Fix freezes in _AsyncLock_ (#1549)

Source/MQTTnet/Adapter/MqttChannelAdapter.cs

@@ -195,7 +195,7 @@ namespace MQTTnet.Adapter
             // This lock makes sure that multiple threads can send packets at the same time.
             // This is required when a disconnect is sent from another thread while the 
             // worker thread is still sending publish packets etc.
-            using (await _syncRoot.WaitAsync(cancellationToken).ConfigureAwait(false))
+            using (await _syncRoot.EnterAsync(cancellationToken).ConfigureAwait(false))
             {
                 // Check for cancellation here again because "WaitAsync" might take some time.
                 cancellationToken.ThrowIfCancellationRequested();
@@ -363,7 +363,7 @@ namespace MQTTnet.Adapter
                 var fixedHeader = readFixedHeaderResult.FixedHeader;
                 if (fixedHeader.RemainingLength == 0)
                 {
-                    return new ReceivedMqttPacket(fixedHeader.Flags, PlatformAbstractionLayer.EmptyByteArraySegment, 2);
+                    return new ReceivedMqttPacket(fixedHeader.Flags, EmptyBuffer.ArraySegment, 2);
                 }
 
                 var bodyLength = fixedHeader.RemainingLength;

Source/MQTTnet/Client/IMqttClient.cs

@@ -8,6 +8,7 @@ namespace MQTTnet.Client
     public interface IMqttClient : IDisposable
     {
         event Func<MqttApplicationMessageReceivedEventArgs, Task> ApplicationMessageReceivedAsync;
+        
         event Func<MqttClientConnectedEventArgs, Task> ConnectedAsync;
 
         event Func<MqttClientConnectingEventArgs, Task> ConnectingAsync;

Source/MQTTnet/Client/MqttClient.cs

@@ -9,7 +9,6 @@ using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.PacketDispatcher;
 using MQTTnet.Packets;
@@ -224,17 +223,17 @@ namespace MQTTnet.Client
 
         public async Task PingAsync(CancellationToken cancellationToken = default)
         {
-            if (!cancellationToken.CanBeCanceled)
+            if (cancellationToken.CanBeCanceled)
+            {
+                await SendAndReceiveAsync<MqttPingRespPacket>(MqttPingReqPacket.Instance, cancellationToken).ConfigureAwait(false);
+            }
+            else
             {
                 using (var timeout = new CancellationTokenSource(Options.Timeout))
                 {
                     await SendAndReceiveAsync<MqttPingRespPacket>(MqttPingReqPacket.Instance, timeout.Token).ConfigureAwait(false);
                 }
             }
-            else
-            {
-                await SendAndReceiveAsync<MqttPingRespPacket>(MqttPingReqPacket.Instance, cancellationToken).ConfigureAwait(false);
-            }
         }
 
         public Task<MqttClientPublishResult> PublishAsync(MqttApplicationMessage applicationMessage, CancellationToken cancellationToken = default)
@@ -369,7 +368,7 @@ namespace MQTTnet.Client
                 throw new MqttProtocolViolationException("Received a not supported QoS level.");
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         async Task<MqttClientConnectResult> AuthenticateAsync(MqttClientOptions options, CancellationToken cancellationToken)
@@ -490,7 +489,7 @@ namespace MQTTnet.Client
                 return DisconnectCoreAsync(sender, exception, connectResult, clientWasConnected);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         bool DisconnectIsPendingOrFinished()
@@ -550,7 +549,7 @@ namespace MQTTnet.Client
                 return extendedAuthenticationExchangeHandler.HandleRequestAsync(new MqttExtendedAuthenticationExchangeContext(authPacket, this));
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         Task ProcessReceivedDisconnectPacket(MqttDisconnectPacket disconnectPacket)
@@ -604,7 +603,7 @@ namespace MQTTnet.Client
                 return SendAsync(pubRelPacket, cancellationToken);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         Task ProcessReceivedPubRelPacket(MqttPubRelPacket pubRelPacket, CancellationToken cancellationToken)

Source/MQTTnet/Formatter/MqttBufferReader.cs

@@ -6,13 +6,13 @@ using System;
 using System.Runtime.CompilerServices;
 using System.Text;
 using MQTTnet.Exceptions;
-using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Formatter
 {
     public sealed class MqttBufferReader
     {
-        byte[] _buffer = PlatformAbstractionLayer.EmptyByteArray;
+        byte[] _buffer = EmptyBuffer.Array;
         int _initialPosition;
         int _length;
         int _position;
@@ -29,7 +29,7 @@ namespace MQTTnet.Formatter
 
             if (length == 0)
             {
-                return PlatformAbstractionLayer.EmptyByteArray;
+                return EmptyBuffer.Array;
             }
 
             ValidateReceiveBuffer(length);
@@ -66,7 +66,7 @@ namespace MQTTnet.Formatter
 
             if (bufferLength == 0)
             {
-                return PlatformAbstractionLayer.EmptyByteArray;
+                return EmptyBuffer.Array;
             }
             
             var buffer = new byte[bufferLength];

