802 client isnt being notified if message is not delivered (#1568)

.github/workflows/ReleaseNotes.md

@@ -4,3 +4,4 @@
 * [Server] Fixed duplicated invocation of the event _ClientAcknowledgedPublishPacketAsync_ for QoS level 2 (#1550)
 * [Server] Fixed issue in upgrading and downgrading of QoS levels for subscriptions and retained messages (#1560)
 * [Server] Fixed memory leak when old sessions are discarded (#1553)
+* [Server] When using MQTTv5 the reason code _NoMatchingSubscribers_ is returned when no client is subscribed to a published topic (#802)

Source/MQTTnet.Tests/MQTTv5/Client_Tests.cs

@@ -154,7 +154,7 @@ namespace MQTTnet.Tests.MQTTv5
                 var result = await client.PublishStringAsync("a", "b", MqttQualityOfServiceLevel.AtLeastOnce);
                 await client.DisconnectAsync();
 
-                Assert.AreEqual(MqttClientPublishReasonCode.Success, result.ReasonCode);
+                Assert.AreEqual(MqttClientPublishReasonCode.NoMatchingSubscribers, result.ReasonCode);
             }
         }
 
@@ -169,7 +169,7 @@ namespace MQTTnet.Tests.MQTTv5
                 var result = await client.PublishStringAsync("a", "b", MqttQualityOfServiceLevel.ExactlyOnce);
                 await client.DisconnectAsync();
 
-                Assert.AreEqual(MqttClientPublishReasonCode.Success, result.ReasonCode);
+                Assert.AreEqual(MqttClientPublishReasonCode.NoMatchingSubscribers, result.ReasonCode);
             }
         }
 

Source/MQTTnet.Tests/Server/Publish_Tests.cs

@@ -0,0 +1,58 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// 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.Threading.Tasks;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using MQTTnet.Client;
+using MQTTnet.Formatter;
+using MQTTnet.Protocol;
+
+namespace MQTTnet.Tests.Server
+{
+    [TestClass]
+    public sealed class Publish_Tests : BaseTestClass
+    {
+        [TestMethod]
+        public async Task Return_Success_When_Subscribed()
+        {
+            using (var testEnvironment = CreateTestEnvironment(MqttProtocolVersion.V500))
+            {
+                await testEnvironment.StartServer();
+
+                var sender = await testEnvironment.ConnectClient();
+                var receiver = await testEnvironment.ConnectClient();
+
+                await receiver.SubscribeAsync("A");
+
+                // AtLeastOnce is required to get an ACK packet from the server.
+                var publishResult = await sender.PublishStringAsync("A", "Payload", MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                Assert.AreEqual(MqttClientPublishReasonCode.Success, publishResult.ReasonCode);
+                
+                Assert.AreEqual(true, publishResult.IsSuccess);
+            }
+        }
+        
+        [TestMethod]
+        public async Task Return_NoMatchingSubscribers_When_Not_Subscribed()
+        {
+            using (var testEnvironment = CreateTestEnvironment(MqttProtocolVersion.V500))
+            {
+                await testEnvironment.StartServer();
+
+                var sender = await testEnvironment.ConnectClient();
+                var receiver = await testEnvironment.ConnectClient();
+             
+                await receiver.SubscribeAsync("A");
+
+                // AtLeastOnce is required to get an ACK packet from the server.
+                var publishResult = await sender.PublishStringAsync("B", "Payload", MqttQualityOfServiceLevel.AtLeastOnce);
+                
+                Assert.AreEqual(MqttClientPublishReasonCode.NoMatchingSubscribers, publishResult.ReasonCode);
+                
+                Assert.AreEqual(true, publishResult.IsSuccess);
+            }
+        }
+    }
+}

