code: func (pm *ProtocolManager) getHelperTrieAuxData(req HelperTrieReq) []byte {
  	if req.Type == htCanonical && req.AuxReq == auxHeader && len(req.Key) == 8 {
  		blockNum := binary.BigEndian.Uint64(req.Key)
  		hash := rawdb.ReadCanonicalHash(pm.chainDb, blockNum)
  		return rawdb.ReadHeaderRLP(pm.chainDb, hash, blockNum)
  	}
  	return nil
  }
docs: // getHelperTrieAuxData returns requested auxiliary data for the given HelperTrie request