Add new multi threaded application message handler for server.

Source/MQTTnet/Server/CheckSubscriptionsResult.cs

@@ -4,6 +4,8 @@ namespace MQTTnet.Server
 {
     public struct CheckSubscriptionsResult
     {
+        public static CheckSubscriptionsResult NotSubscribed = new CheckSubscriptionsResult();
+
         public bool IsSubscribed { get; set; }
 
         public MqttQualityOfServiceLevel QualityOfServiceLevel { get; set; }

Source/MQTTnet/Server/MqttApplicationMessageInterceptorContext.cs

@@ -1,16 +1,23 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
+using MQTTnet.Diagnostics;
 
 namespace MQTTnet.Server
 {
-    public class MqttApplicationMessageInterceptorContext
+    public sealed class MqttApplicationMessageInterceptorContext
     {
-        public MqttApplicationMessageInterceptorContext(string clientId, IDictionary<object, object> sessionItems, MqttApplicationMessage applicationMessage)
+        public MqttApplicationMessageInterceptorContext(string clientId, IDictionary<object, object> sessionItems, IMqttNetScopedLogger logger)
         {
             ClientId = clientId;
-            ApplicationMessage = applicationMessage;
             SessionItems = sessionItems;
+            Logger = logger ?? throw new ArgumentNullException(nameof(logger));
         }
 
+        /// <summary>
+        /// Gets the currently used logger.
+        /// </summary>
+        public IMqttNetScopedLogger Logger { get; }
+
         public string ClientId { get; }
 
         public MqttApplicationMessage ApplicationMessage { get; set; }

Source/MQTTnet/Server/MqttClientSessionsManager.cs

@@ -322,19 +322,21 @@ namespace MQTTnet.Server
                 }
 
                 var deliveryCount = 0;
-
+                List<MqttClientSession> sessions;
                 lock (_sessions)
                 {
-                    foreach (var clientSession in _sessions.Values)
+                    sessions = _sessions.Values.ToList();
+                }
+
+                foreach (var clientSession in sessions)
+                {
+                    var isSubscribed = clientSession.EnqueueApplicationMessage(applicationMessage, senderClientId, false);
+                    if (isSubscribed)
                     {
-                        var isSubscribed = clientSession.EnqueueApplicationMessage(applicationMessage, senderClientId, false);
-                        if (isSubscribed)
-                        {
-                            deliveryCount++;
-                        }
+                        deliveryCount++;
                     }
                 }
-                
+
                 if (deliveryCount == 0)
                 {
                     var undeliveredMessageInterceptor = _options.UndeliveredMessageInterceptor;
@@ -445,7 +447,13 @@ namespace MQTTnet.Server
                 sessionItems = clientConnection.Session.Items;
             }
 
-            var interceptorContext = new MqttApplicationMessageInterceptorContext(senderClientId, sessionItems, applicationMessage);
+            var interceptorContext = new MqttApplicationMessageInterceptorContext(senderClientId, sessionItems, _logger)
+            {
+                AcceptPublish = true,
+                ApplicationMessage = applicationMessage,
+                CloseConnection =  false
+            };
+
             await interceptor.InterceptApplicationMessagePublishAsync(interceptorContext).ConfigureAwait(false);
             return interceptorContext;
         }

Source/MQTTnet/Server/MqttClientSubscriptionsManager.cs

@@ -193,10 +193,7 @@ namespace MQTTnet.Server
 
             if (qosLevels.Count == 0)
             {
-                return new CheckSubscriptionsResult
-                {
-                    IsSubscribed = false
-                };
+                return CheckSubscriptionsResult.NotSubscribed;
             }
 
             return CreateSubscriptionResult(qosLevel, qosLevels);

Source/MQTTnet/Server/MqttServerApplicationMessageInterceptorDelegate.cs

@@ -3,9 +3,9 @@ using System.Threading.Tasks;
 
 namespace MQTTnet.Server
 {
-    public class MqttServerApplicationMessageInterceptorDelegate : IMqttServerApplicationMessageInterceptor
+    public sealed class MqttServerApplicationMessageInterceptorDelegate : IMqttServerApplicationMessageInterceptor
     {
-        private readonly Func<MqttApplicationMessageInterceptorContext, Task> _callback;
+        readonly Func<MqttApplicationMessageInterceptorContext, Task> _callback;
 
         public MqttServerApplicationMessageInterceptorDelegate(Action<MqttApplicationMessageInterceptorContext> callback)
         {