Source/MQTTnet/Client/Publishing/MqttClientPublishResult.cs

@@ -10,29 +10,38 @@ namespace MQTTnet.Client
     public sealed class MqttClientPublishResult
     {
         /// <summary>
-        /// Gets the packet identifier which was used for this publish.
+        ///     Returns if the overall status of the publish is a success. This can be the reason code _Success_ or
+        ///     _NoMatchingSubscribers_. _NoMatchingSubscribers_ usually indicates only that no other client is interested in the topic but overall transmit
+        ///     to the server etc. was a success.
+        /// </summary>
+        public bool IsSuccess => ReasonCode == MqttClientPublishReasonCode.Success || ReasonCode == MqttClientPublishReasonCode.NoMatchingSubscribers;
+
+        /// <summary>
+        ///     Gets the packet identifier which was used for this publish.
         /// </summary>
         public ushort? PacketIdentifier { get; set; }
 
         /// <summary>
-        /// Gets or sets the reason code.
+        ///     Gets or sets the reason code.
-        /// Hint: MQTT 5 feature only.
+        ///     Hint: MQTT 5 feature only.
         /// </summary>
         public MqttClientPublishReasonCode ReasonCode { get; set; } = MqttClientPublishReasonCode.Success;
 
         /// <summary>
-        /// Gets or sets the reason string.
+        ///     Gets or sets the reason string.
-        /// Hint: MQTT 5 feature only.
+        ///     Hint: MQTT 5 feature only.
         /// </summary>
         public string ReasonString { get; set; }
 
         /// <summary>
-        /// Gets or sets the user properties.
+        ///     Gets or sets the user properties.
-        /// In MQTT 5, user properties are basic UTF-8 string key-value pairs that you can append to almost every type of MQTT packet.
+        ///     In MQTT 5, user properties are basic UTF-8 string key-value pairs that you can append to almost every type of MQTT
-        /// As long as you don’t exceed the maximum message size, you can use an unlimited number of user properties to add metadata to MQTT messages and pass information between publisher, broker, and subscriber.
+        ///     packet.
-        /// The feature is very similar to the HTTP header concept.
+        ///     As long as you don’t exceed the maximum message size, you can use an unlimited number of user properties to add
-        /// Hint: MQTT 5 feature only.
+        ///     metadata to MQTT messages and pass information between publisher, broker, and subscriber.
+        ///     The feature is very similar to the HTTP header concept.
+        ///     Hint: MQTT 5 feature only.
         /// </summary>
         public IReadOnlyCollection<MqttUserProperty> UserProperties { get; internal set; }
     }
-}
+}

Source/MQTTnet/Formatter/MqttPubAckPacketFactory.cs

@@ -12,7 +12,10 @@ namespace MQTTnet.Formatter
 {
     public sealed class MqttPubAckPacketFactory
     {
-        public MqttPubAckPacket Create(MqttPublishPacket publishPacket, InterceptingPublishEventArgs interceptingPublishEventArgs)
+        public MqttPubAckPacket Create(
+            MqttPublishPacket publishPacket,
+            InterceptingPublishEventArgs interceptingPublishEventArgs,
+            DispatchApplicationMessageResult dispatchApplicationMessageResult)
         {
             if (publishPacket == null)
             {
@@ -21,10 +24,19 @@ namespace MQTTnet.Formatter
 
             var pubAckPacket = new MqttPubAckPacket
             {
-                PacketIdentifier = publishPacket.PacketIdentifier,
+                PacketIdentifier = publishPacket.PacketIdentifier
-                ReasonCode = MqttPubAckReasonCode.Success
             };
 
+            if (dispatchApplicationMessageResult.MatchingSubscribersCount == 0)
+            {
+                // NoMatchingSubscribers is ONLY sent by the server!
+                pubAckPacket.ReasonCode = MqttPubAckReasonCode.NoMatchingSubscribers;
+            }
+            else
+            {
+                pubAckPacket.ReasonCode = MqttPubAckReasonCode.Success;
+            }
+
             if (interceptingPublishEventArgs != null)
             {
                 pubAckPacket.ReasonCode = (MqttPubAckReasonCode)(int)interceptingPublishEventArgs.Response.ReasonCode;
@@ -42,24 +54,12 @@ namespace MQTTnet.Formatter
                 throw new ArgumentNullException(nameof(applicationMessageReceivedEventArgs));
             }
 
