Add debug logging in address binding when IPv6Any fails

KestrelHttpServer.sln

@@ -25,6 +25,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "shared", "shared", "{0EF2AC
 		test\shared\KestrelTestLoggerFactory.cs = test\shared\KestrelTestLoggerFactory.cs
 		test\shared\LifetimeNotImplemented.cs = test\shared\LifetimeNotImplemented.cs
 		test\shared\MockFrameControl.cs = test\shared\MockFrameControl.cs
+		test\shared\MockLogger.cs = test\shared\MockLogger.cs
 		test\shared\MockSocketOutput.cs = test\shared\MockSocketOutput.cs
 		test\shared\MockSystemClock.cs = test\shared\MockSystemClock.cs
 		test\shared\StringExtensions.cs = test\shared\StringExtensions.cs

src/Microsoft.AspNetCore.Server.Kestrel.Core/CoreStrings.resx

@@ -117,6 +117,9 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
+  <data name="FallbackToIPv4Any" xml:space="preserve">
+    <value>Failed to bind to http://[::]:{port} (IPv6Any). Attempting to bind to http://0.0.0.0:{port} instead.</value>
+  </data>
   <data name="ResponseStreamWasUpgraded" xml:space="preserve">
     <value>Cannot write to response body after connection has been upgraded.</value>
   </data>

src/Microsoft.AspNetCore.Server.Kestrel.Core/Internal/AddressBinder.cs

@@ -206,6 +206,8 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal
                     }
                     catch (Exception ex) when (!(ex is IOException))
                     {
+                        context.Logger.LogDebug(CoreStrings.FormatFallbackToIPv4Any(parsedAddress.Port));
+
                         // for machines that do not support IPv6
                         options = new ListenOptions(new IPEndPoint(IPAddress.Any, parsedAddress.Port));
                         await BindEndpointAsync(options, context).ConfigureAwait(false);

src/Microsoft.AspNetCore.Server.Kestrel.Core/Properties/CoreStrings.Designer.cs

@@ -10,6 +10,20 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core
         private static readonly ResourceManager _resourceManager
             = new ResourceManager("Microsoft.AspNetCore.Server.Kestrel.Core.CoreStrings", typeof(CoreStrings).GetTypeInfo().Assembly);
 
+        /// <summary>
+        /// Failed to bind to http://[::]:{port} (IPv6Any). Attempting to bind to http://0.0.0.0:{port} instead.
+        /// </summary>
+        internal static string FallbackToIPv4Any
+        {
+            get => GetString("FallbackToIPv4Any");
+        }
+
+        /// <summary>
+        /// Failed to bind to http://[::]:{port} (IPv6Any). Attempting to bind to http://0.0.0.0:{port} instead.
+        /// </summary>
+        internal static string FormatFallbackToIPv4Any(object port)
+            => string.Format(CultureInfo.CurrentCulture, GetString("FallbackToIPv4Any", "port"), port);
+
         /// <summary>
         /// Cannot write to response body after connection has been upgraded.
         /// </summary>

test/Microsoft.AspNetCore.Server.Kestrel.Core.Tests/AddressBinderTests.cs

@@ -8,6 +8,7 @@ using System.Net;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Internal;
 using Microsoft.AspNetCore.Server.Kestrel.Transport.Abstractions;
+using Microsoft.AspNetCore.Testing;
 using Microsoft.Extensions.Logging.Abstractions;
 using Xunit;
 
@@ -88,6 +89,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core.Tests
         [InlineData("http://contoso.com:80")]
         public async Task FallbackToIPv4WhenIPv6AnyBindFails(string address)
         {
+            var logger = new MockLogger();
             var addresses = new ServerAddressesFeature();
             addresses.Addresses.Add(address);
             var options = new List<ListenOptions>();
@@ -97,7 +99,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core.Tests
 
             await AddressBinder.BindAsync(addresses,
                 options,
-                NullLogger.Instance,
+                logger,
                 endpoint =>
                 {
                     if (endpoint.IPEndPoint.Address == IPAddress.IPv6Any)
@@ -116,6 +118,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Core.Tests
 
             Assert.True(ipV4Attempt, "Should have attempted to bind to IPAddress.Any");
             Assert.True(ipV6Attempt, "Should have attempted to bind to IPAddress.IPv6Any");
+            Assert.Contains(logger.Messages, f => f.Equals(CoreStrings.FormatFallbackToIPv4Any(80)));
         }
     }
 }

test/shared/MockLogger.cs

@@ -0,0 +1,28 @@
+// 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;
+using System.Collections.Generic;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions.Internal;
+
+namespace Microsoft.AspNetCore.Testing
+{
+    public class MockLogger : ILogger
+    {
+        private List<string> _messages = new List<string>();
+
+        public IDisposable BeginScope<TState>(TState state)
+            => NullScope.Instance;
+
+        public bool IsEnabled(LogLevel logLevel)
+            => true;
+
+        public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception exception, Func<TState, Exception, string> formatter)
+        {
+            _messages.Add(formatter(state, exception));
+        }
+
+        public IReadOnlyList<string> Messages => _messages;
+    }
+}