Update DotNetMuxerTest (#31053)

Fixes #24082

Author: pranavkm

src/Shared/CommandLineUtils/Utilities/DotNetMuxer.cs

@@ -22,7 +22,7 @@ internal static class DotNetMuxer
 
         static DotNetMuxer()
         {
-            MuxerPath = TryFindMuxerPath();
+            MuxerPath = TryFindMuxerPath(Process.GetCurrentProcess().MainModule?.FileName);
         }
 
         /// <summary>
@@ -38,19 +38,18 @@ static DotNetMuxer()
         public static string MuxerPathOrDefault()
             => MuxerPath ?? MuxerName;
 
-        private static string? TryFindMuxerPath()
+        internal static string? TryFindMuxerPath(string? mainModule)
         {
             var fileName = MuxerName;
             if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
             {
                 fileName += ".exe";
             }
 
-            var mainModule = Process.GetCurrentProcess().MainModule;
-            if (!string.IsNullOrEmpty(mainModule?.FileName)
-                && string.Equals(Path.GetFileName(mainModule!.FileName), fileName, StringComparison.OrdinalIgnoreCase))
+            if (!string.IsNullOrEmpty(mainModule)
+                && string.Equals(Path.GetFileName(mainModule!), fileName, StringComparison.OrdinalIgnoreCase))
             {
-                return mainModule.FileName;
+                return mainModule;
             }
 
             return null;

src/Shared/test/Shared.Tests/DotNetMuxerTests.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 #if NETCOREAPP
+using System;
 using System.IO;
 using System.Runtime.InteropServices;
 using Xunit;
@@ -10,14 +11,31 @@ namespace Microsoft.Extensions.CommandLineUtils
 {
     public class DotNetMuxerTests
     {
-        [Fact(Skip = "https://github.com/dotnet/aspnetcore/issues/24082")]
+        [Fact]
         public void FindsTheMuxer()
         {
-            var muxerPath = DotNetMuxer.MuxerPath;
+
+            var muxerPath = DotNetMuxer.TryFindMuxerPath(GetDotnetPath());
             Assert.NotNull(muxerPath);
             Assert.True(File.Exists(muxerPath), "The file did not exist");
             Assert.True(Path.IsPathRooted(muxerPath), "The path should be rooted");
             Assert.Equal("dotnet", Path.GetFileNameWithoutExtension(muxerPath), ignoreCase: true);
+
+            static string GetDotnetPath()
+            {
+                // Process.MainModule is app[.exe] and not `dotnet`. We can instead calculate the dotnet SDK path
+                // by looking at the shared fx directory instead.
+                // depsFile = /dotnet/shared/Microsoft.NETCore.App/6.0-preview2/Microsoft.NETCore.App.deps.json
+                var depsFile = (string)AppContext.GetData("FX_DEPS_FILE");
+                return Path.GetFullPath(Path.Combine(Path.GetDirectoryName(depsFile), "..", "..", "..", "dotnet" + (RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? ".exe" : "")));
+            }
+        }
+
+        [Fact]
+        public void ReturnsNullIfMainModuleIsNotDotNet()
+        {
+            var muxerPath = DotNetMuxer.TryFindMuxerPath(@"d:\some-path\testhost.exe");
+            Assert.Null(muxerPath);
         }
     }
 }