Improve session message injection

Source/MQTTnet.Server/Exceptions/MqttPendingMessagesOverflowException.cs

@@ -0,0 +1,19 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace MQTTnet.Server.Exceptions;
+
+public class MqttPendingMessagesOverflowException : Exception
+{
+    public MqttPendingMessagesOverflowException(string sessionId, MqttPendingMessagesOverflowStrategy overflowStrategy) : base(
+        $"Send buffer max pending messages overflow occurred for session '{sessionId}'. Strategy: {overflowStrategy}.")
+    {
+        SessionId = sessionId;
+        OverflowStrategy = overflowStrategy;
+    }
+
+    public MqttPendingMessagesOverflowStrategy OverflowStrategy { get; }
+
+    public string SessionId { get; }
+}

Source/MQTTnet.Server/Internal/MqttSession.cs

@@ -6,6 +6,7 @@
 using MQTTnet.Internal;
 using MQTTnet.Packets;
 using MQTTnet.Protocol;
+using MQTTnet.Server.Exceptions;
 
 namespace MQTTnet.Server.Internal;
 
@@ -111,10 +112,11 @@ public void EnqueueControlPacket(MqttPacketBusItem packetBusItem)
 
     public EnqueueDataPacketResult EnqueueDataPacket(MqttPacketBusItem packetBusItem)
     {
-        if (_packetBus.ItemsCount(MqttPacketBusPartition.Data) >= _serverOptions.MaxPendingMessagesPerClient)
+        if (PendingDataPacketsCount >= _serverOptions.MaxPendingMessagesPerClient)
         {
             if (_serverOptions.PendingMessagesOverflowStrategy == MqttPendingMessagesOverflowStrategy.DropNewMessage)
             {
+                packetBusItem.Fail(new MqttPendingMessagesOverflowException(Id, _serverOptions.PendingMessagesOverflowStrategy));
                 return EnqueueDataPacketResult.Dropped;
             }
 
@@ -123,10 +125,15 @@ public EnqueueDataPacketResult EnqueueDataPacket(MqttPacketBusItem packetBusItem
                 // Only drop from the data partition. Dropping from control partition might break the connection
                 // because the client does not receive PINGREQ packets etc. any longer.
                 var firstItem = _packetBus.DropFirstItem(MqttPacketBusPartition.Data);
-                if (firstItem != null && _eventContainer.QueuedApplicationMessageOverwrittenEvent.HasHandlers)
+                if (firstItem != null)
                 {
-                    var eventArgs = new QueueMessageOverwrittenEventArgs(Id, firstItem.Packet);
-                    _eventContainer.QueuedApplicationMessageOverwrittenEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
+                    firstItem.Fail(new MqttPendingMessagesOverflowException(Id, _serverOptions.PendingMessagesOverflowStrategy));
+
+                    if (_eventContainer.QueuedApplicationMessageOverwrittenEvent.HasHandlers)
+                    {
+                        var eventArgs = new QueueMessageOverwrittenEventArgs(Id, firstItem.Packet);
+                        _eventContainer.QueuedApplicationMessageOverwrittenEvent.InvokeAsync(eventArgs).ConfigureAwait(false);
+                    }
                 }
             }
         }

Source/MQTTnet.Server/Options/MqttServerOptionsBuilder.cs

@@ -115,6 +115,12 @@ public MqttServerOptionsBuilder WithMaxPendingMessagesPerClient(int value)
             return this;
         }
 
+        public MqttServerOptionsBuilder WithPendingMessagesOverflowStrategy(MqttPendingMessagesOverflowStrategy value)
+        {
+            _options.PendingMessagesOverflowStrategy = value;
+            return this;
+        }
+
         public MqttServerOptionsBuilder WithoutDefaultEndpoint()
         {
             _options.DefaultEndpointOptions.IsEnabled = false;

Source/MQTTnet.Server/Status/MqttSessionStatus.cs

@@ -40,22 +40,65 @@ public Task DeleteAsync()
         return _session.DeleteAsync();
     }
 
-    public Task DeliverApplicationMessageAsync(MqttApplicationMessage applicationMessage)
+    /// <summary>
+    /// Delivers an application message immediately to the session.
+    /// </summary>
+    /// <param name="applicationMessage">The application message to deliver.</param>
+    /// <returns>
+    /// A task that represents the asynchronous operation.
+    /// The result contains the <see cref="InjectMqttApplicationMessageResult"/> that includes the packet identifier of the enqueued message.
+    /// </returns>
+    public async Task<InjectMqttApplicationMessageResult> DeliverApplicationMessageAsync(MqttApplicationMessage applicationMessage)
     {
         ArgumentNullException.ThrowIfNull(applicationMessage);
 
