Fix hostfxr resolution failure on musl when HOSTFXR_PATH is not set

src/Cli/dn/DotnetRootResolver.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Runtime.InteropServices;
+using Microsoft.DotNet.HostFxr;
 
 namespace Microsoft.DotNet.Cli;
 
@@ -84,37 +85,11 @@ internal static string ResolveHostfxrPath(
         Func<string, bool> directoryExists,
         Func<string, string[]> getDirectories,
         Func<string, bool> fileExists)
-    {
-        string fxrDir = Path.Combine(dotnetRoot, "host", "fxr");
-        if (!directoryExists(fxrDir))
-        {
-            return string.Empty;
-        }
-
-        // Pick the highest version directory by parsing version numbers
-        string? latestFxr = getDirectories(fxrDir)
-            .Select(path => new
-            {
-                Path = path,
-                Version = Version.TryParse(Path.GetFileName(path), out Version? version) ? version : null
-            })
-            .Where(candidate => candidate.Version is not null)
-            .OrderByDescending(candidate => candidate.Version)
-            .Select(candidate => candidate.Path)
-            .FirstOrDefault();
-
-        if (latestFxr is null)
-        {
-            return string.Empty;
-        }
-
-        string hostfxrName = isWindows
-            ? "hostfxr.dll"
-            : isMacOS
-                ? "libhostfxr.dylib"
-                : "libhostfxr.so";
-
-        string hostfxrPath = Path.Combine(latestFxr, hostfxrName);
-        return fileExists(hostfxrPath) ? hostfxrPath : string.Empty;
-    }
+        => HostFxrPathResolver.ResolveHostFxrPath(
+            dotnetRoot,
+            isWindows,
+            isMacOS,
+            directoryExists,
+            getDirectories,
+            fileExists);
 }

src/Cli/dn/dn.csproj

@@ -22,6 +22,14 @@
     </PackageReference>
   </ItemGroup>
 
+  <ItemGroup>
+    <PackageReference Include="NuGet.Versioning" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Include="$(RepoRoot)src\Common\HostFxrPathResolver.cs" LinkBase="Common" />
+  </ItemGroup>
+
   <!--
     When building inside Visual Studio, publish the AOT binaries and overlay managed dependencies
     so the "Debug dn (AOT native)" launch profile is ready to go.

src/Common/HostFxrPathResolver.cs

@@ -0,0 +1,80 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using NuGet.Versioning;
+
+namespace Microsoft.DotNet.HostFxr;
+
+/// <summary>
+///  Locates the native <c>hostfxr</c> library underneath a .NET installation root.
+///  This is used as a fallback when the host does not publish the <c>HOSTFXR_PATH</c>
+///  runtime property (for example when the SDK is launched via <c>dotnet exec dotnet.dll</c>
+///  rather than as a first-class SDK command, which is how the <c>dnx</c> script works).
+///  Dependencies are injected so the pure path-resolution logic can be unit tested.
+///  <para>
+///   This file is source-shared (via <c>&lt;Compile Include&gt;</c>) into both the
+///   <c>Microsoft.DotNet.NativeWrapper</c> resolver and the Native AOT <c>dn</c> host so
+///   the two stay in lockstep; a prior divergence between separate copies is what caused
+///   https://github.com/dotnet/sdk/issues/55238 to be fixed in only one place.
+///  </para>
+/// </summary>
+internal static class HostFxrPathResolver
+{
+    /// <summary>
+    ///  Finds the highest-versioned <c>hostfxr</c> library under
+    ///  <paramref name="dotnetRoot"/>/host/fxr, returning its full path or
+    ///  <see cref="string.Empty"/> if it cannot be found.
+    /// </summary>
+    internal static string ResolveHostFxrPath(
+        string? dotnetRoot,
+        bool isWindows,
+        bool isMacOS,
+        Func<string, bool> directoryExists,
+        Func<string, string[]> getDirectories,
+        Func<string, bool> fileExists)
+    {
+        if (string.IsNullOrEmpty(dotnetRoot))
+        {
+            return string.Empty;
+        }
+
+        string fxrDir = Path.Combine(dotnetRoot, "host", "fxr");
+        if (!directoryExists(fxrDir))
+        {
+            return string.Empty;
+        }
+
+        // Match the native host's get_latest_fxr behavior: consider every valid
+        // semantic version, including prerelease versions, and select the highest.
+        // SDK and runtime roll-forward settings apply after hostfxr is loaded and
+        // do not affect hostfxr selection.
+        string? latestFxr = getDirectories(fxrDir)
+            .Select(path =>
+            {
+                SemanticVersion.TryParse(Path.GetFileName(path), out SemanticVersion? version);
+                return new
+                {
+                    Path = path,
+                    Version = version
+                };
+            })
+            .Where(candidate => candidate.Version is not null)
+            .OrderByDescending(candidate => candidate.Version)
+            .Select(candidate => candidate.Path)
+            .FirstOrDefault();
+
+        if (latestFxr is null)
+        {
+            return string.Empty;
+        }
+
+        string hostfxrName = isWindows
+            ? "hostfxr.dll"
+            : isMacOS
+                ? "libhostfxr.dylib"
+                : "libhostfxr.so";
+
+        string hostfxrPath = Path.Combine(latestFxr, hostfxrName);
+        return fileExists(hostfxrPath) ? hostfxrPath : string.Empty;
+    }
+}

