fixed parallel writes

Source/MQTTnet.AspnetCore/MqttConnectionContext.cs

@@ -27,6 +27,8 @@ namespace MQTTnet.AspNetCore
         public event EventHandler ReadingPacketStarted;
         public event EventHandler ReadingPacketCompleted;
 
+        private readonly SemaphoreSlim _writerSemaphore = new SemaphoreSlim(1, 1);
+
         public Task ConnectAsync(TimeSpan timeout, CancellationToken cancellationToken)
         {
             if (Connection is TcpConnection tcp && !tcp.IsConnected)
@@ -105,10 +107,20 @@ namespace MQTTnet.AspNetCore
             return null;
         }
 
-        public Task SendPacketAsync(MqttBasePacket packet, CancellationToken cancellationToken)
+        public async Task SendPacketAsync(MqttBasePacket packet, CancellationToken cancellationToken)
         {
-            var buffer = PacketSerializer.Serialize(packet);
-            return Connection.Transport.Output.WriteAsync(buffer.AsMemory(), cancellationToken).AsTask();
+            var buffer = PacketSerializer.Serialize(packet).AsMemory();
+            var output = Connection.Transport.Output;
+
+            await _writerSemaphore.WaitAsync(cancellationToken).ConfigureAwait(false);
+            try
+            {
+                await output.WriteAsync(buffer, cancellationToken);
+            }
+            finally
+            {
+                _writerSemaphore.Release();
+            }
         }
 
         public void Dispose()

Tests/MQTTnet.AspNetCore.Tests/MqttConnectionContextTest.cs

@@ -2,8 +2,10 @@
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using MQTTnet.AspNetCore.Tests.Mockups;
 using MQTTnet.Exceptions;
+using MQTTnet.Packets;
 using MQTTnet.Serializer;
 using System;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 
@@ -25,5 +27,25 @@ namespace MQTTnet.AspNetCore.Tests
 
             await Assert.ThrowsExceptionAsync<MqttCommunicationException>(() => ctx.ReceivePacketAsync(TimeSpan.FromSeconds(1), CancellationToken.None));
         }
+        
+        [TestMethod]
+        public async Task TestParallelWrites()
+        {
+            var serializer = new MqttPacketSerializer { };
+            var pipe = new DuplexPipeMockup();
+            var connection = new DefaultConnectionContext();
+            connection.Transport = pipe;
+            var ctx = new MqttConnectionContext(serializer, connection);
+
+            var tasks = Enumerable.Range(1, 10).Select(_ => Task.Run(async () => 
+            {
+                for (int i = 0; i < 100; i++)
+                {
+                    await ctx.SendPacketAsync(new MqttPublishPacket(), CancellationToken.None).ConfigureAwait(false);
+                }
+            }));
+
+            await Task.WhenAll(tasks).ConfigureAwait(false);
+        }
     }
 }