Wrap COM exceptions, close client connection when server stops, process retained messages from server

Build/MQTTnet.nuspec

@@ -10,8 +10,11 @@
     <iconUrl>https://raw.githubusercontent.com/chkr1011/MQTTnet/master/Images/Logo_128x128.png</iconUrl>
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <description>MQTTnet is a high performance .NET library for MQTT based communication. It provides a MQTT client and a MQTT server (broker).</description>
-    <releaseNotes>* [Client] Fixed WebSocket sub protocol negotiation (Thanks to @JanEggers)
-
+    <releaseNotes>* [Core] Fixed library reference issues for .NET 4.6 (Thanks to @JanEggers).
+* [Core] Several COM exceptions are now wrapped properly resulting in less warnings in the trace.
+* [Client] Fixed WebSocket sub protocol negotiation for ASP.NET Core 2 servers (Thanks to @JanEggers).
+* [Server] Client connections are now closed when the server is stopped (Thanks to @zhudanfei).
+* [Server] Published messages from the server are now retained (if set) (Thanks to @ChristianRiedl). BREAKING CHANGE!
     </releaseNotes>
     <copyright>Copyright Christian Kratky 2016-2017</copyright>
     <tags>MQTT Message Queue Telemetry Transport MQTTClient MQTTServer Server MQTTBroker Broker NETStandard IoT InternetOfThings Messaging Hardware Arduino Sensor Actuator M2M ESP Smart Home Cities Automation Xamarin</tags> 

Frameworks/MQTTnet.NetStandard/Implementations/MqttWebSocketChannel.cs

@@ -19,9 +19,8 @@ namespace MQTTnet.Implementations
             _options = options ?? throw new ArgumentNullException(nameof(options));
         }
 
-        public Stream SendStream => RawReceiveStream;
-        public Stream ReceiveStream => RawReceiveStream;
-        public Stream RawReceiveStream { get; private set; }
+        public Stream SendStream { get; private set; }
+        public Stream ReceiveStream { get; private set; }
 
         public async Task ConnectAsync()
         {
@@ -32,7 +31,7 @@ namespace MQTTnet.Implementations
             }
 
             _webSocket = new ClientWebSocket();
-
+            
             if (_options.RequestHeaders != null)
             {
                 foreach (var requestHeader in _options.RequestHeaders)
@@ -64,13 +63,13 @@ namespace MQTTnet.Implementations
             }
 
             await _webSocket.ConnectAsync(new Uri(uri), CancellationToken.None).ConfigureAwait(false);
-            RawReceiveStream = new WebSocketStream(_webSocket);
+
+            SendStream = new WebSocketStream(_webSocket);
+            ReceiveStream = SendStream;
         }
 
         public async Task DisconnectAsync()
         {
-            RawReceiveStream = null;
-
             if (_webSocket == null)
             {
                 return;
@@ -80,6 +79,8 @@ namespace MQTTnet.Implementations
             {
                 await _webSocket.CloseAsync(WebSocketCloseStatus.NormalClosure, string.Empty, CancellationToken.None).ConfigureAwait(false);
             }
+
+            _webSocket = null;
         }
 
         public void Dispose()

Frameworks/MQTTnet.NetStandard/Implementations/WebSocketStream.cs

@@ -43,7 +43,7 @@ namespace MQTTnet.Implementations
                 var response = await _webSocket.ReceiveAsync(new ArraySegment<byte>(buffer, currentOffset, count), cancellationToken).ConfigureAwait(false);
                 currentOffset += response.Count;
                 count -= response.Count;
-
+                
                 if (response.MessageType == WebSocketMessageType.Close)
                 {
                     await _webSocket.CloseAsync(WebSocketCloseStatus.NormalClosure, string.Empty, cancellationToken).ConfigureAwait(false);

