code: func (pa *AuthorityImpl) loadHostnamePolicy(unmarshal unmarshalHandler) func([]byte) error {
  	// Return a reloader dataCallback that uses the provided unmarshalHandler.
  	return func(contents []byte) error {
  		hash := sha256.Sum256(contents)
  		pa.log.Infof("loading hostname policy, sha256: %s", hex.EncodeToString(hash[:]))
  		var policy blockedNamesPolicy
  		err := unmarshal(contents, &policy)
  		if err != nil {
  			return err
  		}
  		if len(policy.HighRiskBlockedNames) == 0 {
  			return fmt.Errorf("No entries in HighRiskBlockedNames.")
  		}
  		if len(policy.ExactBlockedNames) == 0 {
  			return fmt.Errorf("No entries in ExactBlockedNames.")
  		}
  		return pa.processHostnamePolicy(policy)
  	}
  }
docs: // loadHostnamePolicy returns a reloader dataCallback function that uses the
  // unmarshalHandler to load a hostname policy. The returned callback is suitable
  // for use with reloader.New()