os: add Root.Chtimes

For #67002

Change-Id: I9b10ac30f852052c85d6d21eb1752a9de5474346
Reviewed-on: https://go-review.googlesource.com/c/go/+/649515
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Kirill Kolyshkin <kolyshkin@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

api/next/67002.txt

@@ -1,2 +1,3 @@
 pkg os, method (*Root) Chmod(string, fs.FileMode) error #67002
 pkg os, method (*Root) Chown(string, int, int) error #67002
+pkg os, method (*Root) Chtimes(string, time.Time, time.Time) error #67002

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

@@ -2,3 +2,4 @@ The [os.Root] type supports the following additional methods:
 
   * [os.Root.Chmod]
   * [os.Root.Chown]
+  * [os.Root.Chtimes]

src/internal/syscall/unix/utimes.go

@@ -0,0 +1,15 @@
+// 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 unix && !wasip1
+
+package unix
+
+import (
+	"syscall"
+	_ "unsafe" // for //go:linkname
+)
+
+//go:linkname Utimensat syscall.utimensat
+func Utimensat(dirfd int, path string, times *[2]syscall.Timespec, flag int) error

src/internal/syscall/unix/utimes_wasip1.go

@@ -0,0 +1,42 @@
+// 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 wasip1
+
+package unix
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+//go:wasmimport wasi_snapshot_preview1 path_filestat_set_times
+//go:noescape
+func path_filestat_set_times(fd int32, flags uint32, path *byte, pathLen size, atim uint64, mtim uint64, fstflags uint32) syscall.Errno
+
+func Utimensat(dirfd int, path string, times *[2]syscall.Timespec, flag int) error {
+	if path == "" {
+		return syscall.EINVAL
+	}
+	atime := syscall.TimespecToNsec(times[0])
+	mtime := syscall.TimespecToNsec(times[1])
+
+	var fflag uint32
+	if times[0].Nsec != UTIME_OMIT {
+		fflag |= syscall.FILESTAT_SET_ATIM
+	}
+	if times[1].Nsec != UTIME_OMIT {
+		fflag |= syscall.FILESTAT_SET_MTIM
+	}
+	errno := path_filestat_set_times(
+		int32(dirfd),
+		syscall.LOOKUP_SYMLINK_FOLLOW,
+		unsafe.StringData(path),
+		size(len(path)),
+		uint64(atime),
+		uint64(mtime),
+		fflag,
+	)
+	return errnoErr(errno)
+}

src/os/file_posix.go

@@ -177,6 +177,14 @@ func (f *File) Sync() error {
 // less precise time unit.
 // If there is an error, it will be of type [*PathError].
 func Chtimes(name string, atime time.Time, mtime time.Time) error {
+	utimes := chtimesUtimes(atime, mtime)
+	if e := syscall.UtimesNano(fixLongPath(name), utimes[0:]); e != nil {
+		return &PathError{Op: "chtimes", Path: name, Err: e}
+	}
+	return nil
+}
+
+func chtimesUtimes(atime, mtime time.Time) [2]syscall.Timespec {
 	var utimes [2]syscall.Timespec
 	set := func(i int, t time.Time) {
 		if t.IsZero() {
@@ -187,10 +195,7 @@ func Chtimes(name string, atime time.Time, mtime time.Time) error {
 	}
 	set(0, atime)
 	set(1, mtime)
-	if e := syscall.UtimesNano(fixLongPath(name), utimes[0:]); e != nil {
-		return &PathError{Op: "chtimes", Path: name, Err: e}
-	}
-	return nil
+	return utimes
 }
 
 // Chdir changes the current working directory to the file,

src/os/root.go

@@ -12,6 +12,7 @@ import (
 	"io/fs"
 	"runtime"
 	"slices"
+	"time"
 )
 
 // OpenInRoot opens the file name in the directory dir.
@@ -54,7 +55,7 @@ func OpenInRoot(dir, name string) (*File, error) {
 //
 //   - When GOOS=windows, file names may not reference Windows reserved device names
 //     such as NUL and COM1.
-//   - On Unix, [Root.Chmod] and [Root.Chown] are vulnerable to a race condition.
+//   - On Unix, [Root.Chmod], [Root.Chown], and [Root.Chtimes] are vulnerable to a race condition.
 //     If the target of the operation is changed from a regular file to a symlink
 //     while the operation is in progress, the operation may be peformed on the link
 //     rather than the link target.
@@ -158,6 +159,12 @@ func (r *Root) Chown(name string, uid, gid int) error {
 	return rootChown(r, name, uid, gid)
 }
 
+// Chtimes changes the access and modification times of the named file in the root.
+// See [Chtimes] for more details.
+func (r *Root) Chtimes(name string, atime time.Time, mtime time.Time) error {
+	return rootChtimes(r, name, atime, mtime)
+}
+
 // Remove removes the named file or (empty) directory in the root.
 // See [Remove] for more details.
 func (r *Root) Remove(name string) error {

src/os/root_noopenat.go

@@ -9,6 +9,7 @@ package os
 import (
 	"errors"
 	"sync/atomic"
+	"time"
 )
 
 // root implementation for platforms with no openat.
@@ -115,6 +116,16 @@ func rootChown(r *Root, name string, uid, gid int) error {
 	return nil
 }
 
+func rootChtimes(r *Root, name string, atime time.Time, mtime time.Time) error {
+	if err := checkPathEscapes(r, name); err != nil {
+		return &PathError{Op: "chtimesat", Path: name, Err: err}
+	}
+	if err := Chtimes(joinPath(r.root.name, name), atime, mtime); err != nil {
+		return &PathError{Op: "chtimesat", Path: name, Err: underlyingError(err)}
+	}
+	return nil
+}
+
 func rootMkdir(r *Root, name string, perm FileMode) error {
 	if err := checkPathEscapes(r, name); err != nil {
 		return &PathError{Op: "mkdirat", Path: name, Err: err}

src/os/root_openat.go

@@ -11,6 +11,7 @@ import (
 	"slices"
 	"sync"
 	"syscall"
+	"time"
 )
 