Source/MQTTnet/Formatter/MqttPacketBuffer.cs

@@ -5,12 +5,13 @@
 using System;
 using System.Linq;
 using MQTTnet.Implementations;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Formatter
 {
     public readonly struct MqttPacketBuffer
     {
-        static readonly ArraySegment<byte> EmptyPayload = PlatformAbstractionLayer.EmptyByteArraySegment;
+        static readonly ArraySegment<byte> EmptyPayload = EmptyBuffer.ArraySegment;
         
         public MqttPacketBuffer(ArraySegment<byte> packet, ArraySegment<byte> payload)
         {

Source/MQTTnet/Implementations/CrossPlatformSocket.cs

@@ -15,14 +15,20 @@ namespace MQTTnet.Implementations
     public sealed class CrossPlatformSocket : IDisposable
     {
         readonly Socket _socket;
+
+#if !NET5_0_OR_GREATER
         readonly Action _socketDisposeAction;
+#endif
 
         NetworkStream _networkStream;
 
         public CrossPlatformSocket(AddressFamily addressFamily)
         {
             _socket = new Socket(addressFamily, SocketType.Stream, ProtocolType.Tcp);
+
+#if !NET5_0_OR_GREATER
             _socketDisposeAction = _socket.Dispose;
+#endif
         }
 
         public CrossPlatformSocket()
@@ -31,17 +37,76 @@ namespace MQTTnet.Implementations
             // will make use of dual mode in the .net framework.
             _socket = new Socket(SocketType.Stream, ProtocolType.Tcp);
 
+#if !NET5_0_OR_GREATER
             _socketDisposeAction = _socket.Dispose;
+#endif
         }
 
-        public CrossPlatformSocket(Socket socket)
+        CrossPlatformSocket(Socket socket)
         {
             _socket = socket ?? throw new ArgumentNullException(nameof(socket));
             _networkStream = new NetworkStream(socket, true);
 
+#if !NET5_0_OR_GREATER
             _socketDisposeAction = _socket.Dispose;
+#endif
         }
 
+        public bool DualMode
+        {
+            get => _socket.DualMode;
+            set => _socket.DualMode = value;
+        }
+
+        public bool IsConnected => _socket.Connected;
+
+        public bool KeepAlive
+        {
+            get => _socket.GetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive) as int? == 1;
+            set => _socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.KeepAlive, value ? 1 : 0);
+        }
+
+        public int TcpKeepAliveInterval
+        {
+#if NETCOREAPP3_0_OR_GREATER
+            get => _socket.GetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval) as int? ?? 0;
+            set => _socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveInterval, value);
+#else
+            get { throw new NotSupportedException("TcpKeepAliveInterval requires at least net5.0."); }
+            set { throw new NotSupportedException("TcpKeepAliveInterval requires at least net5.0."); }
+#endif
+        }
+
+        public int TcpKeepAliveRetryCount
+        {
+#if NETCOREAPP3_0_OR_GREATER
+            get => _socket.GetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount) as int? ?? 0;
+            set => _socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveRetryCount, value);
+#else
+            get { throw new NotSupportedException("TcpKeepAliveRetryCount requires at least net5.0."); }
+            set { throw new NotSupportedException("TcpKeepAliveRetryCount requires at least net5.0."); }
+#endif
+        }
+
+        public int TcpKeepAliveTime
+        {
+#if NETCOREAPP3_0_OR_GREATER
+            get => _socket.GetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime) as int? ?? 0;
+            set => _socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.TcpKeepAliveTime, value);
+#else
+            get { throw new NotSupportedException("TcpKeepAliveTime requires at least net5.0."); }
+            set { throw new NotSupportedException("TcpKeepAliveTime requires at least net5.0."); }
+#endif
+        }
+
+        public LingerOption LingerState
+        {
+            get => _socket.LingerState;
+            set => _socket.LingerState = value;
+        }
+
+        public EndPoint LocalEndPoint => _socket.LocalEndPoint;
+
         public bool NoDelay
         {
             // We cannot use the _NoDelay_ property from the socket because there is an issue in .NET 4.5.2, 4.6.
@@ -51,24 +116,20 @@ namespace MQTTnet.Implementations
             set => _socket.SetSocketOption(SocketOptionLevel.Tcp, SocketOptionName.NoDelay, value ? 1 : 0);
         }
 
-        public LingerOption LingerState
-        {
-            get => _socket.LingerState;
-            set => _socket.LingerState = value;
-        }
-
-        public bool DualMode
-        {
-            get => _socket.DualMode;
-            set => _socket.DualMode = value;
-        }
-
         public int ReceiveBufferSize
         {
             get => _socket.ReceiveBufferSize;
             set => _socket.ReceiveBufferSize = value;
         }
 
+        public EndPoint RemoteEndPoint => _socket.RemoteEndPoint;
+
+        public bool ReuseAddress
+        {
+            get => _socket.GetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress) as int? != 0;
+            set => _socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, value ? 1 : 0);
+        }
+
         public int SendBufferSize
         {
             get => _socket.SendBufferSize;
@@ -81,16 +142,6 @@ namespace MQTTnet.Implementations
             set => _socket.SendTimeout = value;
         }
 
