code: func (h *Handler) updateCache(request *Request) (*cacheEntry, error) {
  
  	updateAddr := request.Addr()
  	log.Trace("feed cache update", "topic", request.Topic.Hex(), "updateaddr", updateAddr, "epoch time", request.Epoch.Time, "epoch level", request.Epoch.Level)
  
  	feedUpdate := h.get(&request.Feed)
  	if feedUpdate == nil {
  		feedUpdate = &cacheEntry{}
  		h.set(&request.Feed, feedUpdate)
  	}
  
  	// update our rsrcs entry map
  	feedUpdate.lastKey = updateAddr
  	feedUpdate.Update = request.Update
  	feedUpdate.Reader = bytes.NewReader(feedUpdate.data)
  	return feedUpdate, nil
  }
docs: // update feed updates cache with specified content