os: add Root.Rename

For #67002

Change-Id: Ifb1042bc5ceaeea64296763319b24634bbcb0bf0
Reviewed-on: https://go-review.googlesource.com/c/go/+/659416
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>

api/next/67002.txt

@@ -3,3 +3,4 @@ pkg os, method (*Root) Chown(string, int, int) error #67002
 pkg os, method (*Root) Chtimes(string, time.Time, time.Time) error #67002
 pkg os, method (*Root) Lchown(string, int, int) error #67002
 pkg os, method (*Root) Readlink(string) (string, error) #67002
+pkg os, method (*Root) Rename(string, string) error #67002

doc/next/6-stdlib/99-minor/os/67002.md

@@ -5,3 +5,4 @@ The [os.Root] type supports the following additional methods:
   * [os.Root.Chtimes]
   * [os.Root.Lchown]
   * [os.Root.Readlink]
+  * [os.Root.Rename]

src/internal/syscall/unix/asm_darwin.s

@@ -27,3 +27,4 @@ TEXT ·libc_readlinkat_trampoline(SB),NOSPLIT,$0-0; JMP libc_readlinkat(SB)
 TEXT ·libc_mkdirat_trampoline(SB),NOSPLIT,$0-0; JMP libc_mkdirat(SB)
 TEXT ·libc_fchmodat_trampoline(SB),NOSPLIT,$0-0; JMP libc_fchmodat(SB)
 TEXT ·libc_fchownat_trampoline(SB),NOSPLIT,$0-0; JMP libc_fchownat(SB)
+TEXT ·libc_renameat_trampoline(SB),NOSPLIT,$0-0; JMP libc_renameat(SB)

src/internal/syscall/unix/asm_openbsd.s

@@ -18,3 +18,5 @@ TEXT ·libc_fchmodat_trampoline(SB),NOSPLIT,$0-0
         JMP	libc_fchmodat(SB)
 TEXT ·libc_fchownat_trampoline(SB),NOSPLIT,$0-0
         JMP	libc_fchownat(SB)
+TEXT ·libc_renameat_trampoline(SB),NOSPLIT,$0-0
+        JMP	libc_renameat(SB)

src/internal/syscall/unix/at.go

@@ -114,3 +114,25 @@ func Fchownat(dirfd int, path string, uid, gid int, flags int) error {
 	}
 	return nil
 }
+
+func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error {
+	oldp, err := syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	newp, err := syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall.Syscall6(renameatTrap,
+		uintptr(olddirfd),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		0,
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

src/internal/syscall/unix/at_darwin.go

@@ -102,3 +102,29 @@ func Fchownat(dirfd int, path string, uid, gid int, flags int) error {
 	}
 	return nil
 }
+
+func libc_renameat_trampoline()
+
+//go:cgo_import_dynamic libc_renameat renameat "/usr/lib/libSystem.B.dylib"
+
+func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error {
+	oldp, err := syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	newp, err := syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_renameat_trampoline),
+		uintptr(olddirfd),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		0,
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

src/internal/syscall/unix/at_libc.go

@@ -18,6 +18,7 @@ import (
 //go:linkname procMkdirat libc_mkdirat
 //go:linkname procFchmodat libc_fchmodat
 //go:linkname procFchownat libc_fchownat
+//go:linkname procRenameat libc_renameat
 
 var (
 	procFstatat,
@@ -26,7 +27,8 @@ var (
 	procReadlinkat,
 	procMkdirat,
 	procFchmodat,
-	procFchownat uintptr
+	procFchownat,
+	procRenameat uintptr
 )
 
 func Unlinkat(dirfd int, path string, flags int) error {
@@ -160,3 +162,25 @@ func Fchownat(dirfd int, path string, uid, gid int, flags int) error {
 	}
 	return nil
 }
+
+func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error {
+	oldp, err := syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	newp, err := syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall6(uintptr(unsafe.Pointer(&procRenameat)), 4,
+		uintptr(olddirfd),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		0,
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

src/internal/syscall/unix/at_openbsd.go

@@ -93,3 +93,29 @@ func Fchownat(dirfd int, path string, uid, gid int, flags int) error {
 	}
 	return nil
 }
+
+//go:cgo_import_dynamic libc_renameat renameat "libc.so"
+
+func libc_renameat_trampoline()
+
+func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error {
+	oldp, err := syscall.BytePtrFromString(oldpath)
+	if err != nil {
+		return err
+	}
+	newp, err := syscall.BytePtrFromString(newpath)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_renameat_trampoline),
+		uintptr(olddirfd),
+		uintptr(unsafe.Pointer(oldp)),
+		uintptr(newdirfd),
+		uintptr(unsafe.Pointer(newp)),
+		0,
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