Frameworks/MQTTnet.NetStandard/MqttFactory.cs

@@ -85,9 +85,7 @@ namespace MQTTnet
                 clientSessionsManager,
                 _serviceProvider.GetRequiredService<MqttClientSubscriptionsManager>(),
                 _serviceProvider.GetRequiredService<ILogger<MqttClientSession>>(),
-                _serviceProvider.GetRequiredService<ILogger<MqttClientPendingMessagesQueue>>(),
-                _serviceProvider.GetRequiredService<IMqttClientRetainedMessageManager>()
-                );
+                _serviceProvider.GetRequiredService<ILogger<MqttClientPendingMessagesQueue>>());
         }
 
         public IMqttClient CreateMqttClient()

MQTTnet.Core/Adapter/MqttChannelCommunicationAdapter.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Runtime.InteropServices;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Core.Channel;
@@ -14,10 +15,12 @@ namespace MQTTnet.Core.Adapter
 {
     public class MqttChannelCommunicationAdapter : IMqttCommunicationAdapter
     {
+        private const uint ErrorOperationAborted = 0x800703E3;
+
         private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
         private readonly ILogger<MqttChannelCommunicationAdapter> _logger;
         private readonly IMqttCommunicationChannel _channel;
-        
+
         public MqttChannelCommunicationAdapter(IMqttCommunicationChannel channel, IMqttPacketSerializer serializer, ILogger<MqttChannelCommunicationAdapter> logger)
         {
             _logger = logger ?? throw new ArgumentNullException(nameof(logger));
@@ -29,166 +32,87 @@ namespace MQTTnet.Core.Adapter
 
         public async Task ConnectAsync(TimeSpan timeout)
         {
-            try
-            {
-                await _channel.ConnectAsync().TimeoutAfter(timeout).ConfigureAwait(false);
-            }
-            catch (TaskCanceledException)
-            {
-                throw;
-            }
-            catch (OperationCanceledException)
-            {
-                throw;
-            }
-            catch (MqttCommunicationTimedOutException)
-            {
-                throw;
-            }
-            catch (MqttCommunicationException)
-            {
-                throw;
-            }
-            catch (Exception exception)
-            {
-                throw new MqttCommunicationException(exception);
-            }
+            await ExecuteAndWrapExceptionAsync(() => _channel.ConnectAsync().TimeoutAfter(timeout));
         }
 
         public async Task DisconnectAsync(TimeSpan timeout)
         {
-            try
-            {
-                await _channel.DisconnectAsync().TimeoutAfter(timeout).ConfigureAwait(false);
-            }
-            catch (TaskCanceledException)
-            {
-                throw;
-            }
-            catch (OperationCanceledException)
-            {
-                throw;
-            }
-            catch (MqttCommunicationTimedOutException)
-            {
-                throw;
-            }
-            catch (MqttCommunicationException)
-            {
-                throw;
-            }
-            catch (Exception exception)
-            {
-                throw new MqttCommunicationException(exception);
-            }
+            await ExecuteAndWrapExceptionAsync(() => _channel.DisconnectAsync().TimeoutAfter(timeout));
         }
 
         public async Task SendPacketsAsync(TimeSpan timeout, CancellationToken cancellationToken, IEnumerable<MqttBasePacket> packets)
         {
-            try
+            await ExecuteAndWrapExceptionAsync(async () =>
             {
                 await _semaphore.WaitAsync(cancellationToken).ConfigureAwait(false);
-
-                foreach (var packet in packets)
+                try
                 {
-                    if (packet == null)
+                    foreach (var packet in packets)
                     {
-                        continue;
-                    }
+                        if (packet == null)
+                        {
+                            continue;
+                        }
 
-                    _logger.LogInformation("TX >>> {0} [Timeout={1}]", packet, timeout);
+                        _logger.LogInformation("TX >>> {0} [Timeout={1}]", packet, timeout);
 
-                    var writeBuffer = PacketSerializer.Serialize(packet);
-                    await _channel.SendStream.WriteAsync(writeBuffer, 0, writeBuffer.Length, cancellationToken).ConfigureAwait(false);
-                }
+                        var writeBuffer = PacketSerializer.Serialize(packet);
+                        await _channel.SendStream.WriteAsync(writeBuffer, 0, writeBuffer.Length, cancellationToken).ConfigureAwait(false);
+                    }
 