-        public EndPoint RemoteEndPoint => _socket.RemoteEndPoint;
-
-        public bool ReuseAddress
-        {
-            get => (int)_socket.GetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress) != 0;
-            set => _socket.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, value ? 1 : 0);
-        }
-
-        public bool IsConnected => _socket.Connected;
-
         public async Task<CrossPlatformSocket> AcceptAsync()
         {
             try
@@ -109,8 +160,6 @@ namespace MQTTnet.Implementations
             }
         }
 
-        public EndPoint LocalEndPoint => _socket.LocalEndPoint;
-
         public void Bind(EndPoint localEndPoint)
         {
             if (localEndPoint is null)
@@ -121,11 +170,6 @@ namespace MQTTnet.Implementations
             _socket.Bind(localEndPoint);
         }
 
-        public void Listen(int connectionBacklog)
-        {
-            _socket.Listen(connectionBacklog);
-        }
-
         public async Task ConnectAsync(string host, int port, CancellationToken cancellationToken)
         {
             if (host is null)
@@ -137,11 +181,17 @@ namespace MQTTnet.Implementations
 
             try
             {
-                _networkStream?.Dispose();
-
 #if NET5_0_OR_GREATER
+
+                if (_networkStream != null)
+                {
+                    await _networkStream.DisposeAsync().ConfigureAwait(false);
+                }
+
                 await _socket.ConnectAsync(host, port, cancellationToken).ConfigureAwait(false);
 #else
+                _networkStream?.Dispose();
+
                 // Workaround for: https://github.com/dotnet/corefx/issues/24430
                 using (cancellationToken.Register(_socketDisposeAction))
                 {
@@ -175,20 +225,26 @@ namespace MQTTnet.Implementations
             }
         }
 
-        public async Task SendAsync(ArraySegment<byte> buffer, SocketFlags socketFlags)
+        public void Dispose()
         {
-            try
+            _networkStream?.Dispose();
+            _socket?.Dispose();
+        }
+
+        public NetworkStream GetStream()
+        {
+            var networkStream = _networkStream;
+            if (networkStream == null)
             {
-#if NET452 || NET461
-                await Task.Factory.FromAsync(SocketWrapper.BeginSend, _socket.EndSend, new SocketWrapper(_socket, buffer, socketFlags)).ConfigureAwait(false);
-#else
-                await _socket.SendAsync(buffer, socketFlags).ConfigureAwait(false);
-#endif
-            }
-            catch (ObjectDisposedException)
-            {
-                // This will happen when _socket.EndConnect gets called by Task library but the socket is already disposed.
+                throw new IOException("The socket is not connected.");
             }
+
+            return networkStream;
+        }
+
+        public void Listen(int connectionBacklog)
+        {
+            _socket.Listen(connectionBacklog);
         }
 
         public async Task<int> ReceiveAsync(ArraySegment<byte> buffer, SocketFlags socketFlags)
@@ -208,28 +264,27 @@ namespace MQTTnet.Implementations
             }
         }
 
-        public NetworkStream GetStream()
+        public async Task SendAsync(ArraySegment<byte> buffer, SocketFlags socketFlags)
         {
-            var networkStream = _networkStream;
-            if (networkStream == null)
+            try
             {
-                throw new IOException("The socket is not connected.");
+#if NET452 || NET461
+                await Task.Factory.FromAsync(SocketWrapper.BeginSend, _socket.EndSend, new SocketWrapper(_socket, buffer, socketFlags)).ConfigureAwait(false);
+#else
+                await _socket.SendAsync(buffer, socketFlags).ConfigureAwait(false);
+#endif
+            }
+            catch (ObjectDisposedException)
+            {
+                // This will happen when _socket.EndConnect gets called by Task library but the socket is already disposed.
             }
-
-            return networkStream;
-        }
-
-        public void Dispose()
-        {
-            _networkStream?.Dispose();
-            _socket?.Dispose();
         }
 
 #if NET452 || NET461
         sealed class SocketWrapper
         {
-            readonly Socket _socket;
             readonly ArraySegment<byte> _buffer;
+            readonly Socket _socket;
             readonly SocketFlags _socketFlags;
 
             public SocketWrapper(Socket socket, ArraySegment<byte> buffer, SocketFlags socketFlags)
@@ -239,16 +294,28 @@ namespace MQTTnet.Implementations
                 _socketFlags = socketFlags;
             }
 
