// the directory itself, if the dirPath doesn't exist this function doesn't return
// an error.
func readDirFn(dirPath string, fn func(name string, typ os.FileMode) error) error {
	fd, err := openFileWithFD(dirPath, readMode, 0o666)
	if err != nil {
		if osErrToFileErr(err) == errFileNotFound {
			return nil
		}
		if !osIsPermission(err) {
			return osErrToFileErr(err)
		}
		// There may be permission error when dirPath