src/internal/syscall/unix/at_solaris.go

@@ -17,6 +17,7 @@ func rawSyscall6(trap, nargs, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, e
 //go:cgo_import_dynamic libc_fchownat fchownat "libc.so"
 //go:cgo_import_dynamic libc_fstatat fstatat "libc.so"
 //go:cgo_import_dynamic libc_openat openat "libc.so"
+//go:cgo_import_dynamic libc_renameat renameat "libc.so"
 //go:cgo_import_dynamic libc_unlinkat unlinkat "libc.so"
 //go:cgo_import_dynamic libc_readlinkat readlinkat "libc.so"
 //go:cgo_import_dynamic libc_mkdirat mkdirat "libc.so"

src/internal/syscall/unix/at_sysnum_dragonfly.go

@@ -14,6 +14,7 @@ const (
 	mkdiratTrap    uintptr = syscall.SYS_MKDIRAT
 	fchmodatTrap   uintptr = syscall.SYS_FCHMODAT
 	fchownatTrap   uintptr = syscall.SYS_FCHOWNAT
+	renameatTrap   uintptr = syscall.SYS_RENAMEAT
 
 	AT_EACCESS          = 0x4
 	AT_FDCWD            = 0xfffafdcd

src/internal/syscall/unix/at_sysnum_freebsd.go

@@ -21,4 +21,5 @@ const (
 	mkdiratTrap        uintptr = syscall.SYS_MKDIRAT
 	fchmodatTrap       uintptr = syscall.SYS_FCHMODAT
 	fchownatTrap       uintptr = syscall.SYS_FCHOWNAT
+	renameatTrap       uintptr = syscall.SYS_RENAMEAT
 )

src/internal/syscall/unix/at_sysnum_netbsd.go

@@ -14,6 +14,7 @@ const (
 	mkdiratTrap    uintptr = syscall.SYS_MKDIRAT
 	fchmodatTrap   uintptr = syscall.SYS_FCHMODAT
 	fchownatTrap   uintptr = syscall.SYS_FCHOWNAT
+	renameatTrap   uintptr = syscall.SYS_RENAMEAT
 )
 
 const (

src/internal/syscall/unix/at_wasip1.go

@@ -111,6 +111,24 @@ func Fchownat(dirfd int, path string, uid, gid int, flags int) error {
 	return syscall.ENOSYS
 }
 
+//go:wasmimport wasi_snapshot_preview1 path_rename
+//go:noescape
+func path_rename(oldFd int32, oldPath *byte, oldPathLen size, newFd int32, newPath *byte, newPathLen size) syscall.Errno
+
+func Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) error {
+	if oldpath == "" || newpath == "" {
+		return syscall.EINVAL
+	}
+	return errnoErr(path_rename(
+		int32(olddirfd),
+		unsafe.StringData(oldpath),
+		size(len(oldpath)),
+		int32(newdirfd),
+		unsafe.StringData(newpath),
+		size(len(newpath)),
+	))
+}
+
 //go:wasmimport wasi_snapshot_preview1 path_create_directory
 //go:noescape
 func path_create_directory(fd int32, path *byte, pathLen size) syscall.Errno

src/internal/syscall/unix/renameat2_sysnum_linux.go

@@ -0,0 +1,16 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux && (loong64 || riscv64)
+
+package unix
+
+import "syscall"
+
+const (
+	// loong64 and riscv64 only have renameat2.
+	// renameat2 has an extra flags parameter.
+	// When called with a 0 flags it is identical to renameat.
+	renameatTrap uintptr = syscall.SYS_RENAMEAT2
+)

src/internal/syscall/unix/renameat_sysnum_linux.go

@@ -0,0 +1,13 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build linux && !(loong64 || riscv64)
+
+package unix
+
+import "syscall"
+
+const (
+	renameatTrap uintptr = syscall.SYS_RENAMEAT
+)