-            public static IAsyncResult BeginSend(AsyncCallback callback, object state)
-            {
-                var socketWrapper = (SocketWrapper)state;
-                return socketWrapper._socket.BeginSend(socketWrapper._buffer.Array, socketWrapper._buffer.Offset, socketWrapper._buffer.Count, socketWrapper._socketFlags, callback, state);
-            }
-
             public static IAsyncResult BeginReceive(AsyncCallback callback, object state)
             {
                 var socketWrapper = (SocketWrapper)state;
-                return socketWrapper._socket.BeginReceive(socketWrapper._buffer.Array, socketWrapper._buffer.Offset, socketWrapper._buffer.Count, socketWrapper._socketFlags, callback, state);
+                return socketWrapper._socket.BeginReceive(
+                    socketWrapper._buffer.Array,
+                    socketWrapper._buffer.Offset,
+                    socketWrapper._buffer.Count,
+                    socketWrapper._socketFlags,
+                    callback,
+                    state);
+            }
+
+            public static IAsyncResult BeginSend(AsyncCallback callback, object state)
+            {
+                var socketWrapper = (SocketWrapper)state;
+                return socketWrapper._socket.BeginSend(
+                    socketWrapper._buffer.Array,
+                    socketWrapper._buffer.Offset,
+                    socketWrapper._buffer.Count,
+                    socketWrapper._socketFlags,
+                    callback,
+                    state);
             }
         }
 #endif

Source/MQTTnet/Implementations/MqttTcpChannel.cs

@@ -14,6 +14,7 @@ using System.Runtime.ExceptionServices;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Implementations
 {
@@ -154,7 +155,7 @@ namespace MQTTnet.Implementations
         public Task DisconnectAsync(CancellationToken cancellationToken)
         {
             Dispose();
-            return Task.FromResult(0);
+            return CompletedTask.Instance;
         }
 
         public void Dispose()

Source/MQTTnet/Implementations/MqttTcpServerAdapter.cs

@@ -13,6 +13,7 @@ using System.Net.Sockets;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading;
 using System.Threading.Tasks;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Implementations
 {
@@ -56,13 +57,13 @@ namespace MQTTnet.Implementations
                 RegisterListeners(options.TlsEndpointOptions, tlsCertificate, logger, _cancellationTokenSource.Token);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         public Task StopAsync()
         {
             Cleanup();
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         public void Dispose()
@@ -124,7 +125,7 @@ namespace MQTTnet.Implementations
             var clientHandler = ClientHandler;
             if (clientHandler == null)
             {
-                return Task.FromResult(0);
+                return CompletedTask.Instance;
             }
 
             return clientHandler(channelAdapter);

Source/MQTTnet/Implementations/MqttWebSocketChannel.cs

@@ -117,7 +117,7 @@ namespace MQTTnet.Implementations
                 return;
             }
 
-            using (await _sendLock.WaitAsync(cancellationToken).ConfigureAwait(false))
+            using (await _sendLock.EnterAsync(cancellationToken).ConfigureAwait(false))
             {
                 await _webSocket.SendAsync(buffer, WebSocketMessageType.Binary, isEndOfPacket, cancellationToken).ConfigureAwait(false);
             }

Source/MQTTnet/Implementations/PlatformAbstractionLayer.cs

@@ -1,43 +0,0 @@
-// 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 System;
-using System.Threading;
-using System.Threading.Tasks;
-
-namespace MQTTnet.Implementations
-{
-    public static class PlatformAbstractionLayer
-    {
-#if NET452
-        public static Task CompletedTask { get; } = Task.FromResult(0);
-
-        public static byte[] EmptyByteArray { get; } = new byte[0];
-#else
-        public static Task CompletedTask { get; } = Task.CompletedTask;
-
-        public static byte[] EmptyByteArray { get; } = Array.Empty<byte>();
-#endif
-
-        public static ArraySegment<byte> EmptyByteArraySegment { get; } = new ArraySegment<byte>(EmptyByteArray);
-
-        public static void Sleep(TimeSpan timeout)
-        {
-#if !NETSTANDARD1_3 && !WINDOWS_UWP
-            try
-            {
-                System.Threading.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();
-            }
-#else
-            Task.Delay(timeout).Wait();
-#endif
-        }
-    }
-}

Source/MQTTnet/Internal/AsyncEvent.cs

@@ -4,7 +4,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Linq;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
 
@@ -37,7 +36,7 @@ namespace MQTTnet.Internal
                 _handlers.Add(new AsyncEventInvocator<TEventArgs>(null, handler));
 
                 HasHandlers = true;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers.ToList());
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
             }
         }
 
@@ -53,7 +52,7 @@ namespace MQTTnet.Internal
                 _handlers.Add(new AsyncEventInvocator<TEventArgs>(handler, null));
 
                 HasHandlers = true;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers.ToList());
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
             }
         }
 
@@ -86,7 +85,7 @@ namespace MQTTnet.Internal
                 _handlers.RemoveAll(h => h.WrapsHandler(handler));
 
                 HasHandlers = _handlers.Count > 0;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers.ToList());
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
             }
         }
 
@@ -102,7 +101,7 @@ namespace MQTTnet.Internal
                 _handlers.RemoveAll(h => h.WrapsHandler(handler));
 
                 HasHandlers = _handlers.Count > 0;
-                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers.ToList());
+                _handlersForInvoke = new List<AsyncEventInvocator<TEventArgs>>(_handlers);
             }
         }
 

Source/MQTTnet/Internal/AsyncEventInvocator.cs

@@ -36,7 +36,7 @@ namespace MQTTnet.Internal
             if (_handler != null)
             {
                 _handler.Invoke(eventArgs);
-                return PlatformAbstractionLayer.CompletedTask;
+                return CompletedTask.Instance;
             }
 
             if (_asyncHandler != null)

