rethrow MqttClientUnexpectedDisconnectReceivedException when Publish … (#1974)

* rethrow MqttClientUnexpectedDisconnectReceivedException when Publish errors out because server disconnects the client and QoS is 0

* use a local var to cache _unexpectedDisconnectPacket

* Add inner exception to new exception

* Update release notes

* set the default inner exception as null

---------

Co-authored-by: christian <6939810+chkr1011@users.noreply.github.com>

.github/workflows/ReleaseNotes.md

@@ -1,6 +1,7 @@
 * [Core] Optimized packet serialization of PUBACK and PUBREC packets for protocol version 5.0.0 (#1939, thanks to @Y-Sindo).
 * [Core] The package inspector is now fully async (#1941).
 * [Client] Added a dedicated exception when the client is not connected (#1954, thanks to @marcpiulachs).
+* [Client] The client will now throw a _MqttClientUnexpectedDisconnectReceivedException_ when publishing a QoS 0 message which leads to a server disconnect (BREAKING CHANGE!, #1974, thanks to @fazho).
 * [Client] Exposed the certificate selection event handler in client options (#1984).
 * [Server] The server will no longer send _NoMatchingSubscribers_ when the actual subscription was non success (#1965, BREAKING CHANGE!). 
 * [Server] Fixed broken support for _null_ in _AddServer_ method in ASP.NET integration (#1981).

Source/MQTTnet/Client/Exceptions/MqttClientUnexpectedDisconnectReceivedException.cs

@@ -2,6 +2,7 @@
 // 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.Collections.Generic;
 using MQTTnet.Exceptions;
 using MQTTnet.Packets;
@@ -11,8 +12,9 @@ namespace MQTTnet.Client
 {
     public sealed class MqttClientUnexpectedDisconnectReceivedException : MqttCommunicationException
     {
-        public MqttClientUnexpectedDisconnectReceivedException(MqttDisconnectPacket disconnectPacket) 
-            : base($"Unexpected DISCONNECT (Reason code={disconnectPacket.ReasonCode}) received.")
+        public MqttClientUnexpectedDisconnectReceivedException(MqttDisconnectPacket disconnectPacket, Exception innerExcpetion = null) : base(
+            $"Unexpected DISCONNECT (Reason code={disconnectPacket.ReasonCode}) received.",
+            innerExcpetion)
         {
             ReasonCode = disconnectPacket.ReasonCode;
             SessionExpiryInterval = disconnectPacket.SessionExpiryInterval;
@@ -23,12 +25,12 @@ namespace MQTTnet.Client
 
         public MqttDisconnectReasonCode? ReasonCode { get; }
 
-        public uint? SessionExpiryInterval { get; }
-
         public string ReasonString { get; }
 
-        public List<MqttUserProperty> UserProperties { get; }
-
         public string ServerReference { get; }
+
+        public uint? SessionExpiryInterval { get; }
+
+        public List<MqttUserProperty> UserProperties { get; }
     }
-}
+}

Source/MQTTnet/Client/MqttClient.cs

@@ -48,6 +48,7 @@ namespace MQTTnet.Client
         Task _packetReceiverTask;
         AsyncQueue<MqttPublishPacket> _publishPacketReceiverQueue;
         Task _publishPacketReceiverTask;
+        MqttDisconnectPacket _unexpectedDisconnectPacket;
 
         public MqttClient(IMqttClientAdapterFactory channelFactory, IMqttNetLogger logger)
         {
@@ -123,6 +124,8 @@ namespace MQTTnet.Client
                 var adapter = _adapterFactory.CreateClientAdapter(options, new MqttPacketInspector(_events.InspectPacketEvent, _rootLogger), _rootLogger);
                 _adapter = adapter ?? throw new InvalidOperationException("The adapter factory did not provide an adapter.");
 
+                _unexpectedDisconnectPacket = null;
+
                 if (cancellationToken.CanBeCanceled)
                 {
                     connectResult = await ConnectInternal(adapter, cancellationToken).ConfigureAwait(false);
@@ -239,7 +242,7 @@ namespace MQTTnet.Client
         public async Task PingAsync(CancellationToken cancellationToken = default)
         {
             cancellationToken.ThrowIfCancellationRequested();
-            
+
             ThrowIfDisposed();
             ThrowIfNotConnected();
 
@@ -330,7 +333,7 @@ namespace MQTTnet.Client
 
             ThrowIfDisposed();
             ThrowIfNotConnected();
-            
+
             if (Options.ValidateFeatures)
             {
                 MqttClientSubscribeOptionsValidator.ThrowIfNotSupported(options, _adapter.PacketFormatterAdapter.ProtocolVersion);
@@ -682,9 +685,10 @@ namespace MQTTnet.Client
             _disconnectReason = (int)disconnectPacket.ReasonCode;
             _disconnectReasonString = disconnectPacket.ReasonString;
             _disconnectUserProperties = disconnectPacket.UserProperties;
+            _unexpectedDisconnectPacket = disconnectPacket;
 
             // Also dispatch disconnect to waiting threads to generate a proper exception.
-            _packetDispatcher.Dispose(new MqttClientUnexpectedDisconnectReceivedException(disconnectPacket));
+            _packetDispatcher.Dispose(new MqttClientUnexpectedDisconnectReceivedException(disconnectPacket, null));
 
             return DisconnectInternal(_packetReceiverTask, null, null);
         }
@@ -751,10 +755,27 @@ namespace MQTTnet.Client
 
         async Task<MqttClientPublishResult> PublishAtMostOnce(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
         {
-            // No packet identifier is used for QoS 0 [3.3.2.2 Packet Identifier]
-            await Send(publishPacket, cancellationToken).ConfigureAwait(false);
+            try
+            {
+                // No packet identifier is used for QoS 0 [3.3.2.2 Packet Identifier]
+                await Send(publishPacket, cancellationToken).ConfigureAwait(false);
 
-            return MqttClientResultFactory.PublishResult.Create(null);
+                return MqttClientResultFactory.PublishResult.Create(null);
+            }
+            catch (Exception exception)
+            {
+                // We have to check if the server has sent a disconnect packet in response to the published message.
+                // Since we are in QoS 0 we do not get a direct response via an PUBACK packet and thus basically no
+                // feedback at all.
+                var localUnexpectedDisconnectPacket = _unexpectedDisconnectPacket;
+
+                if (localUnexpectedDisconnectPacket != null)
+                {
+                    throw new MqttClientUnexpectedDisconnectReceivedException(localUnexpectedDisconnectPacket, exception);
+                }
+
+                throw;
+            }
         }
 
         async Task<MqttClientPublishResult> PublishExactlyOnce(MqttPublishPacket publishPacket, CancellationToken cancellationToken)
@@ -1071,4 +1092,4 @@ namespace MQTTnet.Client
             }
         }
     }
-}
+}