os: implement File.Chmod on Windows

Fixes: #39606

Change-Id: I4def67ef18bd3ff866b140f6e76cdabe5d51a1c5
Reviewed-on: https://go-review.googlesource.com/c/go/+/250077
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

Author: iwdgo

src/internal/poll/fd_posix.go

@@ -29,17 +29,6 @@ func (fd *FD) Shutdown(how int) error {
 	return syscall.Shutdown(fd.Sysfd, how)
 }
 
-// Fchmod wraps syscall.Fchmod.
-func (fd *FD) Fchmod(mode uint32) error {
-	if err := fd.incref(); err != nil {
-		return err
-	}
-	defer fd.decref()
-	return ignoringEINTR(func() error {
-		return syscall.Fchmod(fd.Sysfd, mode)
-	})
-}
-
 // Fchown wraps syscall.Fchown.
 func (fd *FD) Fchown(uid, gid int) error {
 	if err := fd.incref(); err != nil {

src/internal/poll/fd_unix.go

@@ -437,6 +437,17 @@ func (fd *FD) ReadDirent(buf []byte) (int, error) {
 	}
 }
 
+// Fchmod wraps syscall.Fchmod.
+func (fd *FD) Fchmod(mode uint32) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+	return ignoringEINTR(func() error {
+		return syscall.Fchmod(fd.Sysfd, mode)
+	})
+}
+
 // Fchdir wraps syscall.Fchdir.
 func (fd *FD) Fchdir() error {
 	if err := fd.incref(); err != nil {

src/internal/poll/fd_windows.go

@@ -886,6 +886,33 @@ func (fd *FD) FindNextFile(data *syscall.Win32finddata) error {
 	return syscall.FindNextFile(fd.Sysfd, data)
 }
 
+// Fchmod updates syscall.ByHandleFileInformation.Fileattributes when needed.
+func (fd *FD) Fchmod(mode uint32) error {
+	if err := fd.incref(); err != nil {
+		return err
+	}
+	defer fd.decref()
+
+	var d syscall.ByHandleFileInformation
+	if err := syscall.GetFileInformationByHandle(fd.Sysfd, &d); err != nil {
+		return err
+	}
+	attrs := d.FileAttributes
+	if mode&syscall.S_IWRITE != 0 {
+		attrs &^= syscall.FILE_ATTRIBUTE_READONLY
+	} else {
+		attrs |= syscall.FILE_ATTRIBUTE_READONLY
+	}
+	if attrs == d.FileAttributes {
+		return nil
+	}
+
+	var du windows.FILE_BASIC_INFO
+	du.FileAttributes = attrs
+	l := uint32(unsafe.Sizeof(d))
+	return windows.SetFileInformationByHandle(fd.Sysfd, windows.FileBasicInfo, uintptr(unsafe.Pointer(&du)), l)
+}
+
 // Fchdir wraps syscall.Fchdir.
 func (fd *FD) Fchdir() error {
 	if err := fd.incref(); err != nil {

src/internal/syscall/windows/syscall_windows.go

@@ -131,6 +131,14 @@ type IpAdapterAddresses struct {
 	/* more fields might be present here. */
 }
 
+type FILE_BASIC_INFO struct {
+	CreationTime   syscall.Filetime
+	LastAccessTime syscall.Filetime
+	LastWriteTime  syscall.Filetime
+	ChangedTime    syscall.Filetime
+	FileAttributes uint32
+}
+
 const (
 	IfOperStatusUp             = 1
 	IfOperStatusDown           = 2
@@ -145,6 +153,7 @@ const (
 //sys	GetComputerNameEx(nameformat uint32, buf *uint16, n *uint32) (err error) = GetComputerNameExW
 //sys	MoveFileEx(from *uint16, to *uint16, flags uint32) (err error) = MoveFileExW
 //sys	GetModuleFileName(module syscall.Handle, fn *uint16, len uint32) (n uint32, err error) = kernel32.GetModuleFileNameW
+//sys	SetFileInformationByHandle(handle syscall.Handle, fileInformationClass uint32, buf uintptr, bufsize uint32) (err error) = kernel32.SetFileInformationByHandle
 
 const (
 	WSA_FLAG_OVERLAPPED        = 0x01

src/internal/syscall/windows/zsyscall_windows.go

@@ -1099,29 +1099,34 @@ func checkMode(t *testing.T, path string, mode FileMode) {
 	if err != nil {
 		t.Fatalf("Stat %q (looking for mode %#o): %s", path, mode, err)
 	}
-	if dir.Mode()&0777 != mode {
+	if dir.Mode()&ModePerm != mode {
 		t.Errorf("Stat %q: mode %#o want %#o", path, dir.Mode(), mode)
 	}
 }
 
 func TestChmod(t *testing.T) {
-	// Chmod is not supported under windows.
-	if runtime.GOOS == "windows" {
-		return
-	}
 	f := newFile("TestChmod", t)
 	defer Remove(f.Name())
 	defer f.Close()
+	// Creation mode is read write
 
-	if err := Chmod(f.Name(), 0456); err != nil {
-		t.Fatalf("chmod %s 0456: %s", f.Name(), err)
+	fm := FileMode(0456)
+	if runtime.GOOS == "windows" {
+		fm = FileMode(0444) // read-only file
 	}
-	checkMode(t, f.Name(), 0456)
+	if err := Chmod(f.Name(), fm); err != nil {
+		t.Fatalf("chmod %s %#o: %s", f.Name(), fm, err)
+	}
+	checkMode(t, f.Name(), fm)
 
-	if err := f.Chmod(0123); err != nil {
-		t.Fatalf("chmod %s 0123: %s", f.Name(), err)
+	fm = FileMode(0123)
+	if runtime.GOOS == "windows" {
+		fm = FileMode(0666) // read-write file
+	}
+	if err := f.Chmod(fm); err != nil {
+		t.Fatalf("chmod %s %#o: %s", f.Name(), fm, err)
 	}
-	checkMode(t, f.Name(), 0123)
+	checkMode(t, f.Name(), fm)
 }
 
 func checkSize(t *testing.T, f *File, size int64) {