Added IMqttRetainedMessagesManager

* Added interface IMqttRetainedMessagesManager to allow for different
retained messages manager implementations. The interface copies almost
exactly the current MqttRetainedMessagesManager implementation.

* Added IMqttRetainedMessagesManager.Start() to configure the provided
IMqttRetainedMessagesManager outside the constructor. This method returns
Task because some implementations that use external storage may be slow on
initialization (e.g., connect to a database).

* Modified MqttRetainedMessagesManager to implement new interface.

Source/MQTTnet/Server/IMqttRetainedMessagesManager.cs

@@ -0,0 +1,21 @@
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using MQTTnet.Diagnostics;
+
+namespace MQTTnet.Server
+{
+    public interface IMqttRetainedMessagesManager
+    {
+        Task Start(IMqttServerOptions options, IMqttNetChildLogger logger);
+
+        Task LoadMessagesAsync();
+
+        Task ClearMessagesAsync();
+
+        Task HandleMessageAsync(string clientId, MqttApplicationMessage applicationMessage);
+
+        Task<IList<MqttApplicationMessage>> GetMessagesAsync();
+
+        Task<List<MqttApplicationMessage>> GetSubscribedMessagesAsync(ICollection<TopicFilter> topicFilters);
+    }
+}

Source/MQTTnet/Server/IMqttServerOptions.cs

@@ -22,8 +22,8 @@ namespace MQTTnet.Server
         MqttServerTcpEndpointOptions DefaultEndpointOptions { get; }
         MqttServerTlsTcpEndpointOptions TlsEndpointOptions { get; }
 
-        IMqttServerStorage Storage { get; }       
-        
+        IMqttServerStorage Storage { get; }
 
+        IMqttRetainedMessagesManager RetainedMessagesManager { get; }
     }
 }

Source/MQTTnet/Server/MqttClientConnection.cs

@@ -21,7 +21,7 @@ namespace MQTTnet.Server
         private readonly MqttPacketDispatcher _packetDispatcher = new MqttPacketDispatcher();
         private readonly CancellationTokenSource _cancellationToken = new CancellationTokenSource();
 
-        private readonly MqttRetainedMessagesManager _retainedMessagesManager;
+        private readonly IMqttRetainedMessagesManager _retainedMessagesManager;
         private readonly MqttClientKeepAliveMonitor _keepAliveMonitor;
         private readonly MqttClientSessionsManager _sessionsManager;
 
@@ -36,7 +36,7 @@ namespace MQTTnet.Server
         private Task<MqttClientDisconnectType> _packageReceiverTask;
         private DateTime _lastPacketReceivedTimestamp;
         private DateTime _lastNonKeepAlivePacketReceivedTimestamp;
-        
+
         private long _receivedPacketsCount;
         private long _sentPacketsCount = 1; // Start with 1 because the CONNECT packet is not counted anywhere.
         private long _receivedApplicationMessagesCount;
@@ -48,14 +48,14 @@ namespace MQTTnet.Server
             MqttClientSession session,
             IMqttServerOptions serverOptions,
             MqttClientSessionsManager sessionsManager,
-            MqttRetainedMessagesManager retainedMessagesManager,
+            IMqttRetainedMessagesManager retainedMessagesManager,
             IMqttNetChildLogger logger)
         {
             Session = session ?? throw new ArgumentNullException(nameof(session));
             _serverOptions = serverOptions ?? throw new ArgumentNullException(nameof(serverOptions));
             _sessionsManager = sessionsManager ?? throw new ArgumentNullException(nameof(sessionsManager));
             _retainedMessagesManager = retainedMessagesManager ?? throw new ArgumentNullException(nameof(retainedMessagesManager));
-            
+
             _channelAdapter = channelAdapter ?? throw new ArgumentNullException(nameof(channelAdapter));
             _dataConverter = _channelAdapter.PacketFormatterAdapter.DataConverter;
             _endpoint = _channelAdapter.Endpoint;
@@ -76,7 +76,7 @@ namespace MQTTnet.Server
         public string ClientId => ConnectPacket.ClientId;
 
         public MqttClientSession Session { get; }
-        
+
         public async Task StopAsync()
         {
             StopInternal();
@@ -112,7 +112,7 @@ namespace MQTTnet.Server
             status.BytesSent = _channelAdapter.BytesSent;
             status.BytesReceived = _channelAdapter.BytesReceived;
         }
-        
+
         public void Dispose()
         {
             _cancellationToken.Dispose();
@@ -130,7 +130,7 @@ namespace MQTTnet.Server
             try
             {
                 _logger.Info("Client '{0}': Session started.", ClientId);
-                
+
                 _channelAdapter.ReadingPacketStartedCallback = OnAdapterReadingPacketStarted;
                 _channelAdapter.ReadingPacketCompletedCallback = OnAdapterReadingPacketCompleted;
 
