[BUG发现]回测代码段重复执行两遍，第二次就无法正常交易下单了无反馈，无交易了。代码可以直接测试复现。

from datetime import date
from contextlib import closing
from tqsdk import TqApi, TqSim, TqBacktest, TargetPosTask
import sys
 def test():
    # 在创建 api 实例时传入 TqBacktest 就会进入回测模式
    api = TqApi(TqSim(), backtest=TqBacktest(start_dt=date(2018, 9, 1), end_dt=date(2018, 10, 1)))
    # 获得 m1901 5分钟K线的引用
    klines = api.get_kline_serial("DCE.m1901", 60 * 60, data_length=15)
    # 创建 m1901 的目标持仓 task，该 task 负责调整 m1901 的仓位到指定的目标仓位
    target_pos = TargetPosTask(api, "DCE.m1901")
     # 使用with closing机制确保回测完成后释放对应的资源
    with closing(api):
        while True:
            api.wait_update()
            if api.is_changing(klines):
                ma = sum(klines.close.iloc[-15:]) / 15
                #print("最新价", klines.close.iloc[-1], "MA", ma)
                if klines.close.iloc[-1] > ma:
                    #print("最新价大于MA: 目标多头5手")
                    # 设置目标持仓为多头5手
                    target_pos.set_target_volume(5)
                elif klines.close.iloc[-1] < ma:
                    #print("最新价小于MA: 目标空仓")
                    # 设置目标持仓为空仓
                    target_pos.set_target_volume(0)
                sys.stdout.flush()
    if api != None:
        del api
        api = None
    if klines != None:
        del klines
        klines = None
    if target_pos != None:
        del target_pos
        target_pos = None
 if __name__ == '__main__':
    try:
        test()
    except:
        print("Unexpected error1:", sys.exc_info()[0])
        sys.stdout.flush()
    print(" 1:", 1)
    sys.stdout.flush()
    try:
        test()
    except:
        print("Unexpected error2:", sys.exc_info()[0])
        sys.stdout.flush()
    print(" 2:", 2)
    sys.stdout.flush()