code: func (s *ChainService) getFilterFromCache(blockHash *chainhash.Hash,
  	filterType filterdb.FilterType) (*gcs.Filter, error) {
  
  	cacheKey := cache.FilterCacheKey{*blockHash, filterType}
  
  	filterValue, err := s.FilterCache.Get(cacheKey)
  	if err != nil {
  		return nil, err
  	}
  
  	return filterValue.(*cache.CacheableFilter).Filter, nil
  }
docs: // getFilterFromCache returns a filter from ChainService's FilterCache if it
  // exists, returning nil and error if it doesn't.