线程和 asyncio：任务已销毁，但处于挂起状态

Posted 2023-03-14

【中文标题】线程和 asyncio：任务已销毁，但处于挂起状态

【英文标题】：Thread and asyncio: Task was destroyed but it is pending

【发布时间】：2017-11-27 15:09:16

【问题描述】：

我有一个运行异步循环的线程。我开始了一个未来的任务，做与这里无关的事情。当我停止线程时，我也停止了 asyncio 循环。但是，我似乎无法取消池任务并获得Task was destroyed but it is pending!

这是一个玩具示例：

from contextlib import suppress
from threading import Thread
from time import sleep
import asyncio


class Hardware(Thread):

    def __init__(self, *args, **kwargs):
        super(Hardware, self).__init__(*args, **kwargs)
        self.loop = None
        self._poll_task = None

    def run(self):
        self.loop = asyncio.new_event_loop()
        asyncio.set_event_loop(self.loop)
        self.loop.create_task(self._poll())
        self.loop.run_forever()

    async def _poll(self):
        print('ook')
        await asyncio.sleep(1.0)
        self._poll_task = asyncio.ensure_future(self._poll())
        return self._poll_task

    def stop(self):
        if self._poll_task is not None:
            self.loop.call_soon_threadsafe(self._poll_task.cancel)
        with suppress(asyncio.CancelledError):
            self.loop.call_soon_threadsafe(self.loop.stop)


hw = Hardware()
try:
    hw.start()
    while True:
        sleep(.1)
except KeyboardInterrupt:
    hw.stop()
    hw.join()

运行它输出：

; python ook.py
ook
ook
^CTask was destroyed but it is pending!
task: <Task pending coro=<Hardware._poll() running at ook.py:22> wait_for=<Future cancelled>>  

我做错了什么？

【问题讨论】：

我对代码进行了一些重构，以便更好地启动/关闭。

【参考方案1】：

你不仅应该在任务上调用cancel()，还应该调用await its cancellation，而不是像你一样只是停止循环。

from contextlib import suppress
from threading import Thread
from time import sleep
import asyncio


class Hardware(Thread):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.loop = None
        self._poll_task = None

    def run(self):
        self.loop = asyncio.new_event_loop()
        loop = self.loop
        asyncio.set_event_loop(loop)
        try:
            # create task:
            self._poll_task = asyncio.ensure_future(self._poll())

            # run loop:
            loop.run_forever()
            loop.run_until_complete(loop.shutdown_asyncgens())

            # cancel task:
            self._poll_task.cancel()
            with suppress(asyncio.CancelledError):
                loop.run_until_complete(self._poll_task)
        finally:
            loop.close()

    def stop(self):
        self.loop.call_soon_threadsafe(self.loop.stop)

    async def _poll(self):
        while True:  # you don't need to create new task each time
            print('ook')
            await asyncio.sleep(1.0)


hw = Hardware()
try:
    hw.start()
    while True:
        sleep(.1)
except KeyboardInterrupt:
    hw.stop()
    hw.join()