 // root implementation for platforms with a function to open a file
@@ -87,6 +88,16 @@ func rootChown(r *Root, name string, uid, gid int) error {
 	return nil
 }
 
+func rootChtimes(r *Root, name string, atime time.Time, mtime time.Time) error {
+	_, err := doInRoot(r, name, func(parent sysfdType, name string) (struct{}, error) {
+		return struct{}{}, chtimesat(parent, name, atime, mtime)
+	})
+	if err != nil {
+		return &PathError{Op: "chtimesat", Path: name, Err: err}
+	}
+	return err
+}
+
 func rootMkdir(r *Root, name string, perm FileMode) error {
 	_, err := doInRoot(r, name, func(parent sysfdType, name string) (struct{}, error) {
 		return struct{}{}, mkdirat(parent, name, perm)

src/os/root_test.go

@@ -426,6 +426,53 @@ func TestRootChmod(t *testing.T) {
 	}
 }
 
+func TestRootChtimes(t *testing.T) {
+	for _, test := range rootTestCases {
+		test.run(t, func(t *testing.T, target string, root *os.Root) {
+			if target != "" {
+				if err := os.WriteFile(target, nil, 0o666); err != nil {
+					t.Fatal(err)
+				}
+			}
+			for _, times := range []struct {
+				atime, mtime time.Time
+			}{{
+				atime: time.Now().Add(-1 * time.Minute),
+				mtime: time.Now().Add(-1 * time.Minute),
+			}, {
+				atime: time.Now().Add(1 * time.Minute),
+				mtime: time.Now().Add(1 * time.Minute),
+			}, {
+				atime: time.Time{},
+				mtime: time.Now(),
+			}, {
+				atime: time.Now(),
+				mtime: time.Time{},
+			}} {
+				if runtime.GOOS == "js" {
+					times.atime = times.atime.Truncate(1 * time.Second)
+					times.mtime = times.mtime.Truncate(1 * time.Second)
+				}
+
+				err := root.Chtimes(test.open, times.atime, times.mtime)
+				if errEndsTest(t, err, test.wantError, "root.Chtimes(%q)", test.open) {
+					return
+				}
+				st, err := os.Stat(target)
+				if err != nil {
+					t.Fatalf("os.Stat(%q) = %v", target, err)
+				}
+				if got := st.ModTime(); !times.mtime.IsZero() && !got.Equal(times.mtime) {
+					t.Errorf("after root.Chtimes(%q, %v, %v): got mtime=%v, want %v", test.open, times.atime, times.mtime, got, times.mtime)
+				}
+				if got := os.Atime(st); !times.atime.IsZero() && !got.Equal(times.atime) {
+					t.Errorf("after root.Chtimes(%q, %v, %v): got atime=%v, want %v", test.open, times.atime, times.mtime, got, times.atime)
+				}
+			}
+		})
+	}
+}
+
 func TestRootMkdir(t *testing.T) {
 	for _, test := range rootTestCases {
 		test.run(t, func(t *testing.T, target string, root *os.Root) {

src/os/root_unix.go

@@ -11,6 +11,7 @@ import (
 	"internal/syscall/unix"
 	"runtime"
 	"syscall"
+	"time"
 )
 
 type sysfdType = int
@@ -165,6 +166,15 @@ func chownat(parent int, name string, uid, gid int) error {
 	})
 }
 
+func chtimesat(parent int, name string, atime time.Time, mtime time.Time) error {
+	return afterResolvingSymlink(parent, name, func() error {
+		return ignoringEINTR(func() error {
+			utimes := chtimesUtimes(atime, mtime)
+			return unix.Utimensat(parent, name, &utimes, unix.AT_SYMLINK_NOFOLLOW)
+		})
+	})
+}
+
 func mkdirat(fd int, name string, perm FileMode) error {
 	return ignoringEINTR(func() error {
 		return unix.Mkdirat(fd, name, syscallMode(perm))

src/os/root_windows.go

@@ -13,6 +13,7 @@ import (
 	"internal/syscall/windows"
 	"runtime"
 	"syscall"
+	"time"
 	"unsafe"
 )
 
@@ -287,3 +288,25 @@ func mkdirat(dirfd syscall.Handle, name string, perm FileMode) error {
 func removeat(dirfd syscall.Handle, name string) error {
 	return windows.Deleteat(dirfd, name)
 }
+
+func chtimesat(dirfd syscall.Handle, name string, atime time.Time, mtime time.Time) error {
+	h, err := windows.Openat(dirfd, name, syscall.O_CLOEXEC|windows.O_NOFOLLOW_ANY|windows.O_WRITE_ATTRS, 0)
+	if err == syscall.ELOOP || err == syscall.ENOTDIR {
+		if link, err := readReparseLinkAt(dirfd, name); err == nil {
+			return errSymlink(link)
+		}
+	}
+	if err != nil {
+		return err
+	}
+	defer syscall.CloseHandle(h)
+	a := syscall.Filetime{}
+	w := syscall.Filetime{}
+	if !atime.IsZero() {
+		a = syscall.NsecToFiletime(atime.UnixNano())
+	}
+	if !mtime.IsZero() {
+		w = syscall.NsecToFiletime(mtime.UnixNano())
+	}
+	return syscall.SetFileTime(h, nil, &a, &w)
+}