Source/MQTTnet/Server/MqttServerMultiThreadedApplicationMessageInterceptorDelegate.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Threading.Tasks;
+using MQTTnet.Diagnostics;
+using MQTTnet.Implementations;
+using MQTTnet.Internal;
+
+namespace MQTTnet.Server
+{
+    public sealed class MqttServerMultiThreadedApplicationMessageInterceptorDelegate : IMqttServerApplicationMessageInterceptor
+    {
+        readonly Func<MqttApplicationMessageInterceptorContext, Task> _callback;
+
+        public MqttServerMultiThreadedApplicationMessageInterceptorDelegate(Action<MqttApplicationMessageInterceptorContext> callback)
+        {
+            if (callback == null) throw new ArgumentNullException(nameof(callback));
+
+            _callback = context =>
+            {
+                callback(context);
+                return Task.FromResult(0);
+            };
+        }
+
+        public MqttServerMultiThreadedApplicationMessageInterceptorDelegate(Func<MqttApplicationMessageInterceptorContext, Task> callback)
+        {
+            _callback = callback ?? throw new ArgumentNullException(nameof(callback));
+        }
+
+        public Task InterceptApplicationMessagePublishAsync(MqttApplicationMessageInterceptorContext context)
+        {
+            Task.Run(async () =>
+            {
+                try
+                {
+                    await _callback.Invoke(context).ConfigureAwait(false);
+                }
+                catch (Exception exception)
+                {
+                    context.Logger.Error(exception, "Error while intercepting application message.");
+                }
+            }).RunInBackground();
+
+            return PlatformAbstractionLayer.CompletedTask;
+        }
+    }
+}

Source/MQTTnet/Server/MqttServerOptionsBuilder.cs

@@ -3,11 +3,13 @@ using System.Net;
 using System.Net.Security;
 using System.Security.Authentication;
 using MQTTnet.Certificates;
+using System.Threading.Tasks;
 
 #if !WINDOWS_UWP
 using System.Security.Cryptography.X509Certificates;
 #endif
 
+// ReSharper disable UnusedMember.Global
 namespace MQTTnet.Server
 {
     public class MqttServerOptionsBuilder
@@ -187,6 +189,24 @@ namespace MQTTnet.Server
             return this;
         }
 
+        public MqttServerOptionsBuilder WithApplicationMessageInterceptor(Func<MqttApplicationMessageInterceptorContext, Task> value)
+        {
+            _options.ApplicationMessageInterceptor = new MqttServerApplicationMessageInterceptorDelegate(value);
+            return this;
+        }
+
+        public MqttServerOptionsBuilder WithMultiThreadedApplicationMessageInterceptor(Action<MqttApplicationMessageInterceptorContext> value)
+        {
+            _options.ApplicationMessageInterceptor = new MqttServerMultiThreadedApplicationMessageInterceptorDelegate(value);
+            return this;
+        }
+
+        public MqttServerOptionsBuilder WithMultiThreadedApplicationMessageInterceptor(Func<MqttApplicationMessageInterceptorContext, Task> value)
+        {
+            _options.ApplicationMessageInterceptor = new MqttServerMultiThreadedApplicationMessageInterceptorDelegate(value);
+            return this;
+        }
+
         public MqttServerOptionsBuilder WithClientMessageQueueInterceptor(IMqttServerClientMessageQueueInterceptor value)
         {
             _options.ClientMessageQueueInterceptor = value;
@@ -217,6 +237,12 @@ namespace MQTTnet.Server
             return this;
         }
 
+        public MqttServerOptionsBuilder WithUndeliveredMessageInterceptor(Action<MqttApplicationMessageInterceptorContext> value)
+        {
+            _options.UndeliveredMessageInterceptor = new MqttServerApplicationMessageInterceptorDelegate(value);
+            return this;
+        }
+
         public MqttServerOptionsBuilder WithDefaultEndpointReuseAddress()
         {
             _options.DefaultEndpointOptions.ReuseAddress = true;
@@ -248,11 +274,5 @@ namespace MQTTnet.Server
         {
             return _options;
         }
-
-        public MqttServerOptionsBuilder WithUndeliveredMessageInterceptor(Action<MqttApplicationMessageInterceptorContext> value)
-        {
-            _options.UndeliveredMessageInterceptor = new MqttServerApplicationMessageInterceptorDelegate(value);
-            return this;
-        }
     }
 }