Source/MQTTnet/Internal/AsyncLock.cs

@@ -4,7 +4,7 @@
 
 using System;
 using System.Collections.Generic;
-using System.Diagnostics;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -12,173 +12,151 @@ namespace MQTTnet.Internal
 {
     public sealed class AsyncLock : IDisposable
     {
-        /*
-         * This async supporting lock does not support reentrancy!
-         */
-
-        readonly List<Releaser> _queuedTasks = new List<Releaser>(64);
-        readonly Task<IDisposable> _releaserTaskWithDirectApproval;
-
-        readonly Releaser _releaserWithDirectApproval;
-
+        readonly Task<IDisposable> _completedTask;
+        readonly IDisposable _releaser;
         readonly object _syncRoot = new object();
+        readonly Queue<AsyncLockWaiter> _waiters = new Queue<AsyncLockWaiter>(64);
+
+        volatile bool _isDisposed;
+        bool _isLocked;
 
-        bool _isDisposed;
-        
         public AsyncLock()
         {
-            _releaserWithDirectApproval = new Releaser(this, null, CancellationToken.None);
-            _releaserTaskWithDirectApproval = Task.FromResult((IDisposable)_releaserWithDirectApproval);
+            _releaser = new Releaser(this);
+            _completedTask = Task.FromResult(_releaser);
         }
 
         public void Dispose()
         {
             lock (_syncRoot)
             {
-                foreach (var waitingTask in _queuedTasks)
-                {
-                    waitingTask.Fail(new ObjectDisposedException(nameof(AsyncLock)));
-                }
-
-                _queuedTasks.Clear();
-                
                 _isDisposed = true;
+
+                while (_waiters.Any())
+                {
+                    _waiters.Dequeue().Dispose();
+                }
             }
         }
 
-        public Task<IDisposable> WaitAsync(CancellationToken cancellationToken)
+        public Task<IDisposable> EnterAsync(CancellationToken cancellationToken = default)
         {
-            var hasDirectApproval = false;
-            Releaser releaser;
+            cancellationToken.ThrowIfCancellationRequested();
+            
+            if (_isDisposed)
+            {
+                throw new ObjectDisposedException(nameof(AsyncLock));
+            }
 
             lock (_syncRoot)
             {
-                if (_isDisposed)
+                if (!_isLocked)
                 {
-                    throw new ObjectDisposedException(nameof(AsyncLock));
-                }
-                
-                if (_queuedTasks.Count == 0)
-                {
-                    // There is no other waiting task apart from the current one.
-                    // So we can approve the current task directly.
-                    releaser = _releaserWithDirectApproval;
-                    hasDirectApproval = true;
-                    Debug.WriteLine("AsyncLock: Task -1 directly approved.");
-                }
-                else
-                {
-                    releaser = new Releaser(this, new TaskCompletionSource<IDisposable>(), cancellationToken);
+                    _isLocked = true;
+                    return _completedTask;
                 }
 
-                _queuedTasks.Add(releaser);
+                var waiter = new AsyncLockWaiter(cancellationToken);
+                _waiters.Enqueue(waiter);
+
+                return waiter.Task;
             }
-
-            if (!hasDirectApproval)
-            {
-                return releaser.Task;
-            }
-
-            return _releaserTaskWithDirectApproval;
         }
 
-        void Release(Releaser releaser)
+        void Release()
         {
             lock (_syncRoot)
             {
                 if (_isDisposed)
                 {
-                    // There is no much left to do!
+                    // All waiters have been canceled with a ObjectDisposedException.
+                    // So there is nothing left to do.
                     return;
                 }
 
-                var activeTask = _queuedTasks[0];
-                if (!ReferenceEquals(activeTask, releaser))
-                {
-                    throw new InvalidOperationException("The active task must be the current releaser.");
-                }
+                // Assume that there is no waiter left first.
+                _isLocked = false;
 
-                _queuedTasks.RemoveAt(0);
-                
-                while (_queuedTasks.Count > 0)
+                // Try to find the next waiter which can be approved.
+                // Some of them might be canceled already so it is not
+                // guaranteed that the very next waiter is the correct one.
+                while (_waiters.Any())
                 {
-                    var nextTask = _queuedTasks[0];
-                    if (!nextTask.IsPending)
+                    var waiter = _waiters.Dequeue();
+                    var isApproved = waiter.Approve(_releaser);
+                    waiter.Dispose();
+
+                    if (isApproved)
                     {
-                        // Dequeue all canceled or failed tasks.
-                        _queuedTasks.RemoveAt(0);
-                        continue;
+                        _isLocked = true;
+                        return;
                     }
-
-                    nextTask.Approve();
-                    return;
                 }
-
-                Debug.WriteLine("AsyncLock: No Task pending.");
             }
         }
 
-        sealed class Releaser : IDisposable
+        sealed class AsyncLockWaiter : IDisposable
         {
-            readonly AsyncLock _asyncLock;
-            readonly CancellationToken _cancellationToken;
-            readonly int _id;
-            readonly TaskCompletionSource<IDisposable> _promise;
+            readonly CancellationTokenRegistration _cancellationRegistration;
+            readonly bool _hasCancellationRegistration;
+            readonly AsyncTaskCompletionSource<IDisposable> _promise = new AsyncTaskCompletionSource<IDisposable>();
 
