code: func funlock(db *DB) error {
  	var lock syscall.Flock_t
  	lock.Start = 0
  	lock.Len = 0
  	lock.Type = syscall.F_UNLCK
  	lock.Whence = 0
  	return syscall.FcntlFlock(uintptr(db.file.Fd()), syscall.F_SETLK, &lock)
  }
docs: // funlock releases an advisory lock on a file descriptor.