-                if (timeout > TimeSpan.Zero)
-                {
-                    await _channel.SendStream.FlushAsync(cancellationToken).TimeoutAfter(timeout).ConfigureAwait(false);
+                    if (timeout > TimeSpan.Zero)
+                    {
+                        await _channel.SendStream.FlushAsync(cancellationToken).TimeoutAfter(timeout).ConfigureAwait(false);
+                    }
+                    else
+                    {
+                        await _channel.SendStream.FlushAsync(cancellationToken).ConfigureAwait(false);
+                    }
                 }
-                else
+                finally
                 {
-                    await _channel.SendStream.FlushAsync(cancellationToken).ConfigureAwait(false);
+                    _semaphore.Release();
                 }
-            }
-            catch (TaskCanceledException)
-            {
-                throw;
-            }
-            catch (OperationCanceledException)
-            {
-                throw;
-            }
-            catch (MqttCommunicationTimedOutException)
-            {
-                throw;
-            }
-            catch (MqttCommunicationException)
-            {
-                throw;
-            }
-            catch (Exception exception)
-            {
-                throw new MqttCommunicationException(exception);
-            }
-            finally
-            {
-                _semaphore.Release();
-            }
+            });
         }
 
         public async Task<MqttBasePacket> ReceivePacketAsync(TimeSpan timeout, CancellationToken cancellationToken)
         {
-            ReceivedMqttPacket receivedMqttPacket = null;
-            try
+            MqttBasePacket packet = null;
+            await ExecuteAndWrapExceptionAsync(async () =>
             {
-                if (timeout > TimeSpan.Zero)
-                {
-                    receivedMqttPacket = await ReceiveAsync(_channel.ReceiveStream, cancellationToken).TimeoutAfter(timeout).ConfigureAwait(false);
-                }
-                else
+                ReceivedMqttPacket receivedMqttPacket = null;
+                try
                 {
-                    receivedMqttPacket = await ReceiveAsync(_channel.ReceiveStream, cancellationToken).ConfigureAwait(false);
-                }
+                    if (timeout > TimeSpan.Zero)
+                    {
+                        receivedMqttPacket = await ReceiveAsync(_channel.ReceiveStream, cancellationToken).TimeoutAfter(timeout).ConfigureAwait(false);
+                    }
+                    else
+                    {
+                        receivedMqttPacket = await ReceiveAsync(_channel.ReceiveStream, cancellationToken).ConfigureAwait(false);
+                    }
 
-                if (receivedMqttPacket == null || cancellationToken.IsCancellationRequested)
-                {
-                    throw new TaskCanceledException();
-                }
+                    if (receivedMqttPacket == null || cancellationToken.IsCancellationRequested)
+                    {
+                        throw new TaskCanceledException();
+                    }
+
+                    packet = PacketSerializer.Deserialize(receivedMqttPacket);
+                    if (packet == null)
+                    {
+                        throw new MqttProtocolViolationException("Received malformed packet.");
+                    }
 
-                var packet = PacketSerializer.Deserialize(receivedMqttPacket);
-                if (packet == null)
+                    _logger.LogInformation("RX <<< {0}", packet);
+                }
+                finally
                 {
-                    throw new MqttProtocolViolationException("Received malformed packet.");
+                    receivedMqttPacket?.Dispose();
                 }
+            });
 