-            // ReSharper disable once FieldCanBeMadeReadOnly.Local
-            CancellationTokenRegistration _cancellationTokenRegistration;
-
-            internal Releaser(AsyncLock asyncLock, TaskCompletionSource<IDisposable> promise, CancellationToken cancellationToken)
+            public AsyncLockWaiter(CancellationToken cancellationToken)
             {
-                _asyncLock = asyncLock ?? throw new ArgumentNullException(nameof(asyncLock));
-                _promise = promise;
-                _cancellationToken = cancellationToken;
-
+                cancellationToken.ThrowIfCancellationRequested();
+                
                 if (cancellationToken.CanBeCanceled)
                 {
-                    _cancellationTokenRegistration = cancellationToken.Register(Cancel);
+                    _cancellationRegistration = cancellationToken.Register(Cancel);
+                    _hasCancellationRegistration = true;
                 }
-
-                _id = promise?.Task.Id ?? -1;
-
-                Debug.WriteLine($"AsyncLock: Task {_id} queued.");
             }
 
-            public bool IsPending => _promise != null && !_promise.Task.IsCanceled && !_promise.Task.IsFaulted && !_promise.Task.IsCompleted;
+            public Task<IDisposable> Task => _promise.Task;
 
-            public Task<IDisposable> Task => _promise?.Task;
-
-            public void Approve()
+            public bool Approve(IDisposable scope)
             {
-                _promise?.TrySetResult(this);
+                if (scope == null)
+                {
+                    throw new ArgumentNullException(nameof(scope));
+                }
 
-                Debug.WriteLine($"AsyncLock: Task {_id} approved.");
+                if (_promise.Task.IsCompleted)
+                {
+                    return false;
+                }
+
+                return _promise.TrySetResult(scope);
             }
 
             public void Dispose()
             {
-                if (_cancellationToken.CanBeCanceled)
+                if (_hasCancellationRegistration)
                 {
-                    _cancellationTokenRegistration.Dispose();
+                    _cancellationRegistration.Dispose();
                 }
 
-                Debug.WriteLine($"AsyncLock: Task {_id} completed.");
-
-                _asyncLock.Release(this);
-            }
-
-            public void Fail(Exception exception)
-            {
-                _promise?.TrySetException(exception);
-
-                Debug.WriteLine($"AsyncLock: Task {_id} failed ({exception.GetType().Name}).");
+                _promise.TrySetException(new ObjectDisposedException(nameof(AsyncLockWaiter)));
             }
 
             void Cancel()
             {
-                _promise?.TrySetCanceled();
+                _promise.TrySetCanceled();
+            }
+        }
 
-                Debug.WriteLine($"AsyncLock: Task {_id} canceled.");
+        readonly struct Releaser : IDisposable
+        {
+            readonly AsyncLock _asyncLock;
+
+            public Releaser(AsyncLock asyncLock)
+            {
+                _asyncLock = asyncLock ?? throw new ArgumentNullException(nameof(asyncLock));
+            }
+
+            public void Dispose()
+            {
+                _asyncLock.Release();
             }
         }
     }

Source/MQTTnet/Internal/AsyncTaskCompletionSource.cs

@@ -3,6 +3,7 @@
 // See the LICENSE file in the project root for more information.
 
 using System;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace MQTTnet.Internal
@@ -22,7 +23,7 @@ namespace MQTTnet.Internal
 
         public Task<TResult> Task => _taskCompletionSource.Task;
 
-        public bool TrySetCanceled()
+        public void TrySetCanceled()
         {
 #if NET452
             // To prevent deadlocks it is required to call the _TrySetCanceled_ method
@@ -31,19 +32,14 @@ namespace MQTTnet.Internal
             // do it. But _this_ thread is also reading incoming packets -> deadlock.
             // NET452 does not support RunContinuationsAsynchronously
             System.Threading.Tasks.Task.Run(() => _taskCompletionSource.TrySetCanceled());
-            return true;
+            SpinWait.SpinUntil(() => _taskCompletionSource.Task.IsCompleted);
 #else
-            return _taskCompletionSource.TrySetCanceled();
+            _taskCompletionSource.TrySetCanceled();
 #endif
         }
 
-        public bool TrySetException(Exception exception)
+        public void TrySetException(Exception exception)
         {
-            if (exception == null)
-            {
-                throw new ArgumentNullException(nameof(exception));
-            }
-
 #if NET452
             // To prevent deadlocks it is required to call the _TrySetException_ method
             // from a new thread because the awaiting code will not(!) be executed in
@@ -51,9 +47,9 @@ namespace MQTTnet.Internal
             // do it. But _this_ thread is also reading incoming packets -> deadlock.
             // NET452 does not support RunContinuationsAsynchronously
             System.Threading.Tasks.Task.Run(() => _taskCompletionSource.TrySetException(exception));
-            return true;
+            SpinWait.SpinUntil(() => _taskCompletionSource.Task.IsCompleted);
 #else
-            return _taskCompletionSource.TrySetException(exception);
+            _taskCompletionSource.TrySetException(exception);
 #endif
         }
 