src/Resolvers/Microsoft.DotNet.NativeWrapper/HostFxrLocator.cs

@@ -0,0 +1,51 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#if NET
+using Microsoft.DotNet.HostFxr;
+
+namespace Microsoft.DotNet.NativeWrapper
+{
+    /// <summary>
+    ///  NativeWrapper-facing entry point for locating the native <c>hostfxr</c> library.
+    ///  The actual path-resolution logic lives in the source-shared
+    ///  <see cref="HostFxrPathResolver"/> (under <c>src/Common</c>) so this resolver and the
+    ///  Native AOT <c>dn</c> host stay in lockstep.
+    /// </summary>
+    internal static class HostFxrLocator
+    {
+        /// <summary>
+        ///  Finds the highest-versioned <c>hostfxr</c> library under
+        ///  <paramref name="dotnetRoot"/>/host/fxr, returning its full path or
+        ///  <see cref="string.Empty"/> if it cannot be found.
+        /// </summary>
+        internal static string ResolveHostFxrPath(
+            string? dotnetRoot,
+            bool isWindows,
+            bool isMacOS,
+            Func<string, bool> directoryExists,
+            Func<string, string[]> getDirectories,
+            Func<string, bool> fileExists)
+            => HostFxrPathResolver.ResolveHostFxrPath(
+                dotnetRoot,
+                isWindows,
+                isMacOS,
+                directoryExists,
+                getDirectories,
+                fileExists);
+
+        /// <summary>
+        ///  Resolves the <c>hostfxr</c> path against the live filesystem, deriving the .NET
+        ///  installation root from the current process (the running <c>dotnet</c> host).
+        /// </summary>
+        internal static string ResolveHostFxrPath()
+            => ResolveHostFxrPath(
+                EnvironmentProvider.GetDotnetExeDirectory(),
+                isWindows: OperatingSystem.IsWindows(),
+                isMacOS: OperatingSystem.IsMacOS(),
+                directoryExists: Directory.Exists,
+                getDirectories: Directory.GetDirectories,
+                fileExists: File.Exists);
+    }
+}
+#endif

src/Resolvers/Microsoft.DotNet.NativeWrapper/Interop.cs

@@ -68,14 +68,26 @@ private static IntPtr HostFxrResolver(Assembly assembly, string libraryName)
                 return IntPtr.Zero;
             }
 
-            if (string.IsNullOrEmpty(s_hostFxrPath))
+            string? hostFxrPath = s_hostFxrPath;
+
+            // The host only publishes the HOSTFXR_PATH runtime property for first-class SDK
+            // commands (e.g. `dotnet build`). When the SDK is launched via `dotnet exec dotnet.dll`
+            // (as the `dnx` script does), the property is absent. On glibc the bare `libhostfxr`
+            // load succeeds against the already-loaded library so this resolver never runs, but on
+            // musl it does not, so fall back to locating hostfxr under the running .NET root.
+            if (string.IsNullOrEmpty(hostFxrPath))
+            {
+                hostFxrPath = HostFxrLocator.ResolveHostFxrPath();
+            }
+
+            if (string.IsNullOrEmpty(hostFxrPath))
             {
                 throw new HostFxrRuntimePropertyNotSetException();
             }
 
