Use os.Open for Windows realpath to handle long paths

internal/vfs/osvfs/realpath_test.go

@@ -16,24 +16,11 @@ import (
 func TestSymlinkRealpath(t *testing.T) {
 	t.Parallel()
 
-	tmp := t.TempDir()
-
-	target := filepath.Join(tmp, "target")
-	targetFile := filepath.Join(target, "file")
-
-	link := filepath.Join(tmp, "link")
-	linkFile := filepath.Join(link, "file")
-
-	const expectedContents = "hello"
-
-	assert.NilError(t, os.MkdirAll(target, 0o777))
-	assert.NilError(t, os.WriteFile(targetFile, []byte(expectedContents), 0o666))
-
-	mklink(t, target, link, true)
+	targetFile, linkFile := setupSymlinks(t)
 
 	gotContents, err := os.ReadFile(linkFile)
 	assert.NilError(t, err)
-	assert.Equal(t, string(gotContents), expectedContents)
+	assert.Equal(t, string(gotContents), "hello")
 
 	fs := FS()
 
@@ -48,22 +35,65 @@ func TestSymlinkRealpath(t *testing.T) {
 	}
 }
 
-func mklink(t *testing.T, target, link string, isDir bool) {
-	t.Helper()
+func setupSymlinks(tb testing.TB) (targetFile, linkFile string) {
+	tb.Helper()
+
+	tmp := tb.TempDir()
+
+	target := filepath.Join(tmp, "target")
+	targetFile = filepath.Join(target, "file")
+
+	link := filepath.Join(tmp, "link")
+	linkFile = filepath.Join(link, "file")
+
+	assert.NilError(tb, os.MkdirAll(target, 0o777))
+	assert.NilError(tb, os.WriteFile(targetFile, []byte("hello"), 0o666))
+
+	mklink(tb, target, link, true)
+
+	return targetFile, linkFile
+}
+
+func mklink(tb testing.TB, target, link string, isDir bool) {
+	tb.Helper()
 
 	if runtime.GOOS == "windows" && isDir {
 		// Don't use os.Symlink on Windows, as it creates a "real" symlink, not a junction.
-		assert.NilError(t, exec.Command("cmd", "/c", "mklink", "/J", link, target).Run())
+		assert.NilError(tb, exec.Command("cmd", "/c", "mklink", "/J", link, target).Run())
 	} else {
 		err := os.Symlink(target, link)
 		if err != nil && !isDir && runtime.GOOS == "windows" && strings.Contains(err.Error(), "A required privilege is not held by the client") {
-			t.Log(err)
-			t.Skip("file symlink support is not enabled without elevation or developer mode")
+			tb.Log(err)
+			tb.Skip("file symlink support is not enabled without elevation or developer mode")
 		}
-		assert.NilError(t, err)
+		assert.NilError(tb, err)
 	}
 }
 
+func BenchmarkRealpath(b *testing.B) {
+	targetFile, linkFile := setupSymlinks(b)
+
+	fs := FS()
+	normalizedTargetFile := tspath.NormalizePath(targetFile)
+	normalizedLinkFile := tspath.NormalizePath(linkFile)
+
+	b.Run("target", func(b *testing.B) {
+		b.ReportAllocs()
+
+		for b.Loop() {
+			fs.Realpath(normalizedTargetFile)
+		}
+	})
+
+	b.Run("link", func(b *testing.B) {
+		b.ReportAllocs()
+
+		for b.Loop() {
+			fs.Realpath(normalizedLinkFile)
+		}
+	})
+}
+
 func TestGetAccessibleEntries(t *testing.T) {
 	t.Parallel()
 

internal/vfs/osvfs/realpath_windows.go

@@ -11,11 +11,25 @@ import (
 // This implementation is based on what Node's fs.realpath.native does, via libuv: https://github.com/libuv/libuv/blob/ec5a4b54f7da7eeb01679005c615fee9633cdb3b/src/win/fs.c#L2937
 
 func realpath(path string) (string, error) {
-	h, err := openMetadata(path)
-	if err != nil {
-		return "", err
+	var h windows.Handle
+	if len(path) < 248 {
+		var err error
+		h, err = openMetadata(path)
+		if err != nil {
+			return "", err
+		}
+		defer windows.CloseHandle(h) //nolint:errcheck
+	} else {
+		// For long paths, defer to os.Open to run the path through fixLongPath.
+		f, err := os.Open(path)
+		if err != nil {
+			return "", err
+		}
+		defer f.Close()
+
+		// Works on directories too since https://go.dev/cl/405275.
+		h = windows.Handle(f.Fd())
 	}
-	defer windows.CloseHandle(h) //nolint:errcheck
 
 	// based on https://github.com/golang/go/blob/f4e3ec3dbe3b8e04a058d266adf8e048bab563f2/src/os/file_windows.go#L389