger.Error(exception, "Error while disconnecting client channel.");
+            }
+            finally
+            {
+                channelAdapter.Dispose();
+            }
+        }
     }
 }

Source/MQTTnet/Server/MqttServer.cs

@@ -56,7 +56,7 @@ namespace MQTTnet.Server
             return _retainedMessagesManager.GetMessagesAsync().GetAwaiter().GetResult();
         }
 
-        public Task SubscribeAsync(string clientId, IList<TopicFilter> topicFilters)
+        public Task SubscribeAsync(string clientId, IEnumerable<TopicFilter> topicFilters)
         {
             if (clientId == null) throw new ArgumentNullException(nameof(clientId));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
@@ -64,7 +64,7 @@ namespace MQTTnet.Server
             return _clientSessionsManager.SubscribeAsync(clientId, topicFilters);
         }
 
-        public Task UnsubscribeAsync(string clientId, IList<string> topicFilters)
+        public Task UnsubscribeAsync(string clientId, IEnumerable<string> topicFilters)
         {
             if (clientId == null) throw new ArgumentNullException(nameof(clientId));
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
@@ -118,7 +118,7 @@ namespace MQTTnet.Server
 
                 _cancellationTokenSource.Cancel(false);
                 
-                _clientSessionsManager.StopAsync().ConfigureAwait(false);
+                await _clientSessionsManager.StopAsync().ConfigureAwait(false);
 
                 foreach (var adapter in _adapters)
                 {

Source/MQTTnet/Server/MqttServerExtensions.cs

@@ -0,0 +1,45 @@
+using System;
+using System.Threading.Tasks;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Server
+{
+    public static class MqttServerExtensions
+    {
+        public static Task SubscribeAsync(this IMqttServer server, string clientId, params TopicFilter[] topicFilters)
+        {
+            if (server == null) throw new ArgumentNullException(nameof(server));
+            if (clientId == null) throw new ArgumentNullException(nameof(clientId));
+            if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
+
+            return server.SubscribeAsync(clientId, topicFilters);
+        }
+
+        public static Task SubscribeAsync(this IMqttServer server, string clientId, string topic, MqttQualityOfServiceLevel qualityOfServiceLevel)
+        {
+            if (server == null) throw new ArgumentNullException(nameof(server));
+            if (clientId == null) throw new ArgumentNullException(nameof(clientId));
+            if (topic == null) throw new ArgumentNullException(nameof(topic));
+
+            return server.SubscribeAsync(clientId, new TopicFilterBuilder().WithTopic(topic).WithQualityOfServiceLevel(qualityOfServiceLevel).Build());
+        }
+
+        public static Task SubscribeAsync(this IMqttServer server, string clientId, string topic)
+        {
+            if (server == null) throw new ArgumentNullException(nameof(server));
+            if (clientId == null) throw new ArgumentNullException(nameof(clientId));
+            if (topic == null) throw new ArgumentNullException(nameof(topic));
+
+            return server.SubscribeAsync(clientId, new TopicFilterBuilder().WithTopic(topic).Build());
+        }
+
+        public static Task UnsubscribeAsync(this IMqttServer server, string clientId, params string[] topicFilters)
+        {
+            if (server == null) throw new ArgumentNullException(nameof(server));
+            if (clientId == null) throw new ArgumentNullException(nameof(clientId));
+            if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
+
+            return server.UnsubscribeAsync(clientId, topicFilters);
+        }
+    }
+}

Tests/MQTTnet.Core.Tests/MqttServerTests.cs

@@ -17,7 +17,7 @@ using MQTTnet.Server;
 namespace MQTTnet.Tests
 {
     [TestClass]
-    public class MqttServerTests
+    public partial class MqttServerTests
     {
         [TestMethod]
         public void MqttServer_PublishSimple_AtMostOnce()
@@ -263,6 +263,91 @@ namespace MQTTnet.Tests
             }
         }
 
