[Server] Add new interceptor before messages are added to a queue.

Frameworks/MQTTnet.NetStandard/Server/IMqttServerOptions.cs

@@ -16,6 +16,7 @@ namespace MQTTnet.Server
         Action<MqttConnectionValidatorContext> ConnectionValidator { get; }
         Action<MqttSubscriptionInterceptorContext> SubscriptionInterceptor { get; }
         Action<MqttApplicationMessageInterceptorContext> ApplicationMessageInterceptor { get; }
+        Action<MqttClientMessageQueueInterceptorContext> ClientMessageQueueInterceptor { get; set; }
 
         MqttServerDefaultEndpointOptions DefaultEndpointOptions { get; }
         MqttServerTlsEndpointOptions TlsEndpointOptions { get; }

Frameworks/MQTTnet.NetStandard/Server/MqttClientMessageQueueInterceptorContext.cs

@@ -0,0 +1,20 @@
+namespace MQTTnet.Server
+{
+    public class MqttClientMessageQueueInterceptorContext
+    {
+        public MqttClientMessageQueueInterceptorContext(string senderClientId, string receiverClientId, MqttApplicationMessage applicationMessage)
+        {
+            SenderClientId = senderClientId;
+            ReceiverClientId = receiverClientId;
+            ApplicationMessage = applicationMessage;
+        }
+
+        public string SenderClientId { get; }
+
+        public string ReceiverClientId { get; }
+
+        public MqttApplicationMessage ApplicationMessage { get; set; }
+
+        public bool AcceptEnqueue { get; set; } = true;
+    }
+}

Frameworks/MQTTnet.NetStandard/Server/MqttClientPendingMessagesQueue.cs

@@ -19,7 +19,6 @@ namespace MQTTnet.Server
         private readonly IMqttNetChildLogger _logger;
 
         private ConcurrentQueue<MqttBasePacket> _queue = new ConcurrentQueue<MqttBasePacket>();
-        private Task _workerTask;
 
         public MqttClientPendingMessagesQueue(IMqttServerOptions options, MqttClientSession clientSession, IMqttNetChildLogger logger)
         {
@@ -41,7 +40,7 @@ namespace MQTTnet.Server
                 return;
             }
 
-            _workerTask = Task.Run(() => SendQueuedPacketsAsync(adapter, cancellationToken), cancellationToken);
+            Task.Run(() => SendQueuedPacketsAsync(adapter, cancellationToken), cancellationToken);
         }
         
         public void Enqueue(MqttBasePacket packet)

Frameworks/MQTTnet.NetStandard/Server/MqttClientSession.cs

@@ -153,24 +153,39 @@ namespace MQTTnet.Server
             }
         }
 
-        public void EnqueueApplicationMessage(MqttApplicationMessage applicationMessage)
+        public void EnqueueApplicationMessage(MqttClientSession senderClientSession, MqttApplicationMessage applicationMessage)
         {
             if (applicationMessage == null) throw new ArgumentNullException(nameof(applicationMessage));
 
-            var result = _subscriptionsManager.CheckSubscriptions(applicationMessage);
-            if (!result.IsSubscribed)
+            var checkSubscriptionsResult = _subscriptionsManager.CheckSubscriptions(applicationMessage);
+            if (!checkSubscriptionsResult.IsSubscribed)
             {
                 return;
             }
 
             var publishPacket = applicationMessage.ToPublishPacket();
-            publishPacket.QualityOfServiceLevel = result.QualityOfServiceLevel;
+            publishPacket.QualityOfServiceLevel = checkSubscriptionsResult.QualityOfServiceLevel;
 
             if (publishPacket.QualityOfServiceLevel > 0)
             {
                 publishPacket.PacketIdentifier = _packetIdentifierProvider.GetNewPacketIdentifier();
             }
 
+            if (_options.ClientMessageQueueInterceptor != null)
+            {
+                var context = new MqttClientMessageQueueInterceptorContext(
+                    senderClientSession?.ClientId,
+                    ClientId,
+                    publishPacket.ToApplicationMessage());
+               
+                _options.ClientMessageQueueInterceptor?.Invoke(context);
+
+                if (!context.AcceptEnqueue || context.ApplicationMessage == null)
+                {
+                    return;
+                }
+            }
+            
             _pendingMessagesQueue.Enqueue(publishPacket);
         }
 
@@ -276,7 +291,7 @@ namespace MQTTnet.Server
             var retainedMessages = _retainedMessagesManager.GetSubscribedMessages(topicFilters);
             foreach (var applicationMessage in retainedMessages)
             {
-                EnqueueApplicationMessage(applicationMessage);
+                EnqueueApplicationMessage(null, applicationMessage);
             }
         }
 

Frameworks/MQTTnet.NetStandard/Server/MqttClientSessionsManager.cs

@@ -257,7 +257,7 @@ namespace MQTTnet.Server
 
                 foreach (var clientSession in _sessions.Values)
                 {
-                    clientSession.EnqueueApplicationMessage(applicationMessage);
+                    clientSession.EnqueueApplicationMessage(senderClientSession, applicationMessage);
                 }
             }
             catch (Exception exception)

Frameworks/MQTTnet.NetStandard/Server/MqttServerOptions.cs

@@ -21,6 +21,8 @@ namespace MQTTnet.Server
 
         public Action<MqttApplicationMessageInterceptorContext> ApplicationMessageInterceptor { get; set; }
 
+        public Action<MqttClientMessageQueueInterceptorContext> ClientMessageQueueInterceptor { get; set; }
+
         public Action<MqttSubscriptionInterceptorContext> SubscriptionInterceptor { get; set; }
 
         public IMqttServerStorage Storage { get; set; }