test(batch16): resolve route slow-consumer recover status with evidence

dotnet/src/ZB.MOM.NatsNet.Server/ClientConnection.cs

@@ -1491,6 +1491,7 @@ public sealed partial class ClientConnection
             return false;
 
         Flags = Flags.Set(ClientFlags.FlushOutbound);
+        bool gotWriteTimeout = false;
         try
         {
             if (_nc is null || Server is null || OutPb == 0)
@@ -1515,6 +1516,7 @@ public sealed partial class ClientConnection
                                            (se.SocketErrorCode == SocketError.TimedOut ||
                                             se.SocketErrorCode == SocketError.WouldBlock))
             {
+                gotWriteTimeout = true;
                 if (HandleWriteTimeout(written, attempted, OutWnb.Count))
                     return true;
             }
@@ -1532,6 +1534,9 @@ public sealed partial class ClientConnection
                     OutWnb.Clear();
             }
 
+            if (!gotWriteTimeout && Flags.IsSet(ClientFlags.IsSlowConsumer))
+                Flags = Flags.Clear(ClientFlags.IsSlowConsumer);
+
             return true;
         }
         finally

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/ImplBacklog/RouteHandlerTests.Impltests.cs

@@ -1,5 +1,7 @@
+using System.IO;
 using Shouldly;
 using ZB.MOM.NatsNet.Server;
+using ZB.MOM.NatsNet.Server.Internal;
 
 namespace ZB.MOM.NatsNet.Server.Tests.ImplBacklog;
 
@@ -74,4 +76,27 @@ public sealed partial class RouteHandlerTests
         errors.ShouldBeEmpty();
         options.Cluster.Compression.Mode.ShouldBe(CompressionModes.Off);
     }
+
+    [Fact] // T:2859
+    public void RouteSlowConsumerRecover_ShouldSucceed()
+    {
+        var (server, err) = NatsServer.NewServer(new ServerOptions());
+        err.ShouldBeNull();
+
+        using var outStream = new MemoryStream();
+        var route = new ClientConnection(ClientKind.Router, server, outStream)
+        {
+            OutWtp = WriteTimeoutPolicy.Retry,
+            OutMp = 1024 * 1024,
+        };
+
+        // Detect slow consumer state from write-timeout path.
+        route.HandleWriteTimeout(written: 1, attempted: 1024, numChunks: 2).ShouldBeFalse();
+        route.Flags.IsSet(ClientFlags.IsSlowConsumer).ShouldBeTrue();
+
+        // A successful flush should clear slow-consumer marker (recovered).
+        route.QueueOutbound("MSG test 1 5\r\nhello\r\n"u8.ToArray());
+        route.FlushOutbound().ShouldBeTrue();
+        route.Flags.IsSet(ClientFlags.IsSlowConsumer).ShouldBeFalse();
+    }
 }