@@ -244,7 +244,7 @@ namespace MQTTnet.Server
                 _channelAdapter.ReadingPacketCompletedCallback = null;
 
                 _logger.Info("Client '{0}': Session stopped.", ClientId);
-                
+
                 _packageReceiverTask = null;
             }
 

Source/MQTTnet/Server/MqttClientSession.cs

@@ -52,7 +52,7 @@ namespace MQTTnet.Server
             ApplicationMessagesQueue.Enqueue(applicationMessage, senderClientId, checkSubscriptionsResult.QualityOfServiceLevel, isRetainedApplicationMessage);
         }
 
-        public async Task SubscribeAsync(ICollection<TopicFilter> topicFilters, MqttRetainedMessagesManager retainedMessagesManager)
+        public async Task SubscribeAsync(ICollection<TopicFilter> topicFilters, IMqttRetainedMessagesManager retainedMessagesManager)
         {
             await SubscriptionsManager.SubscribeAsync(topicFilters).ConfigureAwait(false);
 

Source/MQTTnet/Server/MqttClientSessionsManager.cs

@@ -25,13 +25,13 @@ namespace MQTTnet.Server
         private readonly CancellationToken _cancellationToken;
         private readonly MqttServerEventDispatcher _eventDispatcher;
 
-        private readonly MqttRetainedMessagesManager _retainedMessagesManager;
+        private readonly IMqttRetainedMessagesManager _retainedMessagesManager;
         private readonly IMqttServerOptions _options;
         private readonly IMqttNetChildLogger _logger;
 
         public MqttClientSessionsManager(
             IMqttServerOptions options,
-            MqttRetainedMessagesManager retainedMessagesManager,
+            IMqttRetainedMessagesManager retainedMessagesManager,
             CancellationToken cancellationToken,
             MqttServerEventDispatcher eventDispatcher,
             IMqttNetChildLogger logger)
@@ -72,7 +72,7 @@ namespace MQTTnet.Server
             {
                 var clientStatus = new MqttClientStatus(connection);
                 connection.FillStatus(clientStatus);
-                
+
                 var sessionStatus = new MqttSessionStatus(connection.Session, this);
                 connection.Session.FillStatus(sessionStatus);
                 clientStatus.Session = sessionStatus;
@@ -91,7 +91,7 @@ namespace MQTTnet.Server
             {
                 var sessionStatus = new MqttSessionStatus(session, this);
                 session.FillStatus(sessionStatus);
-    
+
                 result.Add(sessionStatus);
             }
 
@@ -259,7 +259,7 @@ namespace MQTTnet.Server
                 var connection = await CreateConnectionAsync(connectPacket, connectionValidatorContext, channelAdapter).ConfigureAwait(false);
 
                 await _eventDispatcher.HandleClientConnectedAsync(clientId).ConfigureAwait(false);
-                
+
                 disconnectType = await connection.RunAsync(connectionValidatorContext).ConfigureAwait(false);
             }
             catch (OperationCanceledException)
@@ -272,7 +272,7 @@ namespace MQTTnet.Server
             finally
             {
                 if (clientWasConnected)
-                { 
+                {
                     if (clientId != null)
                     {
                         _connections.TryRemove(clientId, out _);
@@ -333,13 +333,13 @@ namespace MQTTnet.Server
                 {
                     await existingConnection.StopAsync().ConfigureAwait(false);
                 }
-                
+
                 if (isSessionPresent)
                 {
                     if (connectPacket.CleanSession)
                     {
                         session = null;
-                        
+
                         _logger.Verbose("Deleting existing session of client '{0}'.", connectPacket.ClientId);
                     }
                     else

Source/MQTTnet/Server/MqttRetainedMessagesManager.cs

@@ -7,20 +7,21 @@ using MQTTnet.Internal;
 
 namespace MQTTnet.Server
 {
-    public class MqttRetainedMessagesManager
+    public class MqttRetainedMessagesManager : IMqttRetainedMessagesManager
     {
         private readonly byte[] _emptyArray = new byte[0];
         private readonly AsyncLock _messagesLock = new AsyncLock();
         private readonly Dictionary<string, MqttApplicationMessage> _messages = new Dictionary<string, MqttApplicationMessage>();
 
-        private readonly IMqttNetChildLogger _logger;
-        private readonly IMqttServerOptions _options;
+        private IMqttNetChildLogger _logger;
+        private IMqttServerOptions _options;
 
