code: func handleGetBlockHash(s *rpcServer, cmd interface{}, closeChan <-chan struct{}) (interface{}, error) {
  	c := cmd.(*btcjson.GetBlockHashCmd)
  	hash, err := s.cfg.Chain.BlockHashByHeight(int32(c.Index))
  	if err != nil {
  		return nil, &btcjson.RPCError{
  			Code:    btcjson.ErrRPCOutOfRange,
  			Message: "Block number out of range",
  		}
  	}
  
  	return hash.String(), nil
  }
docs: // handleGetBlockHash implements the getblockhash command.