-            if (!NativeLibrary.TryLoad(s_hostFxrPath, out var handle))
+            if (!NativeLibrary.TryLoad(hostFxrPath, out var handle))
             {
-                throw new HostFxrNotFoundException(s_hostFxrPath);
+                throw new HostFxrNotFoundException(hostFxrPath);
             }
 
             return handle;

src/Resolvers/Microsoft.DotNet.NativeWrapper/Microsoft.DotNet.NativeWrapper.csproj

@@ -15,10 +15,16 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.Deployment.DotNet.Releases" />
+    <PackageReference Include="NuGet.Versioning" Condition="'$(TargetFramework)' != 'net472'" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Include="$(RepoRoot)src\Common\HostFxrPathResolver.cs" LinkBase="Common" Condition="'$(TargetFramework)' != 'net472'" />
   </ItemGroup>
 
   <ItemGroup>
     <InternalsVisibleTo Include="dotnet-aot" PublicKey="0024000004800000940000000602000000240000525341310004000001000100F33A29044FA9D740C9B3213A93E57C84B472C84E0B8A0E1AE48E67A9F8F6DE9D5F7F3D52AC23E48AC51801F1DC950ABE901DA34D2A9E3BAADB141A17C77EF3C565DD5EE5054B91CF63BB3C6AB83F72AB3AAFE93D0FC3C2348B764FAFB0B1C0733DE51459AEAB46580384BF9D74C4E28164B7CDE247F891BA07891C9D872AD2BB" />
+    <InternalsVisibleTo Include="dotnet-aot.Tests" PublicKey="0024000004800000940000000602000000240000525341310004000001000100F33A29044FA9D740C9B3213A93E57C84B472C84E0B8A0E1AE48E67A9F8F6DE9D5F7F3D52AC23E48AC51801F1DC950ABE901DA34D2A9E3BAADB141A17C77EF3C565DD5EE5054B91CF63BB3C6AB83F72AB3AAFE93D0FC3C2348B764FAFB0B1C0733DE51459AEAB46580384BF9D74C4E28164B7CDE247F891BA07891C9D872AD2BB" />
   </ItemGroup>
 
 </Project>

test/dotnet-aot.Tests/DotnetRootResolverTests.cs

@@ -180,24 +180,126 @@ public void ResolveHostfxrPath_PicksHighestVersion()
     }
 