-        public MqttRetainedMessagesManager(IMqttServerOptions options, IMqttNetChildLogger logger)
+        public Task Start(IMqttServerOptions options, IMqttNetChildLogger logger)
         {
             if (logger == null) throw new ArgumentNullException(nameof(logger));
             _logger = logger.CreateChildLogger(nameof(MqttRetainedMessagesManager));
             _options = options ?? throw new ArgumentNullException(nameof(options));
+            return Task.CompletedTask;
         }
 
         public async Task LoadMessagesAsync()
@@ -128,7 +129,7 @@ namespace MQTTnet.Server
                     break;
                 }
             }
-            
+
             return matchingRetainedMessages;
         }
 

Source/MQTTnet/Server/MqttServer.cs

@@ -19,7 +19,7 @@ namespace MQTTnet.Server
         private readonly IMqttNetChildLogger _logger;
 
         private MqttClientSessionsManager _clientSessionsManager;
-        private MqttRetainedMessagesManager _retainedMessagesManager;
+        private IMqttRetainedMessagesManager _retainedMessagesManager;
         private CancellationTokenSource _cancellationTokenSource;
 
         public MqttServer(IEnumerable<IMqttServerAdapter> adapters, IMqttNetChildLogger logger)
@@ -48,7 +48,7 @@ namespace MQTTnet.Server
             get => _eventDispatcher.ClientDisconnectedHandler;
             set => _eventDispatcher.ClientDisconnectedHandler = value;
         }
-        
+
         public IMqttServerClientSubscribedTopicHandler ClientSubscribedTopicHandler
         {
             get => _eventDispatcher.ClientSubscribedTopicHandler;
@@ -60,7 +60,7 @@ namespace MQTTnet.Server
             get => _eventDispatcher.ClientUnsubscribedTopicHandler;
             set => _eventDispatcher.ClientUnsubscribedTopicHandler = value;
         }
-        
+
         public IMqttApplicationMessageReceivedHandler ApplicationMessageReceivedHandler
         {
             get => _eventDispatcher.ApplicationMessageReceivedHandler;
@@ -121,7 +121,8 @@ namespace MQTTnet.Server
 
             _cancellationTokenSource = new CancellationTokenSource();
 
-            _retainedMessagesManager = new MqttRetainedMessagesManager(Options, _logger);
+            _retainedMessagesManager = Options.RetainedMessagesManager ?? new MqttRetainedMessagesManager();
+            await _retainedMessagesManager.Start(Options, _logger);
             await _retainedMessagesManager.LoadMessagesAsync().ConfigureAwait(false);
 
             _clientSessionsManager = new MqttClientSessionsManager(Options, _retainedMessagesManager, _cancellationTokenSource.Token, _eventDispatcher, _logger);
@@ -150,9 +151,9 @@ namespace MQTTnet.Server
                 {
                     return;
                 }
-                
+
                 await _clientSessionsManager.StopAsync().ConfigureAwait(false);
-                
+
                 _cancellationTokenSource.Cancel(false);
 
                 foreach (var adapter in _adapters)

Source/MQTTnet/Server/MqttServerOptions.cs

@@ -21,7 +21,7 @@ namespace MQTTnet.Server
         public IMqttServerConnectionValidator ConnectionValidator { get; set; }
 
         public IMqttServerApplicationMessageInterceptor ApplicationMessageInterceptor { get; set; }
-        
+
         public IMqttServerClientMessageQueueInterceptor ClientMessageQueueInterceptor { get; set; }
 
         public IMqttServerSubscriptionInterceptor SubscriptionInterceptor { get; set; }
@@ -29,5 +29,7 @@ namespace MQTTnet.Server
         public IMqttServerUnsubscriptionInterceptor UnsubscriptionInterceptor { get; set; }
 
         public IMqttServerStorage Storage { get; set; }
+
+        public IMqttRetainedMessagesManager RetainedMessagesManager { get; set; }
     }
 }

Source/MQTTnet/Server/MqttServerOptionsBuilder.cs

@@ -57,7 +57,7 @@ namespace MQTTnet.Server
             _options.DefaultEndpointOptions.IsEnabled = false;
             return this;
         }
-        
+
         public MqttServerOptionsBuilder WithEncryptedEndpoint()
         {
             _options.TlsEndpointOptions.IsEnabled = true;
@@ -118,13 +118,19 @@ namespace MQTTnet.Server
             return this;
         }
 #endif
-        
+
         public MqttServerOptionsBuilder WithStorage(IMqttServerStorage value)
         {
             _options.Storage = value;
             return this;
         }
 
+        public MqttServerOptionsBuilder WithRetainedMessagesManager(IMqttRetainedMessagesManager value)
+        {
+            _options.RetainedMessagesManager = value;
+            return this;
+        }
+
         public MqttServerOptionsBuilder WithConnectionValidator(IMqttServerConnectionValidator value)
         {
             _options.ConnectionValidator = value;