Fix test #3

go/runfiles/fs.go

@@ -17,6 +17,7 @@ package runfiles
 import (
 	"io"
 	"io/fs"
+	"runtime"
 	"sort"
 	"strings"
 	"time"
@@ -29,7 +30,8 @@ import (
 // apparent repo names that are visible to the current source repo
 // (with --enable_bzlmod).
 func (r *Runfiles) Open(name string) (fs.File, error) {
-	if !fs.ValidPath(name) {
+	// Required by testfs.TestFS.
+	if !fs.ValidPath(name) || (runtime.GOOS == "windows" && strings.ContainsRune(name, '\\')) {
 		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrInvalid}
 	}
 	if name == "." {

go/runfiles/manifest.go

@@ -121,9 +121,7 @@ func (m *manifest) open(name string) (fs.File, error) {
 		if err == ErrEmpty {
 			return emptyFile(name), nil
 		} else if err == nil {
-			// Go through os.DirFs to throw the correct error on invalid paths.
-			volumePrefix := filepath.VolumeName(r) + "/"
-			return os.DirFS(volumePrefix).Open(filepath.ToSlash(r[len(volumePrefix):]))
+			return os.Open(r)
 		}
 	}