+        [TestMethod]
+        public async Task MqttServer_No_Messages_If_No_Subscription()
+        {
+            var server = new MqttFactory().CreateMqttServer();
+            try
+            {
+                await server.StartAsync(new MqttServerOptions());
+
+                var client = new MqttFactory().CreateMqttClient();
+                var receivedMessages = new List<MqttApplicationMessage>();
+
+                var options = new MqttClientOptionsBuilder()
+                    .WithTcpServer("localhost").Build();
+
+                client.Connected += async (s, e) =>
+                {
+                    await client.PublishAsync("Connected");
+                };
+
+                client.ApplicationMessageReceived += (s, e) =>
+                {
+                    lock (receivedMessages)
+                    {
+                        receivedMessages.Add(e.ApplicationMessage);
+                    }
+                };
+
+                await client.ConnectAsync(options);
+
+                await Task.Delay(500);
+
+                await client.PublishAsync("Hello");
+
+                await Task.Delay(500);
+                
+                Assert.AreEqual(0, receivedMessages.Count);
+            }
+            finally
+            {
+                await server.StopAsync();
+            }
+        }
+
+        [TestMethod]
+        public async Task MqttServer_Set_Subscription_At_Server()
+        {
+            var server = new MqttFactory().CreateMqttServer();
+            try
+            {
+                await server.StartAsync(new MqttServerOptions());
+                server.ClientConnected += async (s, e) =>
+                {
+                    await server.SubscribeAsync(e.ClientId, "topic1");
+                };
+
+                var client = new MqttFactory().CreateMqttClient();
+                var receivedMessages = new List<MqttApplicationMessage>();
+
+                var options = new MqttClientOptionsBuilder()
+                    .WithTcpServer("localhost").Build();
+
+                client.ApplicationMessageReceived += (s, e) =>
+                {
+                    lock (receivedMessages)
+                    {
+                        receivedMessages.Add(e.ApplicationMessage);
+                    }
+                };
+
+                await client.ConnectAsync(options);
+
+                await Task.Delay(500);
+
+                await client.PublishAsync("Hello");
+
+                await Task.Delay(500);
+
+                Assert.AreEqual(0, receivedMessages.Count);
+            }
+            finally
+            {
+                await server.StopAsync();
+            }
+        }
+
         private static async Task Publish(IMqttClient c1, MqttApplicationMessage message)
         {
             for (int i = 0; i < 1000; i++)
@@ -302,40 +387,29 @@ namespace MQTTnet.Tests
         [TestMethod]
         public async Task MqttServer_Handle_Clean_Disconnect()
         {
-            var s = new MqttFactory().CreateMqttServer();
-            try
+            using (var testSetup = new TestSetup())
             {
+                var server = await testSetup.StartServerAsync(new MqttServerOptionsBuilder());
+
                 var clientConnectedCalled = 0;
                 var clientDisconnectedCalled = 0;
 
-                s.ClientConnected += (_, __) => clientConnectedCalled++;
-                s.ClientDisconnected += (_, __) => clientDisconnectedCalled++;
-
-                var clientOptions = new MqttClientOptionsBuilder()
-                    .WithTcpServer("localhost")
-                    .Build();
+                server.ClientConnected += (_, __) => Interlocked.Increment(ref clientConnectedCalled);
+                server.ClientDisconnected += (_, __) => Interlocked.Increment(ref clientDisconnectedCalled);
+                
+                var c1 = await testSetup.ConnectClient(new MqttClientOptionsBuilder());
 
-                await s.StartAsync(new MqttServerOptions());
+                Assert.AreEqual(1, clientConnectedCalled);
+                Assert.AreEqual(0, clientDisconnectedCalled);
 
-                var c1 = new MqttFactory().CreateMqttClient();
-
-                await c1.ConnectAsync(clientOptions);
-
-                await Task.Delay(100);
+                await Task.Delay(500);
 
                 await c1.DisconnectAsync();
 
-                await Task.Delay(100);
-
-                await s.StopAsync();
-
-                await Task.Delay(100);
+                await Task.Delay(500);
 
-                Assert.AreEqual(clientConnectedCalled, clientDisconnectedCalled);
-            }
-            finally
-            {
-                await s.StopAsync();
+                Assert.AreEqual(1, clientConnectedCalled);
+                Assert.AreEqual(1, clientDisconnectedCalled);
             }
         }
 
@@ -385,7 +459,7 @@ namespace MQTTnet.Tests
         }
 
         [TestMethod]
-        public async Task MqttServer_LotsOfRetainedMessages()
+        public async Task MqttServer_Lots_Of_Retained_Messages()
         {
             const int ClientCount = 100;
 
@@ -745,58 +819,64 @@ namespace MQTTnet.Tests
         }
 
         [TestMethod]
-        public async Task MqttServer_SameClientIdConnectDisconnectEventOrder()
+        public async Task MqttServer_Same_Client_Id_Connect_Disconnect_Event_Order()
         {
-            var s = new MqttFactory().CreateMqttServer();
+            using (var testSetup = new TestSetup())
+            {
+                var server = await testSetup.StartServerAsync(new MqttServerOptionsBuilder());
 
-            var events = new List<string>();
+                var events = new List<string>();
 
-            s.ClientConnected += (_, __) =>
-            {
-                lock (events)
+                server.ClientConnected += (_, __) =>
                 {
-                    events.Add("c");
-                }
-            };
+                    lock (events)
+                    {
+                        events.Add("c");
+                    }
+                };
 