-                _logger.LogInformation("RX <<< {0}", packet);
-                return packet;
-            }
-            catch (TaskCanceledException)
-            {
-                throw;
-            }
-            catch (OperationCanceledException)
-            {
-                throw;
-            }
-            catch (MqttCommunicationTimedOutException)
-            {
-                throw;
-            }
-            catch (MqttCommunicationException)
-            {
-                throw;
-            }
-            catch (Exception exception)
-            {
-                throw new MqttCommunicationException(exception);
-            }
-            finally
-            {
-                receivedMqttPacket?.Dispose();
-            }
+            return packet;
         }
 
         private static async Task<ReceivedMqttPacket> ReceiveAsync(Stream stream, CancellationToken cancellationToken)
@@ -215,5 +139,42 @@ namespace MQTTnet.Core.Adapter
 
             return new ReceivedMqttPacket(header, new MemoryStream(body, 0, body.Length));
         }
+
+        private static async Task ExecuteAndWrapExceptionAsync(Func<Task> action)
+        {
+            try
+            {
+                await action().ConfigureAwait(false);
+            }
+            catch (TaskCanceledException)
+            {
+                throw;
+            }
+            catch (OperationCanceledException)
+            {
+                throw;
+            }
+            catch (MqttCommunicationTimedOutException)
+            {
+                throw;
+            }
+            catch (MqttCommunicationException)
+            {
+                throw;
+            }
+            catch (COMException comException)
+            {
+                if ((uint)comException.HResult == ErrorOperationAborted)
+                {
+                    throw new OperationCanceledException();
+                }
+
+                throw new MqttCommunicationException(comException);
+            }
+            catch (Exception exception)
+            {
+                throw new MqttCommunicationException(exception);
+            }
+        }
     }
 }

MQTTnet.Core/Server/IMqttServer.cs

@@ -10,8 +10,7 @@ namespace MQTTnet.Core.Server
         event EventHandler<MqttClientDisconnectedEventArgs> ClientDisconnected;
         event EventHandler<MqttServerStartedEventArgs> Started;
 
-        IList<ConnectedMqttClient> GetConnectedClients();
-        void Publish(IEnumerable<MqttApplicationMessage> applicationMessages);
+        Task<IList<ConnectedMqttClient>> GetConnectedClientsAsync();
 
         Task StartAsync();
         Task StopAsync();

MQTTnet.Core/Server/MqttClientPendingMessagesQueue.cs

@@ -96,7 +96,7 @@ namespace MQTTnet.Core.Server
                     _pendingPublishPackets.Add(packet, CancellationToken.None);
                 }
 
-                _session.Stop();
+                await _session.StopAsync();
             }
         }
     }

MQTTnet.Core/Server/MqttClientSession.cs

@@ -17,7 +17,6 @@ namespace MQTTnet.Core.Server
     {
         private readonly HashSet<ushort> _unacknowledgedPublishPackets = new HashSet<ushort>();
 
-        private readonly IMqttClientRetainedMessageManager _clientRetainedMessageManager;
         private readonly MqttClientSubscriptionsManager _subscriptionsManager;
         private readonly MqttClientSessionsManager _sessionsManager;
         private readonly MqttClientPendingMessagesQueue _pendingMessagesQueue;
@@ -34,10 +33,8 @@ namespace MQTTnet.Core.Server
             MqttClientSessionsManager sessionsManager,
             MqttClientSubscriptionsManager subscriptionsManager,
             ILogger<MqttClientSession> logger,
-            ILogger<MqttClientPendingMessagesQueue> messageQueueLogger,
-            IMqttClientRetainedMessageManager clientRetainedMessageManager)
+            ILogger<MqttClientPendingMessagesQueue> messageQueueLogger)
         {
-            _clientRetainedMessageManager = clientRetainedMessageManager ?? throw new ArgumentNullException(nameof(clientRetainedMessageManager));
             _sessionsManager = sessionsManager ?? throw new ArgumentNullException(nameof(sessionsManager));
             _subscriptionsManager = subscriptionsManager ?? throw new ArgumentNullException(nameof(subscriptionsManager));
             _logger = logger ?? throw new ArgumentNullException(nameof(logger));
@@ -82,7 +79,7 @@ namespace MQTTnet.Core.Server
             }
         }
 
-        public void Stop()
+        public async Task StopAsync()
         {
             try
             {
@@ -90,17 +87,21 @@ namespace MQTTnet.Core.Server
                 _cancellationTokenSource?.Dispose();
                 _cancellationTokenSource = null;
 
