Client: Tolerate late acknowledgement packets (#2078, #2079) (#2247)

When a publish, subscribe or unsubscribe request was cancelled or timed out client-side, the corresponding awaitable was removed from the packet dispatcher. A subsequent PUBACK / PUBCOMP / SUBACK / UNSUBACK arriving from the broker then fell through to the default case and raised an MqttProtocolViolationException, which forced a disconnect. Under high publish load (issue #2079) and from multiple threads (issue #2078) this manifested as sporadic connection drops. Match these acknowledgement packets explicitly and log a warning when no awaiter is registered, instead of treating it as a protocol violation. The connection is now preserved. Also documents the thread-safety guarantees of IMqttClient: publish, subscribe, unsubscribe and ping operations are safe to invoke concurrently. Connect / disconnect must still be coordinated. Co-authored-by: meisa <meisa@MAFDEL> Co-authored-by: christian <6939810+chkr1011@users.noreply.github.com>
2026-10-30 18:56:30 +08:00 · 2026-06-16 20:28:40 +02:00
parent 745d1d25fe
commit c1b46b0ae7
4 changed files with 137 additions and 1 deletions
@@ -870,4 +870,109 @@ public sealed class MqttClient_Tests : BaseTestClass
        Assert.IsTrue(client1.TryPingAsync().GetAwaiter().GetResult());
        Assert.IsFalse(disconnectedFired);
    }
+
+    // Regression test for issue #2079.
+    // A late PUBACK (one whose matching publish request was already cancelled or timed
+    // out client-side) must NOT raise a protocol violation and must NOT disconnect the
+    // client. Before the fix, the client received the late PUBACK, failed to find a
+    // waiter and threw a MqttProtocolViolationException, tearing down the connection.
+    [TestMethod]
+    [DataRow(MqttQualityOfServiceLevel.AtLeastOnce)]
+    [DataRow(MqttQualityOfServiceLevel.ExactlyOnce)]
+    public async Task Cancelled_Publish_Does_Not_Disconnect_Client(MqttQualityOfServiceLevel qos)
+    {
+        using var testEnvironment = new TestEnvironment(TestContext);
+        var server = await testEnvironment.StartServer();
+
+        // Delay the broker's acknowledgement long enough for the publish caller to give
+        // up. When the caller's cancellation token fires the awaitable is removed; the
+        // (slightly later) PUBACK / PUBREC then arrives without a matching waiter.
+        server.InterceptingPublishAsync += async e =>
+        {
+            await Task.Delay(TimeSpan.FromSeconds(2), CancellationToken.None);
+        };
+
+        var client = await testEnvironment.ConnectClient();
+
+        var disconnected = false;
+        client.DisconnectedAsync += _ =>
+        {
+            disconnected = true;
+            return CompletedTask.Instance;
+        };
+
+        var message = new MqttApplicationMessageBuilder().WithTopic("late-ack").WithQualityOfServiceLevel(qos).Build();
+
+        using (var cts = new CancellationTokenSource(TimeSpan.FromMilliseconds(250)))
+        {
+            await Assert.ThrowsExactlyAsync<MqttCommunicationTimedOutException>(() => client.PublishAsync(message, cts.Token));
+        }
+
+        // Wait long enough for the delayed broker acknowledgement to come back.
+        await Task.Delay(TimeSpan.FromSeconds(3));
+
+        Assert.IsFalse(disconnected, "Client must not disconnect when a late acknowledgement arrives.");
+        Assert.IsTrue(client.IsConnected, "Client must still be connected after a late acknowledgement.");
+
+        // The connection must remain usable for further publishes.
+        var followUp = new MqttApplicationMessageBuilder().WithTopic("late-ack").WithQualityOfServiceLevel(MqttQualityOfServiceLevel.AtMostOnce).Build();
+        await client.PublishAsync(followUp);
+    }
+
+    // Regression test for issue #2078.
+    // The MqttClient must support concurrent publishes from multiple threads without
+    // dropping messages, mismatching acknowledgements or disconnecting the client.
+    [TestMethod]
+    [DataRow(MqttQualityOfServiceLevel.AtMostOnce)]
+    [DataRow(MqttQualityOfServiceLevel.AtLeastOnce)]
+    [DataRow(MqttQualityOfServiceLevel.ExactlyOnce)]
+    public async Task Concurrent_Publish_From_Multiple_Threads(MqttQualityOfServiceLevel qos)
+    {
+        const int Threads = 8;
+        const int MessagesPerThread = 250;
+
+        using var testEnvironment = new TestEnvironment(TestContext);
+        await testEnvironment.StartServer();
+
+        var publisher = await testEnvironment.ConnectClient();
+
+        var subscriber = await testEnvironment.ConnectClient();
+        var receivedCount = 0;
+        subscriber.ApplicationMessageReceivedAsync += _ =>
+        {
+            Interlocked.Increment(ref receivedCount);
+            return CompletedTask.Instance;
+        };
+        await subscriber.SubscribeAsync("concurrent/#", qos);
+
+        var disconnected = false;
+        publisher.DisconnectedAsync += _ =>
+        {
+            disconnected = true;
+            return CompletedTask.Instance;
+        };
+
+        var tasks = Enumerable.Range(0, Threads).Select(threadIndex => Task.Run(async () =>
+        {
+            for (var i = 0; i < MessagesPerThread; i++)
+            {
+                var message = new MqttApplicationMessageBuilder()
+                    .WithTopic($"concurrent/{threadIndex}/{i}")
+                    .WithPayload(BitConverter.GetBytes(i))
+                    .WithQualityOfServiceLevel(qos)
+                    .Build();
+
+                await publisher.PublishAsync(message);
+            }
+        })).ToArray();
+
+        await Task.WhenAll(tasks);
+
+        // Allow the broker to deliver any in-flight messages to the subscriber.
+        SpinWait.SpinUntil(() => receivedCount >= Threads * MessagesPerThread, TimeSpan.FromSeconds(30));
+
+        Assert.IsFalse(disconnected, "Publisher must not disconnect during concurrent publishes.");
+        Assert.IsTrue(publisher.IsConnected);
+        Assert.AreEqual(Threads * MessagesPerThread, receivedCount);
+    }
 }