code: func (bs *blobStore) link(ctx context.Context, path string, dgst digest.Digest) error {
  	// The contents of the "link" file are the exact string contents of the
  	// digest, which is specified in that package.
  	return bs.driver.PutContent(ctx, path, []byte(dgst))
  }
docs: // link links the path to the provided digest by writing the digest into the
  // target file. Caller must ensure that the blob actually exists.