Refactor graceful socket close.

Frameworks/MQTTnet.NetStandard/Adapter/MqttChannelAdapter.cs

@@ -8,6 +8,7 @@ using System.Threading.Tasks;
 using MQTTnet.Channel;
 using MQTTnet.Diagnostics;
 using MQTTnet.Exceptions;
+using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Serializer;
 
@@ -133,18 +134,14 @@ namespace MQTTnet.Adapter
         private async Task<ReceivedMqttPacket> ReceiveAsync(IMqttChannel channel, CancellationToken cancellationToken)
         {
             var fixedHeader = await MqttPacketReader.ReadFixedHeaderAsync(channel, cancellationToken).ConfigureAwait(false);
-            if (!fixedHeader.HasValue)
-            {
-                return null;
-            }
-
+            
             ReadingPacketStarted?.Invoke(this, EventArgs.Empty);
             try
             {
                 var bodyLength = await MqttPacketReader.ReadBodyLengthAsync(channel, cancellationToken).ConfigureAwait(false);
                 if (bodyLength == 0)
                 {
-                    return new ReceivedMqttPacket(fixedHeader.Value, null);
+                    return new ReceivedMqttPacket(fixedHeader, null);
                 }
 
                 var body = new MemoryStream(bodyLength);
@@ -159,11 +156,9 @@ namespace MQTTnet.Adapter
                     }
 
                     var readBytesCount = await channel.ReadAsync(buffer, 0, bytesLeft, cancellationToken).ConfigureAwait(false);
-
-                    // Check if the client closed the connection before sending the full body.
                     if (readBytesCount <= 0)
                     {
-                        throw new MqttCommunicationException("Connection closed while reading remaining packet body.");
+                        ExceptionHelper.ThrowGracefulSocketClose();
                     }
 
                     // Here is no need to await because internally only an array is used and no real I/O operation is made.
@@ -173,7 +168,7 @@ namespace MQTTnet.Adapter
 
                 body.Seek(0L, SeekOrigin.Begin);
 
-                return new ReceivedMqttPacket(fixedHeader.Value, body);
+                return new ReceivedMqttPacket(fixedHeader, body);
             }
             finally
             {

Frameworks/MQTTnet.NetStandard/Internal/ExceptionHelper.cs

@@ -0,0 +1,12 @@
+using MQTTnet.Exceptions;
+
+namespace MQTTnet.Internal
+{
+    public static class ExceptionHelper
+    {
+        public static void ThrowGracefulSocketClose()
+        {
+            throw new MqttCommunicationException("Connection gracefully closed from the remote party.");
+        }
+    }
+}

Frameworks/MQTTnet.NetStandard/Serializer/MqttPacketReader.cs

@@ -5,18 +5,14 @@ using System.Threading;
 using System.Threading.Tasks;
 using MQTTnet.Channel;
 using MQTTnet.Exceptions;
+using MQTTnet.Internal;
 
 namespace MQTTnet.Serializer
 {
     public static class MqttPacketReader
     {
-        public static async Task<byte?> ReadFixedHeaderAsync(IMqttChannel channel, CancellationToken cancellationToken)
+        public static async Task<byte> ReadFixedHeaderAsync(IMqttChannel channel, CancellationToken cancellationToken)
         {
-            if (cancellationToken.IsCancellationRequested)
-            {
-                return null;
-            }
-
             // Wait for the next package which starts with the header. At this point there will probably
             // some large delay and thus the thread should be put back to the pool (await). So ReadByte()
             // is not an option here.
@@ -24,7 +20,7 @@ namespace MQTTnet.Serializer
             var readCount = await channel.ReadAsync(buffer, 0, buffer.Length, cancellationToken).ConfigureAwait(false);
             if (readCount <= 0)
             {
-                return null;
+                ExceptionHelper.ThrowGracefulSocketClose();
             }
 
             return buffer[0];
@@ -68,13 +64,6 @@ namespace MQTTnet.Serializer
             return stream.ReadBytes((int)(stream.Length - stream.Position));
         }
 
-        public static byte[] ReadBytes(this Stream stream, int count)
-        {
-            var buffer = new byte[count];
-            stream.Read(buffer, 0, count);
-            return buffer;
-        }
-
         public static async Task<int> ReadBodyLengthAsync(IMqttChannel channel, CancellationToken cancellationToken)
         {
             // Alorithm taken from https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/errata01/os/mqtt-v3.1.1-errata01-os-complete.html.
@@ -93,7 +82,7 @@ namespace MQTTnet.Serializer
                 var readCount = await channel.ReadAsync(buffer, 0, 1, cancellationToken).ConfigureAwait(false);
                 if (readCount <= 0)
                 {
-                    throw new MqttCommunicationException("Connection closed while reading remaining length data.");
+                    ExceptionHelper.ThrowGracefulSocketClose();
                 }
 
                 encodedByte = buffer[0];
@@ -109,5 +98,12 @@ namespace MQTTnet.Serializer
 
             return value;
         }
+
+        private static byte[] ReadBytes(this Stream stream, int count)
+        {
+            var buffer = new byte[count];
+            stream.Read(buffer, 0, count);
+            return buffer;
+        }
     }
 }