Add interceptor for unsubscriptions.

Source/MQTTnet.Server/Mqtt/MqttServerService.cs

@@ -33,6 +33,7 @@ namespace MQTTnet.Server.Mqtt
         private readonly MqttServerConnectionValidator _mqttConnectionValidator;
         private readonly IMqttServer _mqttServer;
         private readonly MqttSubscriptionInterceptor _mqttSubscriptionInterceptor;
+        private readonly MqttUnsubscriptionInterceptor _mqttUnsubscriptionInterceptor;
         private readonly PythonScriptHostService _pythonScriptHostService;
         private readonly MqttWebSocketServerAdapter _webSocketServerAdapter;
 
@@ -45,6 +46,7 @@ namespace MQTTnet.Server.Mqtt
             MqttClientUnsubscribedTopicHandler mqttClientUnsubscribedTopicHandler,
             MqttServerConnectionValidator mqttConnectionValidator,
             MqttSubscriptionInterceptor mqttSubscriptionInterceptor,
+            MqttUnsubscriptionInterceptor mqttUnsubscriptionInterceptor,
             MqttApplicationMessageInterceptor mqttApplicationMessageInterceptor,
             MqttServerStorage mqttServerStorage,
             PythonScriptHostService pythonScriptHostService,
@@ -57,6 +59,7 @@ namespace MQTTnet.Server.Mqtt
             _mqttClientUnsubscribedTopicHandler = mqttClientUnsubscribedTopicHandler ?? throw new ArgumentNullException(nameof(mqttClientUnsubscribedTopicHandler));
             _mqttConnectionValidator = mqttConnectionValidator ?? throw new ArgumentNullException(nameof(mqttConnectionValidator));
             _mqttSubscriptionInterceptor = mqttSubscriptionInterceptor ?? throw new ArgumentNullException(nameof(mqttSubscriptionInterceptor));
+            _mqttUnsubscriptionInterceptor = mqttUnsubscriptionInterceptor ?? throw new ArgumentNullException(nameof(mqttUnsubscriptionInterceptor));
             _mqttApplicationMessageInterceptor = mqttApplicationMessageInterceptor ?? throw new ArgumentNullException(nameof(mqttApplicationMessageInterceptor));
             _mqttServerStorage = mqttServerStorage ?? throw new ArgumentNullException(nameof(mqttServerStorage));
             _pythonScriptHostService = pythonScriptHostService ?? throw new ArgumentNullException(nameof(pythonScriptHostService));
@@ -178,6 +181,7 @@ namespace MQTTnet.Server.Mqtt
                 .WithConnectionValidator(_mqttConnectionValidator)
                 .WithApplicationMessageInterceptor(_mqttApplicationMessageInterceptor)
                 .WithSubscriptionInterceptor(_mqttSubscriptionInterceptor)
+                .WithUnsubscriptionInterceptor(_mqttUnsubscriptionInterceptor)
                 .WithStorage(_mqttServerStorage);
 
             // Configure unencrypted connections

Source/MQTTnet.Server/Mqtt/MqttUnsubscriptionInterceptor.cs

@@ -0,0 +1,48 @@
+using System;
+using System.Threading.Tasks;
+using IronPython.Runtime;
+using Microsoft.Extensions.Logging;
+using MQTTnet.Server.Scripting;
+
+namespace MQTTnet.Server.Mqtt
+{
+    public class MqttUnsubscriptionInterceptor : IMqttServerUnsubscriptionInterceptor
+    {
+        private readonly PythonScriptHostService _pythonScriptHostService;
+        private readonly ILogger _logger;
+
+        public MqttUnsubscriptionInterceptor(PythonScriptHostService pythonScriptHostService, ILogger<MqttUnsubscriptionInterceptor> logger)
+        {
+            _pythonScriptHostService = pythonScriptHostService ?? throw new ArgumentNullException(nameof(pythonScriptHostService));
+            _logger = logger ?? throw new ArgumentNullException(nameof(logger));
+        }
+
+        public Task InterceptUnsubscriptionAsync(MqttUnsubscriptionInterceptorContext context)
+        {
+            try
+            {
+                var sessionItems = (PythonDictionary)context.SessionItems[MqttServerConnectionValidator.WrappedSessionItemsKey];
+
+                var pythonContext = new PythonDictionary
+                {
+                    { "client_id", context.ClientId },
+                    { "session_items", sessionItems },
+                    { "topic", context.Topic },
+                    { "accept_unsubscription", context.AcceptUnsubscription },
+                    { "close_connection", context.CloseConnection }
+                };
+
+                _pythonScriptHostService.InvokeOptionalFunction("on_intercept_unsubscription", pythonContext);
+
+                context.AcceptUnsubscription = (bool)pythonContext["accept_unsubscription"];
+                context.CloseConnection = (bool)pythonContext["close_connection"];
+            }
+            catch (Exception exception)
+            {
+                _logger.LogError(exception, "Error while intercepting unsubscription.");
+            }
+
+            return Task.CompletedTask;
+        }
+    }
+}

Source/MQTTnet/Server/IMqttServerOptions.cs

@@ -15,6 +15,7 @@ namespace MQTTnet.Server
 
         IMqttServerConnectionValidator ConnectionValidator { get; }
         IMqttServerSubscriptionInterceptor SubscriptionInterceptor { get; }
+        IMqttServerUnsubscriptionInterceptor UnsubscriptionInterceptor { get; }
         IMqttServerApplicationMessageInterceptor ApplicationMessageInterceptor { get; }
         IMqttServerClientMessageQueueInterceptor ClientMessageQueueInterceptor { get; }
 

