code: func (idx *CfIndex) entryByBlockHash(filterTypeKeys [][]byte,
  	filterType wire.FilterType, h *chainhash.Hash) ([]byte, error) {
  
  	if uint8(filterType) > maxFilterType {
  		return nil, errors.New("unsupported filter type")
  	}
  	key := filterTypeKeys[filterType]
  
  	var entry []byte
  	err := idx.db.View(func(dbTx database.Tx) error {
  		var err error
  		entry, err = dbFetchFilterIdxEntry(dbTx, key, h)
  		return err
  	})
  	return entry, err
  }
docs: // entryByBlockHash fetches a filter index entry of a particular type
  // (eg. filter, filter header, etc) for a filter type and block hash.