-                _adapter = null;
+                if (_adapter != null)
+                {
+                    await _adapter.DisconnectAsync(_options.DefaultCommunicationTimeout).ConfigureAwait(false);
+                    _adapter = null;
+                }
 
-                _logger.LogInformation("Client '{0}': Disconnected.", ClientId);
+                _logger.LogInformation("Client '{0}': Session stopped.", ClientId);
             }
             finally
             {
-                var willMessage = _willMessage; 
+                var willMessage = _willMessage;
                 if (willMessage != null)
                 {
                     _willMessage = null; //clear willmessage so it is send just once
-                    _sessionsManager.DispatchApplicationMessage(this, willMessage);
+                    await _sessionsManager.DispatchApplicationMessageAsync(this, willMessage);
                 }
             }
         }
@@ -133,12 +134,12 @@ namespace MQTTnet.Core.Server
             catch (MqttCommunicationException exception)
             {
                 _logger.LogWarning(new EventId(), exception, "Client '{0}': Communication exception while processing client packets.", ClientId);
-                Stop();
+                await StopAsync();
             }
             catch (Exception exception)
             {
                 _logger.LogError(new EventId(), exception, "Client '{0}': Unhandled exception while processing client packets.", ClientId);
-                Stop();
+                await StopAsync();
             }
         }
 
@@ -182,14 +183,11 @@ namespace MQTTnet.Core.Server
 
             if (packet is MqttDisconnectPacket || packet is MqttConnectPacket)
             {
-                Stop();
-                return Task.FromResult(0);
+                return StopAsync();
             }
 
             _logger.LogWarning("Client '{0}': Received not supported packet ({1}). Closing connection.", ClientId, packet);
-            Stop();
-
-            return Task.FromResult(0);
+            return StopAsync();            
         }
 
         private async Task HandleIncomingSubscribePacketAsync(IMqttCommunicationAdapter adapter, MqttSubscribePacket subscribePacket, CancellationToken cancellationToken)
@@ -202,13 +200,13 @@ namespace MQTTnet.Core.Server
             if (subscribeResult.CloseConnection)
             {
                 await adapter.SendPacketsAsync(_options.DefaultCommunicationTimeout, cancellationToken, new MqttDisconnectPacket()).ConfigureAwait(false);
-                Stop();
+                await StopAsync();
             }
         }
 
         private async Task EnqueueSubscribedRetainedMessagesAsync(MqttSubscribePacket subscribePacket)
         {
-            var retainedMessages = await _clientRetainedMessageManager.GetSubscribedMessagesAsync(subscribePacket).ConfigureAwait(false);
+            var retainedMessages = await _sessionsManager.GetRetainedMessagesAsync(subscribePacket).ConfigureAwait(false);
             foreach (var publishPacket in retainedMessages)
             {
                 EnqueuePublishPacket(publishPacket.ToPublishPacket());
@@ -219,29 +217,16 @@ namespace MQTTnet.Core.Server
         {
             var applicationMessage = publishPacket.ToApplicationMessage();
 
-            var interceptorContext = new MqttApplicationMessageInterceptorContext
-            {
-                ApplicationMessage = applicationMessage
-            };
-
-            _options.ApplicationMessageInterceptor?.Invoke(interceptorContext);
-            applicationMessage = interceptorContext.ApplicationMessage;
-
-            if (applicationMessage.Retain)
-            {
-                await _clientRetainedMessageManager.HandleMessageAsync(ClientId, applicationMessage).ConfigureAwait(false);
-            }
-
             switch (applicationMessage.QualityOfServiceLevel)
             {
                 case MqttQualityOfServiceLevel.AtMostOnce:
                     {
-                        _sessionsManager.DispatchApplicationMessage(this, applicationMessage);
+                        await _sessionsManager.DispatchApplicationMessageAsync(this, applicationMessage);
                         return;
                     }
                 case MqttQualityOfServiceLevel.AtLeastOnce:
                     {
-                        _sessionsManager.DispatchApplicationMessage(this, applicationMessage);
+                        await _sessionsManager.DispatchApplicationMessageAsync(this, applicationMessage);
 
