Add support for Maximum QoS property on CONNACK

Source/MQTTnet/Client/Connecting/MqttClientAuthenticateResult.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using MQTTnet.Packets;
+using MQTTnet.Protocol;
 
 namespace MQTTnet.Client.Connecting
 {
@@ -25,6 +26,8 @@ namespace MQTTnet.Client.Connecting
         public string ReasonString { get; set; }
 
         public ushort? ReceiveMaximum { get; set; }
+        
+        public MqttQualityOfServiceLevel? MaximumQoS { get; set; }
 
         public string ResponseInformation { get; set; }
 

Source/MQTTnet/Formatter/V5/MqttV500DataConverter.cs

@@ -93,6 +93,7 @@ namespace MQTTnet.Formatter.V5
                 MaximumPacketSize = connAckPacket.Properties?.MaximumPacketSize,
                 ReasonString = connAckPacket.Properties?.ReasonString,
                 ReceiveMaximum = connAckPacket.Properties?.ReceiveMaximum,
+                MaximumQoS = connAckPacket.Properties?.MaximumQoS,
                 ResponseInformation = connAckPacket.Properties?.ResponseInformation,
                 TopicAliasMaximum = connAckPacket.Properties?.TopicAliasMaximum,
                 ServerReference = connAckPacket.Properties?.ServerReference,

Source/MQTTnet/Formatter/V5/MqttV500PacketDecoder.cs

@@ -254,6 +254,10 @@ namespace MQTTnet.Formatter.V5
                 {
                     packet.Properties.ReceiveMaximum = propertiesReader.ReadReceiveMaximum();
                 }
+                else if (propertiesReader.CurrentPropertyId == MqttPropertyId.MaximumQoS)
+                {
+                    packet.Properties.MaximumQoS = propertiesReader.ReadMaximumQoS();
+                }
                 else if (propertiesReader.CurrentPropertyId == MqttPropertyId.AssignedClientIdentifier)
                 {
                     packet.Properties.AssignedClientIdentifier = propertiesReader.ReadAssignedClientIdentifier();

Source/MQTTnet/Formatter/V5/MqttV500PacketEncoder.cs

@@ -203,6 +203,7 @@ namespace MQTTnet.Formatter.V5
                 propertiesWriter.WriteAuthenticationData(packet.Properties.AuthenticationData);
                 propertiesWriter.WriteRetainAvailable(packet.Properties.RetainAvailable);
                 propertiesWriter.WriteReceiveMaximum(packet.Properties.ReceiveMaximum);
+                propertiesWriter.WriteMaximumQoS(packet.Properties.MaximumQoS);
                 propertiesWriter.WriteAssignedClientIdentifier(packet.Properties.AssignedClientIdentifier);
                 propertiesWriter.WriteTopicAliasMaximum(packet.Properties.TopicAliasMaximum);
                 propertiesWriter.WriteReasonString(packet.Properties.ReasonString);

Source/MQTTnet/Formatter/V5/MqttV500PropertiesReader.cs

@@ -82,6 +82,16 @@ namespace MQTTnet.Formatter.V5
             return _body.ReadTwoByteInteger();
         }
 
+        public MqttQualityOfServiceLevel ReadMaximumQoS()
+        {
+            byte value = _body.ReadByte();
+            if (value > 1)
+            {
+                throw new MqttProtocolViolationException($"Unexpected Maximum QoS value: {value}");
+            }
+            return (MqttQualityOfServiceLevel)value;
+        }
+
         public string ReadAssignedClientIdentifier()
         {
             return _body.ReadStringWithLengthPrefix();

Source/MQTTnet/Formatter/V5/MqttV500PropertiesWriter.cs

@@ -133,6 +133,17 @@ namespace MQTTnet.Formatter.V5
             Write(MqttPropertyId.ReceiveMaximum, value);
         }
 
+        public void WriteMaximumQoS(MqttQualityOfServiceLevel? value)
+        {
+            if (!value.HasValue || value.Value > MqttQualityOfServiceLevel.AtLeastOnce)
+            {
+                return;
+            }
+
+            _packetWriter.Write((byte)MqttPropertyId.MaximumQoS);
+            _packetWriter.Write((byte)value.Value);
+        }
+
         public void WriteMaximumPacketSize(uint? value)
         {
             WriteAsFourByteInteger(MqttPropertyId.MaximumPacketSize, value);

Source/MQTTnet/Packets/MqttConnAckPacketProperties.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using MQTTnet.Protocol;
 
 namespace MQTTnet.Packets
 {
@@ -8,6 +9,8 @@ namespace MQTTnet.Packets
 
         public ushort? ReceiveMaximum { get; set; }
 
+        public MqttQualityOfServiceLevel? MaximumQoS { get; set; }
+
         public bool? RetainAvailable { get; set; }
 
         public uint? MaximumPacketSize { get; set; }