比如我要开多头5手，但是现在有-3手空头，这样只会平掉这3个空头，没有再补开2手多头
如果同一个vt_symbol有多种不同策略在交易，那这样子所有策略的持仓都会乱掉了

    def send_order(
        self,
        algo: SpreadAlgoTemplate,
        vt_symbol: str,
        price: float,
        volume: float,
        direction: Direction,
        lock: bool
    ) -> List[str]:
        """"""
        holding = self.offset_converter.get_position_holding(vt_symbol)
        contract = self.main_engine.get_contract(vt_symbol)

        if direction == Direction.LONG:
            available = holding.short_pos - holding.short_pos_frozen
        else:
            available = holding.long_pos - holding.long_pos_frozen

        # If no position to close, just open new
        if not available:
            offset = Offset.OPEN
        # If enougth position to close, just close old
        elif volume < available:
            offset = Offset.CLOSE
        # Otherwise, just close existing position
        else:
            volume = available
            offset = Offset.CLOSE

帖子删不掉？
看了SpreadTakerAlgo的代码，如果只平掉了3手，后面会再重新下2手的单，从更上层解决了这个问题。