Add regression tests for path casing normalization

PathUtilL0:
- Folder casing normalization (create MiXeDcAsE, query lowercase)
- Idempotency (calling twice returns same result)
- Input casing independence (upper and lower resolve to same canonical)

HostContextL0:
- Root directory returns cached value across calls
- Derived paths (Diag, Externals) share Root prefix casing

src/Runner.Common/HostContext.cs

@@ -64,6 +64,7 @@ namespace GitHub.Runner.Common
         private readonly List<ProductInfoHeaderValue> _userAgents = new() { new ProductInfoHeaderValue($"GitHubActionsRunner-{BuildConstants.RunnerPackage.PackageName}", BuildConstants.RunnerPackage.Version) };
         private CancellationTokenSource _runnerShutdownTokenSource = new();
         private object _perfLock = new();
+        private string _canonicalRootDirectory;
         private Tracing _trace;
         private Tracing _actionsHttpTrace;
         private Tracing _netcoreHttpTrace;
@@ -391,7 +392,12 @@ namespace GitHub.Runner.Common
                     break;
 
                 case WellKnownDirectory.Root:
-                    path = new DirectoryInfo(GetDirectory(WellKnownDirectory.Bin)).Parent.FullName;
+                    if (_canonicalRootDirectory == null)
+                    {
+                        _canonicalRootDirectory = PathUtil.GetCanonicalPath(
+                            new DirectoryInfo(GetDirectory(WellKnownDirectory.Bin)).Parent.FullName);
+                    }
+                    path = _canonicalRootDirectory;
                     break;
 
                 case WellKnownDirectory.Temp:

src/Runner.Sdk/Util/PathUtil.cs

@@ -1,4 +1,7 @@
 using System.IO;
+using System.Runtime.InteropServices;
+using System.Text;
+using Microsoft.Win32.SafeHandles;
 
 namespace GitHub.Runner.Sdk
 {
@@ -6,8 +9,98 @@ namespace GitHub.Runner.Sdk
     {
 #if OS_WINDOWS
         public static readonly string PathVariable = "Path";
+
+        [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
+        private static extern SafeFileHandle CreateFile(
+            string lpFileName,
+            uint dwDesiredAccess,
+            uint dwShareMode,
+            System.IntPtr lpSecurityAttributes,
+            uint dwCreationDisposition,
+            uint dwFlagsAndAttributes,
+            System.IntPtr hTemplateFile);
+
+        [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
+        private static extern uint GetFinalPathNameByHandle(
+            SafeFileHandle hFile,
+            [Out] StringBuilder lpszFilePath,
+            uint cchFilePath,
+            uint dwFlags);
+
+        private const uint FILE_READ_ATTRIBUTES = 0x80;
+        private const uint FILE_SHARE_READ = 0x1;
+        private const uint FILE_SHARE_WRITE = 0x2;
+        private const uint FILE_SHARE_DELETE = 0x4;
+        private const uint OPEN_EXISTING = 3;
+        private const uint FILE_FLAG_BACKUP_SEMANTICS = 0x02000000;
+        private const uint VOLUME_NAME_DOS = 0x0;
+
+        /// <summary>
+        /// Returns the NTFS canonical path for a directory, resolving drive letter
+        /// and folder name casing to match what is stored on disk.
+        /// On non-Windows platforms, returns the path unchanged.
+        /// </summary>
+        public static string GetCanonicalPath(string path)
+        {
+            if (string.IsNullOrEmpty(path) || !Directory.Exists(path))
+            {
+                return path;
+            }
+
+            using var handle = CreateFile(
+                path,
+                FILE_READ_ATTRIBUTES,
+                FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
+                System.IntPtr.Zero,
+                OPEN_EXISTING,
+                FILE_FLAG_BACKUP_SEMANTICS,
+                System.IntPtr.Zero);
+
+            if (handle.IsInvalid)
+            {
+                return path;
+            }
+
+            var buffer = new StringBuilder(1024);
+            var result = GetFinalPathNameByHandle(handle, buffer, (uint)buffer.Capacity, VOLUME_NAME_DOS);
+            if (result == 0)
+            {
+                return path;
+            }
+
+            // Retry with a larger buffer if the path was longer than expected
+            if (result >= buffer.Capacity)
+            {
+                buffer = new StringBuilder((int)result + 1);
+                result = GetFinalPathNameByHandle(handle, buffer, (uint)buffer.Capacity, VOLUME_NAME_DOS);
+                if (result == 0 || result >= buffer.Capacity)
+                {
+                    return path;
+                }
+            }
+
+            var canonicalPath = buffer.ToString();
+
+            // Strip the \\?\UNC\ prefix and convert to standard UNC path
+            if (canonicalPath.StartsWith(@"\\?\UNC\", System.StringComparison.Ordinal))
+            {
+                canonicalPath = @"\\" + canonicalPath.Substring(8);
+            }
+            // Strip the \\?\ prefix for local paths
+            else if (canonicalPath.StartsWith(@"\\?\", System.StringComparison.Ordinal))
+            {
+                canonicalPath = canonicalPath.Substring(4);
+            }
+
+            return canonicalPath;
+        }
 #else
         public static readonly string PathVariable = "PATH";
+
+        public static string GetCanonicalPath(string path)
+        {
+            return path;
+        }
 #endif
 
         public static string PrependPath(string path, string currentPath)

src/Test/L0/HostContextL0.cs

@@ -299,6 +299,52 @@ namespace GitHub.Runner.Common.Tests
             }
         }
 