     [TestMethod]
-    public void ResolveHostfxrPath_SkipsPrereleaseDirectories()
+    public void ResolveHostfxrPath_FindsPrereleaseVersionDirectory()
     {
+        // On a preview-only install, host/fxr contains a single prerelease-named
+        // directory (e.g. "10.0.0-preview.5"). Version.TryParse can't parse that string,
+        // so the numeric core must be used instead of skipping the directory.
         string dotnetRoot = BuildPath(true, "dotnet");
         string fxrDir = Path.Combine(dotnetRoot, "host", "fxr");
         string preview = Path.Combine(fxrDir, "10.0.0-preview.5");
+        string expectedPath = Path.Combine(preview, "hostfxr.dll");
+
+        string result = DotnetRootResolver.ResolveHostfxrPath(
+            dotnetRoot: dotnetRoot,
+            isWindows: true,
+            isMacOS: false,
+            directoryExists: _ => true,
+            getDirectories: _ => new[] { preview },
+            fileExists: _ => true);
+
+        Assert.AreEqual(expectedPath, result);
+    }
+
+    [TestMethod]
+    public void ResolveHostfxrPath_PicksHighestVersion_IncludingPrerelease()
+    {
+        string dotnetRoot = BuildPath(true, "dotnet");
+        string fxrDir = Path.Combine(dotnetRoot, "host", "fxr");
         string v900 = Path.Combine(fxrDir, "9.0.0");
-        string expectedPath = Path.Combine(v900, "hostfxr.dll");
+        string preview = Path.Combine(fxrDir, "10.0.0-preview.5");
+        string expectedPath = Path.Combine(preview, "hostfxr.dll");
+
+        string result = DotnetRootResolver.ResolveHostfxrPath(
+            dotnetRoot: dotnetRoot,
+            isWindows: true,
+            isMacOS: false,
+            directoryExists: _ => true,
+            getDirectories: _ => new[] { v900, preview },
+            fileExists: _ => true);
+
+        Assert.AreEqual(expectedPath, result);
+    }
+
+    [TestMethod]
+    public void ResolveHostfxrPath_PrefersStableOverPrereleaseOfSameCore()
+    {
+        string dotnetRoot = BuildPath(true, "dotnet");
+        string fxrDir = Path.Combine(dotnetRoot, "host", "fxr");
+        string stable = Path.Combine(fxrDir, "10.0.0");
+        string preview = Path.Combine(fxrDir, "10.0.0-preview.5");
+        string expectedPath = Path.Combine(stable, "hostfxr.dll");
+
+        string result = DotnetRootResolver.ResolveHostfxrPath(
+            dotnetRoot: dotnetRoot,
+            isWindows: true,
+            isMacOS: false,
+            directoryExists: _ => true,
+            getDirectories: _ => new[] { preview, stable },
+            fileExists: _ => true);
+
+        Assert.AreEqual(expectedPath, result);
+    }
+
+    [TestMethod]
+    public void ResolveHostfxrPath_TreatsBuildMetadataAsStable()
+    {
+        string dotnetRoot = BuildPath(true, "dotnet");
+        string fxrDir = Path.Combine(dotnetRoot, "host", "fxr");
+        string stable = Path.Combine(fxrDir, "10.0.0+build.1");
+        string preview = Path.Combine(fxrDir, "10.0.0-preview.5");
+        string expectedPath = Path.Combine(stable, "hostfxr.dll");
+
+        string result = DotnetRootResolver.ResolveHostfxrPath(
+            dotnetRoot: dotnetRoot,
+            isWindows: true,
+            isMacOS: false,
+            directoryExists: _ => true,
+            getDirectories: _ => new[] { preview, stable },
+            fileExists: _ => true);
+
+        Assert.AreEqual(expectedPath, result);
+    }
+
+    [TestMethod]
+    public void ResolveHostfxrPath_OrdersPrereleaseSegmentsNumerically()
+    {
+        // "preview.10" must sort after "preview.6"; a plain ordinal compare gets this wrong.
+        string dotnetRoot = BuildPath(true, "dotnet");
+        string fxrDir = Path.Combine(dotnetRoot, "host", "fxr");
+        string preview6 = Path.Combine(fxrDir, "10.0.0-preview.6");
+        string preview10 = Path.Combine(fxrDir, "10.0.0-preview.10");
+        string expectedPath = Path.Combine(preview10, "hostfxr.dll");
+
+        string result = DotnetRootResolver.ResolveHostfxrPath(
+            dotnetRoot: dotnetRoot,
+            isWindows: true,
+            isMacOS: false,
+            directoryExists: _ => true,
+            getDirectories: _ => new[] { preview6, preview10 },
+            fileExists: _ => true);
+
+        Assert.AreEqual(expectedPath, result);
+    }
+
+    [TestMethod]
+    public void ResolveHostfxrPath_SkipsInvalidSemanticVersions()
+    {
+        string dotnetRoot = BuildPath(true, "dotnet");
+        string fxrDir = Path.Combine(dotnetRoot, "host", "fxr");
+        string twoPartVersion = Path.Combine(fxrDir, "99.0");
+        string leadingZero = Path.Combine(fxrDir, "98.0.0-preview.01");
+        string valid = Path.Combine(fxrDir, "10.0.0");
+        string expectedPath = Path.Combine(valid, "hostfxr.dll");
 
-        // Version.TryParse fails for prerelease strings like "10.0.0-preview.5"
         string result = DotnetRootResolver.ResolveHostfxrPath(
             dotnetRoot: dotnetRoot,
             isWindows: true,
             isMacOS: false,
             directoryExists: _ => true,
-            getDirectories: _ => new[] { preview, v900 },
+            getDirectories: _ => new[] { twoPartVersion, leadingZero, valid },
             fileExists: _ => true);
 
-        // Should pick 9.0.0 since the preview dir is skipped
         Assert.AreEqual(expectedPath, result);
     }