Source/MQTTnet/Server/IMqttServerUnsubscriptionInterceptor.cs

@@ -0,0 +1,9 @@
+using System.Threading.Tasks;
+
+namespace MQTTnet.Server
+{
+    public interface IMqttServerUnsubscriptionInterceptor
+    {
+        Task InterceptUnsubscriptionAsync(MqttUnsubscriptionInterceptorContext context);
+    }
+}

Source/MQTTnet/Server/MqttClientSubscriptionsManager.cs

@@ -107,9 +107,16 @@ namespace MQTTnet.Server
                 PacketIdentifier = unsubscribePacket.PacketIdentifier
             };
 
-            lock (_subscriptions)
+            foreach (var topicFilter in unsubscribePacket.TopicFilters)
             {
-                foreach (var topicFilter in unsubscribePacket.TopicFilters)
+                var interceptorContext = await InterceptUnsubscribeAsync(topicFilter).ConfigureAwait(false);
+                if (!interceptorContext.AcceptUnsubscription)
+                {
+                    unsubAckPacket.ReasonCodes.Add(MqttUnsubscribeReasonCode.ImplementationSpecificError);
+                    continue;
+                }
+
+                lock (_subscriptions)
                 {
                     if (_subscriptions.Remove(topicFilter))
                     {
@@ -130,19 +137,23 @@ namespace MQTTnet.Server
             return unsubAckPacket;
         }
 
-        public Task UnsubscribeAsync(IEnumerable<string> topicFilters)
+        public async Task UnsubscribeAsync(IEnumerable<string> topicFilters)
         {
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            lock (_subscriptions)
+            foreach (var topicFilter in topicFilters)
             {
-                foreach (var topicFilter in topicFilters)
+                var interceptorContext = await InterceptUnsubscribeAsync(topicFilter).ConfigureAwait(false);
+                if (!interceptorContext.AcceptUnsubscription)
+                {
+                    continue;
+                }
+
+                lock (_subscriptions)
                 {
                     _subscriptions.Remove(topicFilter);
-                }
+                }    
             }
-
-            return Task.FromResult(0);
         }
 
         public CheckSubscriptionsResult CheckSubscriptions(string topic, MqttQualityOfServiceLevel qosLevel)
@@ -206,6 +217,17 @@ namespace MQTTnet.Server
             return context;
         }
 
+        private async Task<MqttUnsubscriptionInterceptorContext> InterceptUnsubscribeAsync(string topicFilter)
+        {
+            var context = new MqttUnsubscriptionInterceptorContext(_clientSession.ClientId, topicFilter, _clientSession.Items);
+            if (_serverOptions.UnsubscriptionInterceptor != null)
+            {
+                await _serverOptions.UnsubscriptionInterceptor.InterceptUnsubscriptionAsync(context).ConfigureAwait(false);
+            }
+
+            return context;
+        }
+
         private static CheckSubscriptionsResult CreateSubscriptionResult(MqttQualityOfServiceLevel qosLevel, HashSet<MqttQualityOfServiceLevel> subscribedQoSLevels)
         {
             MqttQualityOfServiceLevel effectiveQoS;

Source/MQTTnet/Server/MqttServerOptions.cs

@@ -26,6 +26,8 @@ namespace MQTTnet.Server
 
         public IMqttServerSubscriptionInterceptor SubscriptionInterceptor { get; set; }
 
+        public IMqttServerUnsubscriptionInterceptor UnsubscriptionInterceptor { get; set; }
+
         public IMqttServerStorage Storage { get; set; }
     }
 }

Source/MQTTnet/Server/MqttServerOptionsBuilder.cs

@@ -155,6 +155,12 @@ namespace MQTTnet.Server
             return this;
         }
 
+        public MqttServerOptionsBuilder WithUnsubscriptionInterceptor(IMqttServerUnsubscriptionInterceptor value)
+        {
+            _options.UnsubscriptionInterceptor = value;
+            return this;
+        }
+
         public MqttServerOptionsBuilder WithSubscriptionInterceptor(Action<MqttSubscriptionInterceptorContext> value)
         {
             _options.SubscriptionInterceptor = new MqttServerSubscriptionInterceptorDelegate(value);

Source/MQTTnet/Server/MqttUnsubscriptionInterceptorContext.cs

@@ -0,0 +1,27 @@
+using System.Collections.Generic;
+
+namespace MQTTnet.Server
+{
+    public class MqttUnsubscriptionInterceptorContext
+    {
+        public MqttUnsubscriptionInterceptorContext(string clientId, string topic, IDictionary<object, object> sessionItems)
+        {
+            ClientId = clientId;
+            Topic = topic;
+            SessionItems = sessionItems;
+        }
+
+        public string ClientId { get; }
+
+        public string Topic { get; set; }
+
+        /// <summary>
+        /// Gets or sets a key/value collection that can be used to share data within the scope of this session.
+        /// </summary>
+        public IDictionary<object, object> SessionItems { get; }
+
+        public bool AcceptUnsubscription { get; set; } = true;
+
+        public bool CloseConnection { get; set; }
+    }
+}