func openLast(lg *zap.Logger, dirpath string) (*fileutil.LockedFile, error) {
	names, err := readWALNames(lg, dirpath)
	if err != nil {
		return nil, err
	}
	last := filepath.Join(dirpath, names[len(names)-1])
	return fileutil.LockFile(last, os.O_RDWR, fileutil.PrivateFileMode)
}
func openLast(lg *zap.Logger, dirpath string) (*fileutil.LockedFile, error) {
	names, err := readWALNames(lg, dirpath)
	if err != nil {
		return nil, err
	}
	last := filepath.Join(dirpath, names[len(names)-1])
	return fileutil.LockFile(last, os.O_RDWR, fileutil.PrivateFileMode)
}
// openLast opens the last wal file for read and write.