os: document that Chown with -1 means to leave values unchanged, like POSIX

And fix the nacl implementation.

Fixes #24710

Change-Id: I31ffeea03a72dac5021ffb183fde31e9ffd060ad
Reviewed-on: https://go-review.googlesource.com/106464Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/file_plan9.go

@@ -451,7 +451,11 @@ func Readlink(name string) (string, error) {
 
 // Chown changes the numeric uid and gid of the named file.
 // If the file is a symbolic link, it changes the uid and gid of the link's target.
+// A uid or gid of -1 means to not change that value.
 // If there is an error, it will be of type *PathError.
+//
+// On Windows or Plan 9, Chown always returns the syscall.EWINDOWS or
+// EPLAN9 error, wrapped in *PathError.
 func Chown(name string, uid, gid int) error {
 	return &PathError{"chown", name, syscall.EPLAN9}
 }

src/os/file_posix.go

@@ -65,10 +65,11 @@ func (f *File) chmod(mode FileMode) error {
 
 // Chown changes the numeric uid and gid of the named file.
 // If the file is a symbolic link, it changes the uid and gid of the link's target.
+// A uid or gid of -1 means to not change that value.
 // If there is an error, it will be of type *PathError.
 //
-// On Windows, it always returns the syscall.EWINDOWS error, wrapped
-// in *PathError.
+// On Windows or Plan 9, Chown always returns the syscall.EWINDOWS or
+// EPLAN9 error, wrapped in *PathError.
 func Chown(name string, uid, gid int) error {
 	if e := syscall.Chown(name, uid, gid); e != nil {
 		return &PathError{"chown", name, e}

src/syscall/fs_nacl.go

@@ -582,8 +582,12 @@ func Chown(path string, uid, gid int) error {
 	if err != nil {
 		return err
 	}
+	if uid != -1 {
 		ip.Uid = uint32(uid)
+	}
+	if gid != -1 {
 		ip.Gid = uint32(gid)
+	}
 	return nil
 }