code: func (l *FileLock) ExclusiveLock() error {
  	return syscall.Flock(l.fd, syscall.LOCK_EX)
  }
docs: // ExclusiveLock takes an exclusive lock.
  // This is idempotent when the Lock already represents an exclusive lock,
  // and promotes a shared lock to exclusive atomically.
  // It will block if an exclusive lock is already held.