fixed parallel writes #932

Source/MQTTnet.AspnetCore/MqttConnectionContext.cs

@@ -163,18 +163,16 @@ namespace MQTTnet.AspNetCore
         public async Task SendPacketAsync(MqttBasePacket packet, TimeSpan timeout, CancellationToken cancellationToken)
         {
             var formatter = PacketFormatterAdapter;
-            var buffer = formatter.Encode(packet);
-            var msg = buffer.AsMemory();
-            var output = _output;
-
             using (await _writerLock.WaitAsync(cancellationToken).ConfigureAwait(false))
             {
+                var buffer = formatter.Encode(packet);
+                var msg = buffer.AsMemory();
+                var output = _output;
                 var result = await output.WriteAsync(msg, cancellationToken).ConfigureAwait(false);
                 if (result.IsCompleted)
                 {
                     BytesSent += msg.Length;
                 }
-
                 PacketFormatterAdapter.FreeBuffer();
             }
         }

Tests/MQTTnet.AspNetCore.Tests/MqttConnectionContextTest.cs

@@ -45,7 +45,7 @@ namespace MQTTnet.AspNetCore.Tests
             connection.Transport = pipe;
             var ctx = new MqttConnectionContext(serializer, connection);
 
-            var tasks = Enumerable.Range(1, 10).Select(_ => Task.Run(async () => 
+            var tasks = Enumerable.Range(1, 100).Select(_ => Task.Run(async () => 
             {
                 for (int i = 0; i < 100; i++)
                 {