code: func (l *FileLock) TryExclusiveLock() error {
  	err := syscall.Flock(l.fd, syscall.LOCK_EX|syscall.LOCK_NB)
  	if err == syscall.EWOULDBLOCK {
  		err = ErrLocked
  	}
  	return err
  }
docs: // TryExclusiveLock takes an exclusive lock without blocking.
  // This is idempotent when the Lock already represents an exclusive lock,
  // and tries promote a shared lock to exclusive atomically.
  // It will return ErrLocked if any lock is already held.