                         await adapter.SendPacketsAsync(_options.DefaultCommunicationTimeout, cancellationToken,
                             new MqttPubAckPacket { PacketIdentifier = publishPacket.PacketIdentifier });
@@ -256,7 +241,7 @@ namespace MQTTnet.Core.Server
                             _unacknowledgedPublishPackets.Add(publishPacket.PacketIdentifier);
                         }
 
-                        _sessionsManager.DispatchApplicationMessage(this, applicationMessage);
+                        await _sessionsManager.DispatchApplicationMessageAsync(this, applicationMessage);
 
                         await adapter.SendPacketsAsync(_options.DefaultCommunicationTimeout, cancellationToken,
                             new MqttPubRecPacket { PacketIdentifier = publishPacket.PacketIdentifier }).ConfigureAwait(false);

MQTTnet.Core/Server/MqttClientSessionsManager.cs

@@ -16,19 +16,24 @@ namespace MQTTnet.Core.Server
 {
     public sealed class MqttClientSessionsManager
     {
-        private readonly Dictionary<string, MqttClientSession> _clientSessions = new Dictionary<string, MqttClientSession>();
+        private readonly Dictionary<string, MqttClientSession> _sessions = new Dictionary<string, MqttClientSession>();
+        private readonly SemaphoreSlim _sessionsSemaphore = new SemaphoreSlim(1, 1);
+
+        private readonly MqttServerOptions _options;
         private readonly ILogger<MqttClientSessionsManager> _logger;
         private readonly IMqttClientSesssionFactory _clientSesssionFactory;
-        private readonly MqttServerOptions _options;
+        private readonly IMqttClientRetainedMessageManager _clientRetainedMessageManager;
 
         public MqttClientSessionsManager(
-            IOptions<MqttServerOptions> options, 
+            IOptions<MqttServerOptions> options,
             ILogger<MqttClientSessionsManager> logger,
-            IMqttClientSesssionFactory clientSesssionFactory)
+            IMqttClientSesssionFactory clientSesssionFactory,
+            IMqttClientRetainedMessageManager clientRetainedMessageManager)
         {
             _logger = logger ?? throw new ArgumentNullException(nameof(logger));
             _options = options.Value ?? throw new ArgumentNullException(nameof(options));
             _clientSesssionFactory = clientSesssionFactory ?? throw new ArgumentNullException(nameof(clientSesssionFactory));
+            _clientRetainedMessageManager = clientRetainedMessageManager ?? throw new ArgumentNullException(nameof(clientRetainedMessageManager));
         }
 
         public event EventHandler<MqttClientConnectedEventArgs> ClientConnected;
@@ -61,7 +66,7 @@ namespace MQTTnet.Core.Server
                     return;
                 }
 
-                var clientSession = GetOrCreateClientSession(connectPacket);
+                var clientSession = await GetOrCreateClientSessionAsync(connectPacket);
 
                 await clientAdapter.SendPacketsAsync(_options.DefaultCommunicationTimeout, cancellationToken, new MqttConnAckPacket
                 {
@@ -92,7 +97,7 @@ namespace MQTTnet.Core.Server
                 }
                 catch (Exception)
                 {
-                    //ignored
+                    // ignored
                 }
 
                 ClientDisconnected?.Invoke(this, new MqttClientDisconnectedEventArgs(new ConnectedMqttClient
@@ -103,30 +108,58 @@ namespace MQTTnet.Core.Server
             }
         }
 
-        public void Clear()
+        public async Task StopAsync()
         {
-            lock (_clientSessions)
+            await _sessionsSemaphore.WaitAsync().ConfigureAwait(false);
+            try
+            {
+                foreach (var session in _sessions)
+                {
+                    await session.Value.StopAsync();
+                }
+
+                _sessions.Clear();
+            }
+            finally
             {
-                _clientSessions.Clear();
+                _sessionsSemaphore.Release();
             }
         }
 
