code: func FSTryWriteLock(f *os.File) error {
  	err := syscall.Flock(int(f.Fd()), syscall.LOCK_EX|syscall.LOCK_NB)
  	if err != nil {
  		if err == syscall.EWOULDBLOCK {
  			return trace.CompareFailed("lock %v is acquired by another process", f.Name())
  		}
  		return trace.ConvertSystemError(err)
  	}
  	return nil
  }
docs: // FSTryWriteLock tries to grab write lock, returns CompareFailed
  // if lock is already grabbed