-            var pubAckPacket = Create(applicationMessageReceivedEventArgs.PublishPacket, applicationMessageReceivedEventArgs.ReasonCode);
-            pubAckPacket.UserProperties = applicationMessageReceivedEventArgs.ResponseUserProperties;
-            pubAckPacket.ReasonString = applicationMessageReceivedEventArgs.ResponseReasonString;
-
-            return pubAckPacket;
-        }
-
-        static MqttPubAckPacket Create(MqttPublishPacket publishPacket, MqttApplicationMessageReceivedReasonCode applicationMessageReceivedReasonCode)
-        {
-            if (publishPacket == null)
-            {
-                throw new ArgumentNullException(nameof(publishPacket));
-            }
-
             var pubAckPacket = new MqttPubAckPacket
             {
-                PacketIdentifier = publishPacket.PacketIdentifier,
+                PacketIdentifier = applicationMessageReceivedEventArgs.PublishPacket.PacketIdentifier,
-                ReasonCode = (MqttPubAckReasonCode)(int)applicationMessageReceivedReasonCode
+                ReasonCode = (MqttPubAckReasonCode)(int)applicationMessageReceivedEventArgs.ReasonCode,
+                UserProperties = applicationMessageReceivedEventArgs.ResponseUserProperties,
+                ReasonString = applicationMessageReceivedEventArgs.ResponseReasonString
             };
 
             return pubAckPacket;

Source/MQTTnet/Formatter/MqttPubRecPacketFactory.cs

@@ -25,7 +25,10 @@ namespace MQTTnet.Formatter
             return pubRecPacket;
         }
 
-        public MqttPacket Create(MqttPublishPacket publishPacket, InterceptingPublishEventArgs interceptingPublishEventArgs)
+        public MqttPacket Create(
+            MqttPublishPacket publishPacket,
+            InterceptingPublishEventArgs interceptingPublishEventArgs,
+            DispatchApplicationMessageResult dispatchApplicationMessageResult)
         {
             if (publishPacket == null)
             {
@@ -38,6 +41,11 @@ namespace MQTTnet.Formatter
                 ReasonCode = MqttPubRecReasonCode.Success
             };
 
+            if (dispatchApplicationMessageResult.MatchingSubscribersCount == 0)
+            {
+                pubRecPacket.ReasonCode = MqttPubRecReasonCode.NoMatchingSubscribers;
+            }
+
             if (interceptingPublishEventArgs != null)
             {
                 pubRecPacket.ReasonCode = (MqttPubRecReasonCode)(int)interceptingPublishEventArgs.Response.ReasonCode;

Source/MQTTnet/Protocol/MqttPubAckReasonCode.cs

@@ -7,7 +7,12 @@ namespace MQTTnet.Protocol
     public enum MqttPubAckReasonCode
     {
         Success = 0,
+        
+        /// <summary>
+        /// The message is accepted but there are no subscribers. This is sent only by the Server. If the Server knows that there are no matching subscribers, it MAY use this Reason Code instead of 0x00 (Success).
+        /// </summary>
         NoMatchingSubscribers = 16,
+        
         UnspecifiedError = 128,
         ImplementationSpecificError = 131,
         NotAuthorized = 135,

Source/MQTTnet/Server/Internal/DispatchApplicationMessageResult.cs

@@ -0,0 +1,16 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace MQTTnet.Server
+{
+    public sealed class DispatchApplicationMessageResult
+    {
+        public DispatchApplicationMessageResult(int matchingSubscribersCount)
+        {
+            MatchingSubscribersCount = matchingSubscribersCount;
+        }
+
+        public int MatchingSubscribersCount { get; }
+    }
+}

Source/MQTTnet/Server/Internal/MqttClient.cs

@@ -237,9 +237,10 @@ namespace MQTTnet.Server
                 return;
             }
 