-            s.ClientDisconnected += (_, __) =>
-            {
-                lock (events)
+                server.ClientDisconnected += (_, __) =>
                 {
-                    events.Add("d");
-                }
-            };
-
-            var clientOptions = new MqttClientOptionsBuilder()
-                .WithTcpServer("localhost")
-                .WithClientId("same_id")
-                .Build();
+                    lock (events)
+                    {
+                        events.Add("d");
+                    }
+                };
 
-            await s.StartAsync(new MqttServerOptions());
+                var clientOptions = new MqttClientOptionsBuilder()
+                    .WithClientId("same_id");
 
-            var c1 = new MqttFactory().CreateMqttClient();
-            var c2 = new MqttFactory().CreateMqttClient();
+                // c
+                var c1 = await testSetup.ConnectClient(clientOptions);
+                
+                await Task.Delay(500);
 
-            await c1.ConnectAsync(clientOptions);
+                var flow = string.Join(string.Empty, events);
+                Assert.AreEqual("c", flow);
 
-            await Task.Delay(250);
+                // dc
+                var c2 = await testSetup.ConnectClient(clientOptions);
 
-            await c2.ConnectAsync(clientOptions);
+                await Task.Delay(500);
 
-            await Task.Delay(250);
+                flow = string.Join(string.Empty, events);
+                Assert.AreEqual("cdc", flow);
 
-            await c1.DisconnectAsync();
+                // nothing
+                await c1.DisconnectAsync();
 
-            await Task.Delay(250);
+                await Task.Delay(500);
 
-            await c2.DisconnectAsync();
+                // d
+                await c2.DisconnectAsync();
 
-            await Task.Delay(250);
+                await Task.Delay(500);
 
-            await s.StopAsync();
+                await server.StopAsync();
 
-            var flow = string.Join(string.Empty, events);
-            Assert.AreEqual("cdcd", flow);
+                flow = string.Join(string.Empty, events);
+                Assert.AreEqual("cdcd", flow);
+            }
         }
 
         [TestMethod]

Tests/MQTTnet.Core.Tests/TestSetup.cs

@@ -0,0 +1,92 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using MQTTnet.Client;
+using MQTTnet.Client.Options;
+using MQTTnet.Diagnostics;
+using MQTTnet.Server;
+
+namespace MQTTnet.Tests
+{
+    public class TestSetup : IDisposable
+    {
+        private readonly MqttFactory _mqttFactory = new MqttFactory();
+        private readonly List<IMqttClient> _clients = new List<IMqttClient>();
+        private readonly IMqttNetLogger _serverLogger = new MqttNetLogger("server");
+        private readonly IMqttNetLogger _clientLogger = new MqttNetLogger("client");
+
+        private IMqttServer _server;
+
+        private long _serverErrorsCount;
+        private long _clientErrorsCount;
+
+        public TestSetup()
+        {
+            _serverLogger.LogMessagePublished += (s, e) =>
+            {
+                if (e.TraceMessage.Level == MqttNetLogLevel.Error)
+                {
+                    Interlocked.Increment(ref _serverErrorsCount);
+                }
+            };
+
+            _clientLogger.LogMessagePublished += (s, e) =>
+            {
+                if (e.TraceMessage.Level == MqttNetLogLevel.Error)
+                {
+                    Interlocked.Increment(ref _clientErrorsCount);
+                }
+            };
+        }
+
+        public async Task<IMqttServer> StartServerAsync(MqttServerOptionsBuilder options)
+        {
+            if (_server != null)
+            {
+                throw new InvalidOperationException("Server already started.");
+            }
+
+            _server = _mqttFactory.CreateMqttServer(_serverLogger);
+            await _server.StartAsync(options.WithDefaultEndpointPort(1888).Build());
+
+            return _server;
+        }
+
+        public async Task<IMqttClient> ConnectClient(MqttClientOptionsBuilder options)
+        {
+            var client = _mqttFactory.CreateMqttClient(_clientLogger);
+            _clients.Add(client);
+
+            await client.ConnectAsync(options.WithTcpServer("localhost", 1888).Build());
+
+            return client;
+        }
+
+        public void ThrowIfLogErrors()
+        {
+            if (_serverErrorsCount > 0)
+            {
+                throw new Exception($"Server had {_serverErrorsCount} errors.");
+            }
+
+            if (_clientErrorsCount > 0)
+            {
+                throw new Exception($"Client(s) had {_clientErrorsCount} errors.");
+            }
+        }
+
+        public void Dispose()
+        {
+            ThrowIfLogErrors();
+
+            foreach (var mqttClient in _clients)
+            {
+                mqttClient.DisconnectAsync().GetAwaiter().GetResult();
+                mqttClient.Dispose();
+            }
+
+            _server.StopAsync().GetAwaiter().GetResult();
+        }
+    }
+}