src/internal/syscall/windows/at_windows.go

@@ -254,3 +254,77 @@ func Deleteat(dirfd syscall.Handle, name string) error {
 	}
 	return err
 }
+
+func Renameat(olddirfd syscall.Handle, oldpath string, newdirfd syscall.Handle, newpath string) error {
+	objAttrs := &OBJECT_ATTRIBUTES{}
+	if err := objAttrs.init(olddirfd, oldpath); err != nil {
+		return err
+	}
+	var h syscall.Handle
+	err := NtOpenFile(
+		&h,
+		SYNCHRONIZE|DELETE,
+		objAttrs,
+		&IO_STATUS_BLOCK{},
+		FILE_SHARE_DELETE|FILE_SHARE_READ|FILE_SHARE_WRITE,
+		FILE_OPEN_REPARSE_POINT|FILE_OPEN_FOR_BACKUP_INTENT|FILE_SYNCHRONOUS_IO_NONALERT,
+	)
+	if err != nil {
+		return ntCreateFileError(err, 0)
+	}
+	defer syscall.CloseHandle(h)
+
+	renameInfoEx := FILE_RENAME_INFORMATION_EX{
+		Flags: FILE_RENAME_REPLACE_IF_EXISTS |
+			FILE_RENAME_POSIX_SEMANTICS,
+		RootDirectory: newdirfd,
+	}
+	p16, err := syscall.UTF16FromString(newpath)
+	if err != nil {
+		return err
+	}
+	if len(p16) > len(renameInfoEx.FileName) {
+		return syscall.EINVAL
+	}
+	copy(renameInfoEx.FileName[:], p16)
+	renameInfoEx.FileNameLength = uint32((len(p16) - 1) * 2)
+
+	const (
+		FileRenameInformation   = 10
+		FileRenameInformationEx = 65
+	)
+	err = NtSetInformationFile(
+		h,
+		&IO_STATUS_BLOCK{},
+		uintptr(unsafe.Pointer(&renameInfoEx)),
+		uint32(unsafe.Sizeof(FILE_RENAME_INFORMATION_EX{})),
+		FileRenameInformationEx,
+	)
+	if err == nil {
+		return nil
+	}
+
+	// If the prior rename failed, the filesystem might not support
+	// POSIX semantics (for example, FAT), or might not have implemented
+	// FILE_RENAME_INFORMATION_EX.
+	//
+	// Try again.
+	renameInfo := FILE_RENAME_INFORMATION{
+		ReplaceIfExists: true,
+		RootDirectory:   newdirfd,
+	}
+	copy(renameInfo.FileName[:], p16)
+	renameInfo.FileNameLength = renameInfoEx.FileNameLength
+
+	err = NtSetInformationFile(
+		h,
+		&IO_STATUS_BLOCK{},
+		uintptr(unsafe.Pointer(&renameInfo)),
+		uint32(unsafe.Sizeof(FILE_RENAME_INFORMATION{})),
+		FileRenameInformation,
+	)
+	if st, ok := err.(NTStatus); ok {
+		return st.Errno()
+	}
+	return err
+}

src/internal/syscall/windows/types_windows.go

@@ -216,3 +216,25 @@ const (
 	FILE_DISPOSITION_ON_CLOSE                  = 0x00000008
 	FILE_DISPOSITION_IGNORE_READONLY_ATTRIBUTE = 0x00000010
 )
+
+// Flags for FILE_RENAME_INFORMATION_EX.
+const (
+	FILE_RENAME_REPLACE_IF_EXISTS = 0x00000001
+	FILE_RENAME_POSIX_SEMANTICS   = 0x00000002
+)
+
+// https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_file_rename_information
+type FILE_RENAME_INFORMATION struct {
+	ReplaceIfExists bool
+	RootDirectory   syscall.Handle
+	FileNameLength  uint32
+	FileName        [syscall.MAX_PATH]uint16
+}
+
+// https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_file_rename_information
+type FILE_RENAME_INFORMATION_EX struct {
+	Flags          uint32
+	RootDirectory  syscall.Handle
+	FileNameLength uint32
+	FileName       [syscall.MAX_PATH]uint16
+}

src/os/root.go

@@ -199,6 +199,13 @@ func (r *Root) Readlink(name string) (string, error) {
 	return rootReadlink(r, name)
 }
 
+// Rename renames (moves) oldname to newname.
+// Both paths are relative to the root.
+// See [Rename] for more details.
+func (r *Root) Rename(oldname, newname string) error {
+	return rootRename(r, oldname, newname)
+}
+
 func (r *Root) logOpen(name string) {
 	if log := testlog.Logger(); log != nil {
 		// This won't be right if r's name has changed since it was opened,

src/os/root_noopenat.go

@@ -166,3 +166,17 @@ func rootReadlink(r *Root, name string) (string, error) {
 	}
 	return name, nil
 }
+
+func rootRename(r *Root, oldname, newname string) error {
+	if err := checkPathEscapesLstat(r, oldname); err != nil {
+		return &PathError{Op: "renameat", Path: oldname, Err: err}
+	}
+	if err := checkPathEscapesLstat(r, newname); err != nil {
+		return &PathError{Op: "renameat", Path: newname, Err: err}
+	}
+	err := Rename(joinPath(r.root.name, oldname), joinPath(r.root.name, newname))
+	if err != nil {
+		return &LinkError{"renameat", oldname, newname, underlyingError(err)}
+	}
+	return nil
+}

src/os/root_openat.go

@@ -138,6 +138,19 @@ func rootRemove(r *Root, name string) error {
 	return nil
 }
 
