code: func NewDiskCache(dir string) (Cache, error) {
  	if err := os.MkdirAll(dir, 0777); err != nil {
  		return nil, err
  	}
  	fs, err := vfs.FS(dir)
  	if err != nil {
  		return nil, err
  	}
  	chfs, err := vfs.Chroot("/", fs)
  	if err != nil {
  		return nil, err
  	}
  	return NewVFSCache(chfs), nil
  }
docs: // NewDiskCache returns a disk-backed cache