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetDirectoryRootReturnsCachedValue()
+        {
+            try
+            {
+                Setup();
+
+                // Call GetDirectory(Root) twice — should return the same reference
+                var root1 = _hc.GetDirectory(WellKnownDirectory.Root);
+                var root2 = _hc.GetDirectory(WellKnownDirectory.Root);
+
+                Assert.NotNull(root1);
+                Assert.Equal(root1, root2);
+                Assert.True(Directory.Exists(root1));
+            }
+            finally
+            {
+                Teardown();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetDirectoryDerivedPathsUseRootCasing()
+        {
+            try
+            {
+                Setup();
+
+                var root = _hc.GetDirectory(WellKnownDirectory.Root);
+                var diag = _hc.GetDirectory(WellKnownDirectory.Diag);
+                var externals = _hc.GetDirectory(WellKnownDirectory.Externals);
+
+                // Diag and Externals should start with the same Root prefix
+                Assert.StartsWith(root, diag);
+                Assert.StartsWith(root, externals);
+            }
+            finally
+            {
+                Teardown();
+            }
+        }
+
         private void Setup([CallerMemberName] string testName = "")
         {
             _tokenSource = new CancellationTokenSource();

src/Test/L0/Util/PathUtilL0.cs

@@ -0,0 +1,151 @@
+using GitHub.Runner.Sdk;
+using System.IO;
+using System.Runtime.InteropServices;
+using Xunit;
+
+namespace GitHub.Runner.Common.Tests.Util
+{
+    public sealed class PathUtilL0
+    {
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetCanonicalPath_ReturnsPath_WhenDirectoryDoesNotExist()
+        {
+            var fakePath = Path.Combine(Path.GetTempPath(), "nonexistent_" + Path.GetRandomFileName());
+            var result = PathUtil.GetCanonicalPath(fakePath);
+            Assert.Equal(fakePath, result);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetCanonicalPath_ReturnsPath_WhenNull()
+        {
+            Assert.Null(PathUtil.GetCanonicalPath(null));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetCanonicalPath_ReturnsEmpty_WhenEmpty()
+        {
+            Assert.Equal(string.Empty, PathUtil.GetCanonicalPath(string.Empty));
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetCanonicalPath_ReturnsValidPath_ForExistingDirectory()
+        {
+            var tempDir = Path.Combine(Path.GetTempPath(), "pathutil_test_" + Path.GetRandomFileName());
+            try
+            {
+                Directory.CreateDirectory(tempDir);
+                var result = PathUtil.GetCanonicalPath(tempDir);
+                Assert.NotNull(result);
+                Assert.True(Directory.Exists(result));
+            }
+            finally
+            {
+                if (Directory.Exists(tempDir))
+                {
+                    Directory.Delete(tempDir);
+                }
+            }
+        }
+
+#if OS_WINDOWS
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetCanonicalPath_NormalizesDriveLetter_OnWindows()
+        {
+            var tempDir = Path.GetTempPath().TrimEnd(Path.DirectorySeparatorChar);
+
+            // Skip if temp is a UNC path (no drive letter to normalize)
+            if (tempDir.StartsWith(@"\\"))
+            {
+                return;
+            }
+
+            // Force lowercase drive letter
+            var lowerCased = char.ToLower(tempDir[0]) + tempDir.Substring(1);
+
+            var result = PathUtil.GetCanonicalPath(lowerCased);
+
+            // The canonical path should have an uppercase drive letter
+            Assert.True(char.IsUpper(result[0]),
+                $"Expected uppercase drive letter but got: {result}");
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetCanonicalPath_NormalizesFolderCasing_OnWindows()
+        {
+            // Create a directory with known casing, then query with wrong casing
+            var basePath = Path.GetTempPath();
+            if (basePath.StartsWith(@"\\"))
+            {
+                return; // Skip UNC
+            }
+
+            var realName = "PathUtilTest_MiXeDcAsE_" + Path.GetRandomFileName();
+            var realDir = Path.Combine(basePath, realName);
+            try
+            {
+                Directory.CreateDirectory(realDir);
+
+                // Query with all-lowercase version
+                var wrongCased = Path.Combine(basePath, realName.ToLowerInvariant());
+
+                var result = PathUtil.GetCanonicalPath(wrongCased);
+
+                // The canonical result should contain the original mixed-case name
+                Assert.Contains(realName, result);
+            }
+            finally
+            {
+                if (Directory.Exists(realDir))
+                {
+                    Directory.Delete(realDir);
+                }
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetCanonicalPath_IsIdempotent_OnWindows()
+        {
+            // Calling GetCanonicalPath twice should return the same result
+            var tempDir = Path.GetTempPath().TrimEnd(Path.DirectorySeparatorChar);
+            var first = PathUtil.GetCanonicalPath(tempDir);
+            var second = PathUtil.GetCanonicalPath(first);
+            Assert.Equal(first, second);
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void GetCanonicalPath_ReturnsSameResult_RegardlessOfInputCasing_OnWindows()
+        {
+            var tempDir = Path.GetTempPath().TrimEnd(Path.DirectorySeparatorChar);
+            if (tempDir.StartsWith(@"\\"))
+            {
+                return; // Skip UNC
+            }
+
+            var upper = tempDir.ToUpperInvariant();
+            var lower = tempDir.ToLowerInvariant();
+
+            var resultUpper = PathUtil.GetCanonicalPath(upper);
+            var resultLower = PathUtil.GetCanonicalPath(lower);
+
+            // Both should resolve to the same canonical path
+            Assert.Equal(resultUpper, resultLower);
+        }
+#endif
+    }
+}