+func rootRename(r *Root, oldname, newname string) error {
+	_, err := doInRoot(r, oldname, func(oldparent sysfdType, oldname string) (struct{}, error) {
+		_, err := doInRoot(r, newname, func(newparent sysfdType, newname string) (struct{}, error) {
+			return struct{}{}, renameat(oldparent, oldname, newparent, newname)
+		})
+		return struct{}{}, err
+	})
+	if err != nil {
+		return &LinkError{"renameat", oldname, newname, err}
+	}
+	return err
+}
+
 // doInRoot performs an operation on a path in a Root.
 //
 // It opens the directory containing the final element of the path,

src/os/root_test.go

@@ -699,6 +699,91 @@ func TestRootReadlink(t *testing.T) {
 	}
 }
 
+// TestRootRenameFrom tests renaming the test case target to a known-good path.
+func TestRootRenameFrom(t *testing.T) {
+	want := []byte("target")
+	for _, test := range rootTestCases {
+		test.run(t, func(t *testing.T, target string, root *os.Root) {
+			if target != "" {
+				if err := os.WriteFile(target, want, 0o666); err != nil {
+					t.Fatal(err)
+				}
+			}
+			wantError := test.wantError
+			var linkTarget string
+			if test.ltarget != "" {
+				// Rename will rename the link, not the file linked to.
+				wantError = false
+				var err error
+				linkTarget, err = root.Readlink(test.ltarget)
+				if err != nil {
+					t.Fatalf("root.Readlink(%q) = %v, want success", test.ltarget, err)
+				}
+			}
+
+			const dstPath = "destination"
+
+			// Plan 9 doesn't allow cross-directory renames.
+			if runtime.GOOS == "plan9" && strings.Contains(test.open, "/") {
+				wantError = true
+			}
+
+			err := root.Rename(test.open, dstPath)
+			if errEndsTest(t, err, wantError, "root.Rename(%q, %q)", test.open, dstPath) {
+				return
+			}
+
+			if test.ltarget != "" {
+				got, err := os.Readlink(filepath.Join(root.Name(), dstPath))
+				if err != nil || got != linkTarget {
+					t.Errorf("os.Readlink(%q) = %q, %v, want %q", dstPath, got, err, linkTarget)
+				}
+			} else {
+				got, err := os.ReadFile(filepath.Join(root.Name(), dstPath))
+				if err != nil || !bytes.Equal(got, want) {
+					t.Errorf(`os.ReadFile(%q): read content %q, %v; want %q`, dstPath, string(got), err, string(want))
+				}
+			}
+		})
+	}
+}
+
+// TestRootRenameTo tests renaming a known-good path to the test case target.
+func TestRootRenameTo(t *testing.T) {
+	want := []byte("target")
+	for _, test := range rootTestCases {
+		test.run(t, func(t *testing.T, target string, root *os.Root) {
+			const srcPath = "source"
+			if err := os.WriteFile(filepath.Join(root.Name(), srcPath), want, 0o666); err != nil {
+				t.Fatal(err)
+			}
+
+			target = test.target
+			wantError := test.wantError
+			if test.ltarget != "" {
+				// Rename will overwrite the final link rather than follow it.
+				target = test.ltarget
+				wantError = false
+			}
+
+			// Plan 9 doesn't allow cross-directory renames.
+			if runtime.GOOS == "plan9" && strings.Contains(test.open, "/") {
+				wantError = true
+			}
+
+			err := root.Rename(srcPath, test.open)
+			if errEndsTest(t, err, wantError, "root.Rename(%q, %q)", srcPath, test.open) {
+				return
+			}
+
+			got, err := os.ReadFile(filepath.Join(root.Name(), target))
+			if err != nil || !bytes.Equal(got, want) {
+				t.Errorf(`os.ReadFile(%q): read content %q, %v; want %q`, target, string(got), err, string(want))
+			}
+		})
+	}
+}
+
 // A rootConsistencyTest is a test case comparing os.Root behavior with
 // the corresponding non-Root function.
 //
@@ -927,14 +1012,19 @@ func (test rootConsistencyTest) run(t *testing.T, f func(t *testing.T, path stri
 		}
 
