Fix async issues

Frameworks/MQTTnet.NetStandard/Server/MqttClientSession.cs

@@ -145,16 +145,16 @@ namespace MQTTnet.Server
             PendingMessagesQueue.Enqueue(publishPacket);
         }
 
-        public Task SubscribeAsync(IList<TopicFilter> topicFilters)
+        public async Task SubscribeAsync(IList<TopicFilter> topicFilters)
         {
             if (topicFilters == null) throw new ArgumentNullException(nameof(topicFilters));
 
-            var response = SubscriptionsManager.SubscribeAsync(new MqttSubscribePacket
+            await SubscriptionsManager.SubscribeAsync(new MqttSubscribePacket
             {
                 TopicFilters = topicFilters
-            });
+            }).ConfigureAwait(false);
 
-            return response;
+            await EnqueueSubscribedRetainedMessagesAsync(topicFilters).ConfigureAwait(false);
         }
 
         public Task UnsubscribeAsync(IList<string> topicFilters)
@@ -270,7 +270,7 @@ namespace MQTTnet.Server
                 await StopAsync().ConfigureAwait(false);
             }
 
-            await EnqueueSubscribedRetainedMessagesAsync(subscribePacket).ConfigureAwait(false);
+            await EnqueueSubscribedRetainedMessagesAsync(subscribePacket.TopicFilters).ConfigureAwait(false);
         }
 
         private async Task HandleIncomingUnsubscribePacketAsync(IMqttChannelAdapter adapter, MqttUnsubscribePacket unsubscribePacket, CancellationToken cancellationToken)
@@ -279,12 +279,12 @@ namespace MQTTnet.Server
             await adapter.SendPacketsAsync(_options.DefaultCommunicationTimeout, cancellationToken, unsubscribeResult);
         }
 
-        private async Task EnqueueSubscribedRetainedMessagesAsync(MqttSubscribePacket subscribePacket)
+        private async Task EnqueueSubscribedRetainedMessagesAsync(ICollection<TopicFilter> topicFilters)
         {
-            var retainedMessages = await _retainedMessagesManager.GetSubscribedMessagesAsync(subscribePacket);
-            foreach (var publishPacket in retainedMessages)
+            var retainedMessages = await _retainedMessagesManager.GetSubscribedMessagesAsync(topicFilters);
+            foreach (var applicationMessage in retainedMessages)
             {
-                await EnqueueApplicationMessageAsync(publishPacket);
+                await EnqueueApplicationMessageAsync(applicationMessage).ConfigureAwait(false);
             }
         }