Fall back to DOTNET_HOST_PATH/DOTNET_ROOT when muxer walk fails

Native AOT hosts report the app directory as the runtime directory, so the three-level walk above the shared framework no longer finds dotnet. Probe DOTNET_HOST_PATH and DOTNET_ROOT next, and cover resolution in tests.

Author: kzu

System/DotnetMuxer.cs

@@ -50,6 +50,7 @@ static partial class DotNetMuxer
                 return mainModuleFullPath;
             }
 
+            // Framework-dependent / R2R: muxer is three levels above the shared runtime.
             // The currently running process may not be dotnet(.exe). For example,
             // it might be "testhost(.exe)" when running tests.
             // In this case, we can get the location where the CLR is installed,
@@ -58,8 +59,22 @@ static partial class DotNetMuxer
             var candidateDotNetExePath = Path.Combine(runtimeDirectory, "..", "..", "..", expectedFileName);
             if (File.Exists(candidateDotNetExePath))
             {
-                var normalizedPath = Path.GetFullPath(candidateDotNetExePath);
-                return normalizedPath;
+                return Path.GetFullPath(candidateDotNetExePath);
+            }
+
+            // Native AOT (e.g. dnx go default publish) reports the app dir as the runtime
+            // directory, so that walk is wrong — use DOTNET_HOST_PATH / DOTNET_ROOT.
+            if (Environment.GetEnvironmentVariable("DOTNET_HOST_PATH") is { Length: > 0 } host
+                && File.Exists(host))
+            {
+                return host;
+            }
+
+            if (Environment.GetEnvironmentVariable("DOTNET_ROOT") is { Length: > 0 } root)
+            {
+                var fromRoot = Path.Combine(root, expectedFileName);
+                if (File.Exists(fromRoot))
+                    return fromRoot;
             }
 
             return null;

tests/DotNetMuxerTests.cs

@@ -1,12 +1,72 @@
-using Xunit;
+using System.IO;
+using System.Runtime.InteropServices;
+using Xunit;
 
 namespace System;
 
 public class DotNetMuxerTests
 {
+    static string ExpectedMuxerFileName =>
+        RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? "dotnet.exe" : "dotnet";
+
     [Fact]
     public void MuxerPathIsSet()
     {
         Assert.NotNull(DotNetMuxer.MuxerPath);
     }
+
+    [Fact]
+    public void MuxerPathPointsToExistingFile()
+    {
+        Assert.NotNull(DotNetMuxer.MuxerPath);
+        Assert.True(File.Exists(DotNetMuxer.MuxerPath), $"Muxer not found at '{DotNetMuxer.MuxerPath}'");
+    }
+
+    [Fact]
+    public void MuxerPathIsDotnetExecutable()
+    {
+        Assert.NotNull(DotNetMuxer.MuxerPath);
+        Assert.Equal(ExpectedMuxerFileName, Path.GetFileName(DotNetMuxer.MuxerPath), ignoreCase: true);
+    }
+
+    [Fact]
+    public void MuxerPathOrDefaultReturnsPathWhenResolved()
+    {
+        var path = DotNetMuxer.MuxerPathOrDefault();
+        Assert.False(string.IsNullOrEmpty(path));
+
+        if (DotNetMuxer.MuxerPath is not null)
+            Assert.Equal(DotNetMuxer.MuxerPath, path);
+        else
+            Assert.Equal("dotnet", path);
+    }
+
+    [Fact]
+    public void MuxerPathCanBeResolvedFromDotnetHostPathOrRoot()
+    {
+        // Documents the env-var fallbacks used when the runtime-directory walk fails
+        // (Native AOT). In a normal testhost run the walk already succeeds; still verify
+        // the same locations the fallbacks would use are valid when present.
+        if (Environment.GetEnvironmentVariable("DOTNET_HOST_PATH") is { Length: > 0 } host
+            && File.Exists(host))
+        {
+            Assert.Equal(ExpectedMuxerFileName, Path.GetFileName(host), ignoreCase: true);
+            Assert.NotNull(DotNetMuxer.MuxerPath);
+            return;
+        }
+
+        if (Environment.GetEnvironmentVariable("DOTNET_ROOT") is { Length: > 0 } root)
+        {
+            var fromRoot = Path.Combine(root, ExpectedMuxerFileName);
+            if (File.Exists(fromRoot))
+            {
+                Assert.NotNull(DotNetMuxer.MuxerPath);
+                return;
+            }
+        }
+
+        // No env fallbacks set — still expect the main-module / runtime-dir probes to work.
+        Assert.NotNull(DotNetMuxer.MuxerPath);
+        Assert.True(File.Exists(DotNetMuxer.MuxerPath));
+    }
 }