+            DispatchApplicationMessageResult dispatchResult = null;
             if (processPublish && applicationMessage != null)
             {
-                await _sessionsManager.DispatchApplicationMessage(Id, applicationMessage).ConfigureAwait(false);
+                dispatchResult = await _sessionsManager.DispatchApplicationMessage(Id, applicationMessage).ConfigureAwait(false);
             }
 
             switch (publishPacket.QualityOfServiceLevel)
@@ -251,13 +252,13 @@ namespace MQTTnet.Server
                 }
                 case MqttQualityOfServiceLevel.AtLeastOnce:
                 {
-                    var pubAckPacket = _packetFactories.PubAck.Create(publishPacket, interceptingPublishEventArgs);
+                    var pubAckPacket = _packetFactories.PubAck.Create(publishPacket, interceptingPublishEventArgs, dispatchResult);
                     Session.EnqueueControlPacket(new MqttPacketBusItem(pubAckPacket));
                     break;
                 }
                 case MqttQualityOfServiceLevel.ExactlyOnce:
                 {
-                    var pubRecPacket = _packetFactories.PubRec.Create(publishPacket, interceptingPublishEventArgs);
+                    var pubRecPacket = _packetFactories.PubRec.Create(publishPacket, interceptingPublishEventArgs, dispatchResult);
                     Session.EnqueueControlPacket(new MqttPacketBusItem(pubRecPacket));
                     break;
                 }

Source/MQTTnet/Server/Internal/MqttClientSessionsManager.cs

@@ -127,21 +127,20 @@ namespace MQTTnet.Server
 
             _logger.Verbose("Session for client '{0}' deleted.", clientId);
         }
-
+        
-        public async Task DispatchApplicationMessage(string senderId, MqttApplicationMessage applicationMessage)
+        public async Task<DispatchApplicationMessageResult> DispatchApplicationMessage(string senderId, MqttApplicationMessage applicationMessage)
         {
+            var matchingSubscribersCount = 0;
             try
             {
                 if (applicationMessage.Retain)
                 {
                     await _retainedMessagesManager.UpdateMessage(senderId, applicationMessage).ConfigureAwait(false);
                 }
-
+                
-                var deliveryCount = 0;
                 List<MqttSession> subscriberSessions;
                 lock (_sessionsManagementLock)
                 {
-                    // only subscriber clients are of interest here.
                     subscriberSessions = _subscriberSessions.ToList();
                 }
 
@@ -187,17 +186,22 @@ namespace MQTTnet.Server
                     }
 
                     session.EnqueueDataPacket(new MqttPacketBusItem(newPublishPacket));
-                    deliveryCount++;
+                    matchingSubscribersCount++;
 
                     _logger.Verbose("Client '{0}': Queued PUBLISH packet with topic '{1}'.", session.Id, applicationMessage.Topic);
                 }
 
-                await FireApplicationMessageNotConsumedEvent(applicationMessage, deliveryCount, senderId);
+                if (matchingSubscribersCount == 0)
+                {
+                    await FireApplicationMessageNotConsumedEvent(applicationMessage, senderId).ConfigureAwait(false);
+                }
             }
             catch (Exception exception)
             {
                 _logger.Error(exception, "Unhandled exception while processing next queued application message.");
             }
+
+            return new DispatchApplicationMessageResult(matchingSubscribersCount);
         }
 
         public void Dispose()
@@ -575,13 +579,8 @@ namespace MQTTnet.Server
             return new MqttSession(clientId, isPersistent, sessionItems, _options, _eventContainer, _retainedMessagesManager, this);
         }
 
-        async Task FireApplicationMessageNotConsumedEvent(MqttApplicationMessage applicationMessage, int deliveryCount, string senderId)
+        async Task FireApplicationMessageNotConsumedEvent(MqttApplicationMessage applicationMessage, string senderId)
         {
-            if (deliveryCount > 0)
-            {
-                return;
-            }
-
             if (!_eventContainer.ApplicationMessageNotConsumedEvent.HasHandlers)
             {
                 return;