-        var packetBusItem = new MqttPacketBusItem(MqttPublishPacketFactory.Create(applicationMessage));
+        var publishPacket = MqttPublishPacketFactory.Create(applicationMessage);
+        var packetBusItem = new MqttPacketBusItem(publishPacket);
         _session.EnqueueDataPacket(packetBusItem);
 
-        return packetBusItem.WaitAsync();
+        await packetBusItem.WaitAsync().ConfigureAwait(false);
+
+        var injectResult = new InjectMqttApplicationMessageResult()
+        {
+            PacketIdentifier = publishPacket.PacketIdentifier
+        };
+
+        return injectResult;
     }
 
-    public Task EnqueueApplicationMessageAsync(MqttApplicationMessage applicationMessage)
+    /// <summary>
+    /// Attempts to enqueue an application message to the session's send buffer.
+    /// </summary>
+    /// <param name="applicationMessage">The application message to enqueue.</param>
+    /// <param name="injectResult"><see cref="InjectMqttApplicationMessageResult"/> that includes the packet identifier of the enqueued message.</param>
+    /// <returns><c>true</c> if the message was successfully enqueued; otherwise, <c>false</c>.</returns>
+    /// <remarks>
+    /// When <see cref="MqttServerOptions.PendingMessagesOverflowStrategy"/> is set to <see cref="MqttPendingMessagesOverflowStrategy.DropOldestQueuedMessage"/>,
+    /// this method always returns <c>true</c>.
+    /// However, an existing message in the queue may be <b>dropped later</b> to make room for the newly enqueued message.
+    /// Such dropped messages can be tracked by subscribing to <see cref="MqttServer.QueuedApplicationMessageOverwrittenAsync"/> event.
+    /// </remarks>
+    public bool TryEnqueueApplicationMessage(MqttApplicationMessage applicationMessage, out InjectMqttApplicationMessageResult injectResult)
     {
         ArgumentNullException.ThrowIfNull(applicationMessage);
 
-        _session.EnqueueDataPacket(new MqttPacketBusItem(MqttPublishPacketFactory.Create(applicationMessage)));
+        var publishPacket = MqttPublishPacketFactory.Create(applicationMessage);
+        var enqueueDataPacketResult = _session.EnqueueDataPacket(new MqttPacketBusItem(publishPacket));
+
+        if (enqueueDataPacketResult != EnqueueDataPacketResult.Enqueued)
+        {
+            injectResult = null;
+            return false;
+        }
 
+        injectResult = new InjectMqttApplicationMessageResult() { PacketIdentifier = publishPacket.PacketIdentifier };
+        return true;
+    }
+
+    [Obsolete("This method is obsolete. Use TryEnqueueApplicationMessage instead.")]
+    public Task EnqueueApplicationMessageAsync(MqttApplicationMessage applicationMessage)
+    {
+        TryEnqueueApplicationMessage(applicationMessage, out _);
         return CompletedTask.Instance;
     }
 }

Source/MQTTnet.Tests/BaseTestClass.cs

@@ -13,10 +13,11 @@ namespace MQTTnet.Tests
     public abstract class BaseTestClass
     {
         public TestContext TestContext { get; set; }
-
-        protected TestEnvironment CreateTestEnvironment(MqttProtocolVersion protocolVersion = MqttProtocolVersion.V311)
+
+        protected TestEnvironment CreateTestEnvironment(
+            MqttProtocolVersion protocolVersion = MqttProtocolVersion.V311, bool trackUnobservedTaskException = true)
         {
-            return new TestEnvironment(TestContext, protocolVersion);
+            return new TestEnvironment(TestContext, protocolVersion, trackUnobservedTaskException);
         }
 
         protected Task LongTestDelay()

Source/MQTTnet.Tests/Mockups/TestEnvironment.cs

@@ -32,12 +32,16 @@ public TestEnvironment() : this(null)
         {
         }
 
-        public TestEnvironment(TestContext testContext, MqttProtocolVersion protocolVersion = MqttProtocolVersion.V311)
+        public TestEnvironment(
+            TestContext testContext, MqttProtocolVersion protocolVersion = MqttProtocolVersion.V311, bool trackUnobservedTaskException = true)
         {
             _protocolVersion = protocolVersion;
             TestContext = testContext;
 
-            TaskScheduler.UnobservedTaskException += TrackUnobservedTaskException;
+            if (trackUnobservedTaskException)
+            {
+                TaskScheduler.UnobservedTaskException += TrackUnobservedTaskException;
+            }
 
             ServerLogger.LogMessagePublished += (s, e) =>
             {