@@ -65,7 +61,14 @@ namespace MQTTnet.Internal
             // a new thread automatically (due to await). Furthermore _this_ thread will
             // do it. But _this_ thread is also reading incoming packets -> deadlock.
             // NET452 does not support RunContinuationsAsynchronously
+            if (_taskCompletionSource.Task.IsCompleted)
+            {
+                return false;
+            }
+            
             System.Threading.Tasks.Task.Run(() => _taskCompletionSource.TrySetResult(result));
+            SpinWait.SpinUntil(() => _taskCompletionSource.Task.IsCompleted);
+
             return true;
 #else
             return _taskCompletionSource.TrySetResult(result);

Source/MQTTnet/Internal/CompletedTask.cs

@@ -0,0 +1,17 @@
+// 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 System.Threading.Tasks;
+
+namespace MQTTnet.Internal
+{
+    public static class CompletedTask
+    {
+#if NET452
+        public static readonly Task Instance = Task.FromResult(true);
+#else
+        public static readonly Task Instance = Task.CompletedTask;
+#endif
+    }
+}

Source/MQTTnet/Internal/EmptyBuffer.cs

@@ -0,0 +1,19 @@
+// 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 System;
+
+namespace MQTTnet.Internal
+{
+    public static class EmptyBuffer
+    {
+#if NET452
+        public static readonly byte[] Array = new byte[0];
+#else
+        public static readonly byte[] Array = System.Array.Empty<byte>();
+#endif
+
+        public static readonly ArraySegment<byte> ArraySegment = new ArraySegment<byte>(Array, 0, 0);
+    }
+}

Source/MQTTnet/MQTTnet.csproj

@@ -14,7 +14,6 @@
         <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' ">$(TargetFrameworks);net452;net461</TargetFrameworks>
         <TargetFrameworks Condition=" '$(OS)' == 'Windows_NT' AND '$(MSBuildRuntimeType)' != 'Core' ">$(TargetFrameworks);uap10.0</TargetFrameworks>
         <LangVersion>7.3</LangVersion>
-        
         <AssemblyName>MQTTnet</AssemblyName>
         <RootNamespace>MQTTnet</RootNamespace>
         <GeneratePackageOnBuild>True</GeneratePackageOnBuild>

Source/MQTTnet/Server/Events/ValidatingConnectionEventArgs.cs

@@ -10,6 +10,7 @@ using System.Text;
 using MQTTnet.Adapter;
 using MQTTnet.Formatter;
 using MQTTnet.Implementations;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
 
@@ -88,7 +89,7 @@ namespace MQTTnet.Server
         /// </summary>
         public uint MaximumPacketSize => _connectPacket.MaximumPacketSize;
 
-        public string Password => Encoding.UTF8.GetString(RawPassword ?? PlatformAbstractionLayer.EmptyByteArray);
+        public string Password => Encoding.UTF8.GetString(RawPassword ?? EmptyBuffer.Array);
 
         public MqttProtocolVersion ProtocolVersion => ChannelAdapter.PacketFormatterAdapter.ProtocolVersion;
 

Source/MQTTnet/Server/Internal/MqttClient.cs

@@ -181,7 +181,7 @@ namespace MQTTnet.Server
                 return _eventContainer.ClientAcknowledgedPublishPacketEvent.TryInvokeAsync(eventArgs, _logger);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         Task HandleIncomingPubAckPacket(MqttPubAckPacket pubAckPacket)
@@ -193,7 +193,7 @@ namespace MQTTnet.Server
                 return ClientAcknowledgedPublishPacket(acknowledgedPublishPacket, pubAckPacket);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         Task HandleIncomingPubCompPacket(MqttPubCompPacket pubCompPacket)
@@ -205,7 +205,7 @@ namespace MQTTnet.Server
                 return ClientAcknowledgedPublishPacket(acknowledgedPublishPacket, pubCompPacket);
             }
 
-            return PlatformAbstractionLayer.CompletedTask;
+            return CompletedTask.Instance;
         }
 
         async Task HandleIncomingPublishPacket(MqttPublishPacket publishPacket, CancellationToken cancellationToken)

Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs

@@ -306,6 +306,9 @@ namespace MQTTnet.Server
 
                 await client.RunAsync().ConfigureAwait(false);
             }
+            catch (ObjectDisposedException)
+            {
+            }
             catch (OperationCanceledException)
             {
             }
@@ -479,7 +482,7 @@ namespace MQTTnet.Server
                 sessionShouldPersist = !connectPacket.CleanSession;
             }
 
-            using (await _createConnectionSyncRoot.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+            using (await _createConnectionSyncRoot.EnterAsync().ConfigureAwait(false))
             {
                 MqttSession session;
                 lock (_sessionsManagementLock)
@@ -537,6 +540,7 @@ namespace MQTTnet.Server
                             MqttClientDisconnectType.Takeover,
                             existingClient.Endpoint,
                             existingClient.Session.Items);
+                        
                         await _eventContainer.ClientDisconnectedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
                     }
                 }