-        public IList<ConnectedMqttClient> GetConnectedClients()
+        public async Task<IList<ConnectedMqttClient>> GetConnectedClientsAsync()
         {
-            lock (_clientSessions)
+            await _sessionsSemaphore.WaitAsync().ConfigureAwait(false);
+            try
             {
-                return _clientSessions.Where(s => s.Value.IsConnected).Select(s => new ConnectedMqttClient
+                return _sessions.Where(s => s.Value.IsConnected).Select(s => new ConnectedMqttClient
                 {
                     ClientId = s.Value.ClientId,
                     ProtocolVersion = s.Value.ProtocolVersion ?? MqttProtocolVersion.V311
                 }).ToList();
             }
+            finally
+            {
+                _sessionsSemaphore.Release();
+            }
         }
 
-        public void DispatchApplicationMessage(MqttClientSession senderClientSession, MqttApplicationMessage applicationMessage)
+        public async Task DispatchApplicationMessageAsync(MqttClientSession senderClientSession, MqttApplicationMessage applicationMessage)
         {
             try
             {
+                var interceptorContext = new MqttApplicationMessageInterceptorContext
+                {
+                    ApplicationMessage = applicationMessage
+                };
+
+                _options.ApplicationMessageInterceptor?.Invoke(interceptorContext);
+                applicationMessage = interceptorContext.ApplicationMessage;
+
+                if (applicationMessage.Retain)
+                {
+                    await _clientRetainedMessageManager.HandleMessageAsync(senderClientSession?.ClientId, applicationMessage).ConfigureAwait(false);
+                }
+
                 var eventArgs = new MqttApplicationMessageReceivedEventArgs(senderClientSession?.ClientId, applicationMessage);
                 ApplicationMessageReceived?.Invoke(this, eventArgs);
             }
@@ -135,15 +168,20 @@ namespace MQTTnet.Core.Server
                 _logger.LogError(new EventId(), exception, "Error while processing application message");
             }
 
-            lock (_clientSessions)
+            lock (_sessions)
             {
-                foreach (var clientSession in _clientSessions.Values.ToList())
+                foreach (var clientSession in _sessions.Values.ToList())
                 {
                     clientSession.EnqueuePublishPacket(applicationMessage.ToPublishPacket());
                 }
             }
         }
 
+        public Task<List<MqttApplicationMessage>> GetRetainedMessagesAsync(MqttSubscribePacket subscribePacket)
+        {
+            return _clientRetainedMessageManager.GetSubscribedMessagesAsync(subscribePacket);
+        }
+
         private MqttConnectReturnCode ValidateConnection(MqttConnectPacket connectPacket)
         {
             if (_options.ConnectionValidator != null)
@@ -154,17 +192,18 @@ namespace MQTTnet.Core.Server
             return MqttConnectReturnCode.ConnectionAccepted;
         }
 
-        private GetOrCreateClientSessionResult GetOrCreateClientSession(MqttConnectPacket connectPacket)
+        private async Task<GetOrCreateClientSessionResult> GetOrCreateClientSessionAsync(MqttConnectPacket connectPacket)
         {
-            lock (_clientSessions)
+            await _sessionsSemaphore.WaitAsync().ConfigureAwait(false);
+            try
             {
-                var isSessionPresent = _clientSessions.TryGetValue(connectPacket.ClientId, out var clientSession);
+                var isSessionPresent = _sessions.TryGetValue(connectPacket.ClientId, out var clientSession);
                 if (isSessionPresent)
                 {
                     if (connectPacket.CleanSession)
                     {
-                        _clientSessions.Remove(connectPacket.ClientId);
-                        clientSession.Stop();
+                        _sessions.Remove(connectPacket.ClientId);
+                        await clientSession.StopAsync();
                         clientSession = null;
 
