Lower log severity for opening internal libuv pipe (#6636)

Addresses #4741

Author: halter73

src/Servers/Kestrel/Transport.Libuv/src/Internal/ListenerPrimary.cs

@@ -89,7 +89,7 @@ private void OnListenPipe(UvStreamHandle pipe, int status, UvException error)
                 dispatchPipe.Init(Thread.Loop, Thread.QueueCloseHandle, true);
                 pipe.Accept(dispatchPipe);
 
-                // Ensure client sends "Kestrel" before adding pipe to _dispatchPipes.
+                // Ensure client sends _pipeMessage before adding pipe to _dispatchPipes.
                 var readContext = new PipeReadContext(this);
                 dispatchPipe.ReadStart(
                     (handle, status2, state) => ((PipeReadContext)state).AllocCallback(handle, status2),
@@ -228,6 +228,18 @@ public LibuvFunctions.uv_buf_t AllocCallback(UvStreamHandle dispatchPipe, int su
 
             public void ReadCallback(UvStreamHandle dispatchPipe, int status)
             {
+                if (status == LibuvConstants.EOF && _bytesRead == 0)
+                {
+                    // This is an unexpected immediate termination of the dispatch pipe most likely caused by an
+                    // external process scanning the pipe, so don't we don't log it too severely.
+                    // https://github.com/aspnet/AspNetCore/issues/4741
+
+                    dispatchPipe.Dispose();
+                    _bufHandle.Free();
+                    _listener.Log.LogDebug("An internal pipe was opened unexpectedly.");
+                    return;
+                }
+
                 try
                 {
                     dispatchPipe.Libuv.ThrowIfErrored(status);
@@ -254,7 +266,7 @@ public void ReadCallback(UvStreamHandle dispatchPipe, int status)
                         }
                         else
                         {
-                            throw new IOException("Bad data sent over Kestrel pipe.");
+                            throw new IOException("Bad data sent over an internal pipe.");
                         }
                     }
                 }

src/Servers/Kestrel/Transport.Libuv/test/ListenerPrimaryTests.cs

@@ -1,4 +1,4 @@
-// Copyright (c) .NET Foundation. All rights reserved.
+// Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
@@ -196,9 +196,10 @@ public async Task NonListenerPipeConnectionsAreLoggedAndIgnored()
             await listenerPrimary.DisposeAsync();
             await libuvThreadPrimary.StopAsync(TimeSpan.FromSeconds(5));
 
-            Assert.Equal(1, logger.TotalErrorsLogged);
-            var errorMessage = logger.Messages.First(m => m.LogLevel == LogLevel.Error);
-            Assert.Equal(TestConstants.EOF, Assert.IsType<UvException>(errorMessage.Exception).StatusCode);
+            Assert.Equal(0, logger.TotalErrorsLogged);
+
+            var logMessage = logger.Messages.Single(m => m.Message == "An internal pipe was opened unexpectedly.");
+            Assert.Equal(LogLevel.Debug, logMessage.LogLevel);
         }