Source/MQTTnet/Server/Internal/MqttRetainedMessagesManager.cs

@@ -5,10 +5,8 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
-using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 
 namespace MQTTnet.Server
@@ -85,7 +83,7 @@ namespace MQTTnet.Server
                         }
                         else
                         {
-                            if (existingMessage.QualityOfServiceLevel != applicationMessage.QualityOfServiceLevel || !existingMessage.Payload.SequenceEqual(applicationMessage.Payload ?? PlatformAbstractionLayer.EmptyByteArray))
+                            if (existingMessage.QualityOfServiceLevel != applicationMessage.QualityOfServiceLevel || !existingMessage.Payload.SequenceEqual(applicationMessage.Payload ?? EmptyBuffer.Array))
                             {
                                 _messages[applicationMessage.Topic] = applicationMessage;
                                 saveIsRequired = true;
@@ -103,7 +101,7 @@ namespace MQTTnet.Server
 
                 if (saveIsRequired)
                 {
-                    using (await _storageAccessLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+                    using (await _storageAccessLock.EnterAsync().ConfigureAwait(false))
                     {
                         var eventArgs = new RetainedMessageChangedEventArgs(clientId, applicationMessage, messagesForSave);
                         await _eventContainer.RetainedMessageChangedEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
@@ -132,7 +130,7 @@ namespace MQTTnet.Server
                 _messages.Clear();
             }
 
-            using (await _storageAccessLock.WaitAsync(CancellationToken.None).ConfigureAwait(false))
+            using (await _storageAccessLock.EnterAsync().ConfigureAwait(false))
             {
                 await _eventContainer.RetainedMessagesClearedEvent.InvokeAsync(EventArgs.Empty).ConfigureAwait(false);
             }

Source/MQTTnet/Server/Internal/MqttServerKeepAliveMonitor.cs

@@ -6,7 +6,6 @@ using System;
 using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Diagnostics;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.Protocol;
 
@@ -14,16 +13,20 @@ namespace MQTTnet.Server
 {
     public sealed class MqttServerKeepAliveMonitor
     {
+        readonly MqttNetSourceLogger _logger;
         readonly MqttServerOptions _options;
         readonly MqttClientSessionsManager _sessionsManager;
-        readonly MqttNetSourceLogger _logger;
 
         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));
+
+            if (logger == null)
+            {
+                throw new ArgumentNullException(nameof(logger));
+            }
+
             _logger = logger.WithSource(nameof(MqttServerKeepAliveMonitor));
         }
 
@@ -45,7 +48,7 @@ namespace MQTTnet.Server
                 while (!cancellationToken.IsCancellationRequested)
                 {
                     TryProcessClients();
-                    PlatformAbstractionLayer.Sleep(_options.KeepAliveMonitorInterval);
+                    Sleep(_options.KeepAliveMonitorInterval);
                 }
             }
             catch (OperationCanceledException)
@@ -61,13 +64,22 @@ namespace MQTTnet.Server
             }
         }
 
-        void TryProcessClients()
+        static void Sleep(TimeSpan timeout)
         {
-            var now = DateTime.UtcNow;
-            foreach (var client in _sessionsManager.GetClients())
+#if !NETSTANDARD1_3 && !WINDOWS_UWP
+            try
             {
-                TryProcessClient(client, now);
+                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();
+            }
+#else
+            Task.Delay(timeout).Wait();
+#endif
         }
 
         void TryProcessClient(MqttClient connection, DateTime now)
@@ -119,5 +131,14 @@ namespace MQTTnet.Server
                 _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);
+            }
+        }
     }
-}
+}

Source/MQTTnet/Server/MqttServer.cs

@@ -11,7 +11,6 @@ using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Adapter;
 using MQTTnet.Diagnostics;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
@@ -172,7 +171,7 @@ namespace MQTTnet.Server
         {
             ThrowIfNotStarted();
 
-            return _retainedMessagesManager?.ClearMessages() ?? PlatformAbstractionLayer.CompletedTask;
+            return _retainedMessagesManager?.ClearMessages() ?? CompletedTask.Instance;
         }
 
         public Task DisconnectClientAsync(string id, MqttDisconnectReasonCode reasonCode)

Source/MQTTnet/Server/Status/MqttSessionStatus.cs

@@ -6,7 +6,6 @@ using System;
 using System.Collections;
 using System.Threading.Tasks;
 using MQTTnet.Formatter;
-using MQTTnet.Implementations;
 using MQTTnet.Internal;
 
 namespace MQTTnet.Server
@@ -34,8 +33,8 @@ namespace MQTTnet.Server
             
             var publishPacketFactory = new MqttPublishPacketFactory();
             _session.EnqueueDataPacket(new MqttPacketBusItem(publishPacketFactory.Create(applicationMessage)));
-            
-            return PlatformAbstractionLayer.CompletedTask;
+
+            return CompletedTask.Instance;
         }
 
         public Task DeliverApplicationMessageAsync(MqttApplicationMessage applicationMessage)