                         _logger.LogTrace("Stopped existing session of client '{0}'.", connectPacket.ClientId);
@@ -181,13 +220,17 @@ namespace MQTTnet.Core.Server
                     isExistingSession = false;
 
                     clientSession = _clientSesssionFactory.CreateClientSession(connectPacket.ClientId, this);
-                    _clientSessions[connectPacket.ClientId] = clientSession;
+                    _sessions[connectPacket.ClientId] = clientSession;
 
                     _logger.LogTrace("Created a new session for client '{0}'.", connectPacket.ClientId);
                 }
 
                 return new GetOrCreateClientSessionResult { IsExistingSession = isExistingSession, Session = clientSession };
             }
+            finally
+            {
+                _sessionsSemaphore.Release();
+            }
         }
     }
 }

MQTTnet.Core/Server/MqttServer.cs

@@ -20,12 +20,11 @@ namespace MQTTnet.Core.Server
         private CancellationTokenSource _cancellationTokenSource;
 
         public MqttServer(
-            IOptions<MqttServerOptions> options, 
+            IOptions<MqttServerOptions> options,
             IEnumerable<IMqttServerAdapter> adapters,
             ILogger<MqttServer> logger,
             MqttClientSessionsManager clientSessionsManager,
-            IMqttClientRetainedMessageManager clientRetainedMessageManager
-            )
+            IMqttClientRetainedMessageManager clientRetainedMessageManager)
         {
             _options = options.Value ?? throw new ArgumentNullException(nameof(options));
             _logger = logger ?? throw new ArgumentNullException(nameof(logger));
@@ -35,7 +34,7 @@ namespace MQTTnet.Core.Server
             if (adapters == null)
             {
                 throw new ArgumentNullException(nameof(adapters));
-            }            
+            }
 
             _adapters = adapters.ToList();
 
@@ -44,9 +43,9 @@ namespace MQTTnet.Core.Server
             _clientSessionsManager.ClientDisconnected += OnClientDisconnected;
         }
 
-        public IList<ConnectedMqttClient> GetConnectedClients()
+        public Task<IList<ConnectedMqttClient>> GetConnectedClientsAsync()
         {
-            return _clientSessionsManager.GetConnectedClients();
+            return _clientSessionsManager.GetConnectedClientsAsync();
         }
 
         public event EventHandler<MqttServerStartedEventArgs> Started;
@@ -54,7 +53,7 @@ namespace MQTTnet.Core.Server
         public event EventHandler<MqttClientDisconnectedEventArgs> ClientDisconnected;
         public event EventHandler<MqttApplicationMessageReceivedEventArgs> ApplicationMessageReceived;
 
-        public void Publish(IEnumerable<MqttApplicationMessage> applicationMessages)
+        public async Task PublishAsync(IEnumerable<MqttApplicationMessage> applicationMessages)
         {
             if (applicationMessages == null) throw new ArgumentNullException(nameof(applicationMessages));
 
@@ -65,23 +64,10 @@ namespace MQTTnet.Core.Server
 
             foreach (var applicationMessage in applicationMessages)
             {
-                var interceptorContext = new MqttApplicationMessageInterceptorContext
-                {
-                    ApplicationMessage = applicationMessage
-                };
-
-                _options.ApplicationMessageInterceptor?.Invoke(interceptorContext);
-                
-                _clientSessionsManager.DispatchApplicationMessage(null, interceptorContext.ApplicationMessage);
+                await _clientSessionsManager.DispatchApplicationMessageAsync(null, applicationMessage);
             }
         }
 
-        public Task PublishAsync(IEnumerable<MqttApplicationMessage> applicationMessages)
-        {
-            Publish(applicationMessages);
-            return Task.FromResult(0);
-        }
-
         public async Task StartAsync()
         {
             if (_cancellationTokenSource != null) throw new InvalidOperationException("The MQTT server is already started.");
@@ -113,7 +99,7 @@ namespace MQTTnet.Core.Server
                 await adapter.StopAsync();
             }
 
-            _clientSessionsManager.Clear();
+            await _clientSessionsManager.StopAsync();
 
             _logger.LogInformation("Stopped.");
         }