 		if err1 != nil || err2 != nil {
-			e1, ok := err1.(*os.PathError)
-			if !ok {
-				t.Fatalf("with root, expected PathError; got: %v", err1)
-			}
-			e2, ok := err2.(*os.PathError)
-			if !ok {
-				t.Fatalf("without root, expected PathError; got: %v", err1)
+			underlyingError := func(how string, err error) error {
+				switch e := err1.(type) {
+				case *os.PathError:
+					return e.Err
+				case *os.LinkError:
+					return e.Err
+				default:
+					t.Fatalf("%v, expected PathError or LinkError; got: %v", how, err)
+				}
+				return nil
 			}
+			e1 := underlyingError("with root", err1)
+			e2 := underlyingError("without root", err1)
 			detailedErrorMismatch := false
 			if f := test.detailedErrorMismatch; f != nil {
 				detailedErrorMismatch = f(t)
@@ -943,9 +1033,9 @@ func (test rootConsistencyTest) run(t *testing.T, f func(t *testing.T, path stri
 				// Plan9 syscall errors aren't comparable.
 				detailedErrorMismatch = true
 			}
-			if !detailedErrorMismatch && e1.Err != e2.Err {
-				t.Errorf("with root:    err=%v", e1.Err)
-				t.Errorf("without root: err=%v", e2.Err)
+			if !detailedErrorMismatch && e1 != e2 {
+				t.Errorf("with root:    err=%v", e1)
+				t.Errorf("without root: err=%v", e2)
 				t.Errorf("want consistent results, got mismatch")
 			}
 		}
@@ -1110,6 +1200,64 @@ func TestRootConsistencyReadlink(t *testing.T) {
 	}
 }
 
+func TestRootConsistencyRename(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		// This test depends on moving files between directories.
+		t.Skip("Plan 9 does not support cross-directory renames")
+	}
+	// Run this test in two directions:
+	// Renaming the test path to a known-good path (from),
+	// and renaming a known-good path to the test path (to).
+	for _, name := range []string{"from", "to"} {
+		t.Run(name, func(t *testing.T) {
+			for _, test := range rootConsistencyTestCases {
+				if runtime.GOOS == "windows" {
+					// On Windows, Rename("/path/to/.", x) succeeds,
+					// because Windows cleans the path to just "/path/to".
+					// Root.Rename(".", x) fails as expected.
+					// Don't run this consistency test on Windows.
+					if test.open == "." || test.open == "./" {
+						continue
+					}
+				}
+
+				test.run(t, func(t *testing.T, path string, r *os.Root) (string, error) {
+					rename := os.Rename
+					lstat := os.Lstat
+					if r != nil {
+						rename = r.Rename
+						lstat = r.Lstat
+					}
+
+					otherPath := "other"
+					if r == nil {
+						otherPath = filepath.Join(t.TempDir(), otherPath)
+					}
+
+					var srcPath, dstPath string
+					if name == "from" {
+						srcPath = path
+						dstPath = otherPath
+					} else {
+						srcPath = otherPath
+						dstPath = path
+					}
+
+					if err := rename(srcPath, dstPath); err != nil {
+						return "", err
+					}
+					fi, err := lstat(dstPath)
+					if err != nil {
+						t.Errorf("stat(%q) after successful copy: %v", dstPath, err)
+						return "stat error", err
+					}
+					return fmt.Sprintf("name:%q size:%v mode:%v isdir:%v", fi.Name(), fi.Size(), fi.Mode(), fi.IsDir()), nil
+				})
+			}
+		})
+	}
+}
+
 func TestRootRenameAfterOpen(t *testing.T) {
 	switch runtime.GOOS {
 	case "windows":

src/os/root_unix.go

@@ -209,6 +209,10 @@ func removeat(fd int, name string) error {
 	return e
 }
 
+func renameat(oldfd int, oldname string, newfd int, newname string) error {
+	return unix.Renameat(oldfd, oldname, newfd, newname)
+}
+
 // checkSymlink resolves the symlink name in parent,
 // and returns errSymlink with the link contents.
 //

src/os/root_windows.go

@@ -315,6 +315,10 @@ func chtimesat(dirfd syscall.Handle, name string, atime time.Time, mtime time.Ti
 	return syscall.SetFileTime(h, nil, &a, &w)
 }
 
+func renameat(oldfd syscall.Handle, oldname string, newfd syscall.Handle, newname string) error {
+	return windows.Renameat(oldfd, oldname, newfd, newname)
+}
+
 func readlinkat(dirfd syscall.Handle, name string) (string, error) {
 	fd, err := openat(dirfd, name, windows.O_OPEN_REPARSE, 0)
 	if err != nil {