Storage queue drain test

It's a unit test to ensure the storage queue is drained.

Tests/MQTTnet.Core.Tests/ManagedMqttClient_Tests.cs

@@ -108,5 +108,92 @@ namespace MQTTnet.Tests
                 Assert.AreEqual(0, (await server.GetClientStatusAsync()).Count);
             }
         }
+        
+        [TestMethod]
+        public async Task Storage_Queue_Drains()
+        {
+            using (var testEnvironment = new TestEnvironment())
+            {
+                testEnvironment.IgnoreClientLogErrors = true;
+                testEnvironment.IgnoreServerLogErrors = true;
+
+                var factory = new MqttFactory();
+
+                var server = await testEnvironment.StartServerAsync();
+
+                var managedClient = new ManagedMqttClient(testEnvironment.CreateClient(), new MqttNetLogger().CreateChildLogger());
+                var clientOptions = new MqttClientOptionsBuilder()
+                    .WithTcpServer("localhost", testEnvironment.ServerPort);
+                var storage = new ManagedMqttClientTestStorage();
+
+                TaskCompletionSource<bool> connected = new TaskCompletionSource<bool>();
+                managedClient.ConnectedHandler = new MqttClientConnectedHandlerDelegate(e =>
+                {
+                    managedClient.ConnectedHandler = null;
+                    connected.SetResult(true);
+                });
+
+                await managedClient.StartAsync(new ManagedMqttClientOptionsBuilder()
+                    .WithClientOptions(clientOptions)
+                    .WithStorage(storage)
+                    .WithAutoReconnectDelay(System.TimeSpan.FromSeconds(5))
+                    .Build());
+
+                await connected.Task;
+
+                await testEnvironment.Server.StopAsync();
+
+                await managedClient.PublishAsync(new MqttApplicationMessage { Topic = "1" });
+
+                //Message should have been added to the storage queue in PublishAsync,
+                //and we are awaiting PublishAsync, so the message should already be
+                //in storage at this point (i.e. no waiting).
+                Assert.AreEqual(1, storage.GetMessageCount());
+
+                connected = new TaskCompletionSource<bool>();
+                managedClient.ConnectedHandler = new MqttClientConnectedHandlerDelegate(e =>
+                {
+                    managedClient.ConnectedHandler = null;
+                    connected.SetResult(true);
+                });
+
+                await testEnvironment.Server.StartAsync(new MqttServerOptionsBuilder()
+                    .WithDefaultEndpointPort(testEnvironment.ServerPort).Build());
+
+                await connected.Task;
+
+                //Wait 500ms here so the client has time to publish the queued message
+                await Task.Delay(500);
+
+                Assert.AreEqual(0, storage.GetMessageCount());
+
+                await managedClient.StopAsync();
+            }
+        }
+    }
+    
+    public class ManagedMqttClientTestStorage : IManagedMqttClientStorage
+    {
+        private IList<ManagedMqttApplicationMessage> _messages = null;
+
+        public Task<IList<ManagedMqttApplicationMessage>> LoadQueuedMessagesAsync()
+        {
+            if (_messages == null)
+            {
+                _messages = new List<ManagedMqttApplicationMessage>();
+            }
+            return Task.FromResult(_messages);
+        }
+
+        public Task SaveQueuedMessagesAsync(IList<ManagedMqttApplicationMessage> messages)
+        {
+            _messages = messages;
+            return Task.FromResult(0);
+        }
+
+        public int GetMessageCount()
+        {
+            return _messages.Count;
+        }
     }
-}
+}