python asyncio取消run_in_executor阻塞主线程

Question

当通过异步run_in_executor启动同步功能，然后取消它时，我看到了一个不想要的行为。ThreadPoolExecutor (以及ProcessPoolExecutor)上下文管理器将在退出时调用executor.shutdown，并等待所有挂起的工作完成(在其线程上使用join() )。

我原本希望连接是异步管理的，这样其他任务就不会被阻塞，但我发现连接是同步调用的。

run_in_executor的实现应该考虑到这一点吗？或者这是应该处理的(就像在线程上调用executor.shutdown一样)？

这个示例代码启动了两个任务，并取消了在子线程上运行的任务，从日志中可以看到，在取消之后，子线程是唯一执行的代码：

from time import sleep
from asyncio import (
    get_running_loop, 
    sleep as async_sleep,
    get_event_loop,
    create_task
)
from concurrent.futures import ThreadPoolExecutor
from datetime import datetime
​
​
def sync_watchdog():
    for _ in range(30):
        print(f'{datetime.now()} I am in a thread')
        sleep(1)
​
​
async def inthread():
    with ThreadPoolExecutor() as executor:
        await get_running_loop().run_in_executor(executor, sync_watchdog)
​
async def watchdog():
    for _ in range(30):
        print(f'{datetime.now()} I am an async watchdog')
        await async_sleep(1)
​
​
async def run():
    intread_task = create_task(inthread())
    watchdog_task = create_task(watchdog())
​
    await async_sleep(2)
​
    print('canceling task with subthread')
    intread_task.cancel()
​
    await async_sleep(10)
​
​
get_event_loop().run_until_complete(run())

这将输出：

2021-10-27 17:08:16.796171 I am in a thread
2021-10-27 17:08:16.796248 I am an async watchdog
2021-10-27 17:08:17.797205 I am an async watchdog
2021-10-27 17:08:17.797310 I am in a thread
canceling task with subthread
2021-10-27 17:08:18.797990 I am an async watchdog
2021-10-27 17:08:18.798430 I am in a thread
2021-10-27 17:08:19.799037 I am in a thread
2021-10-27 17:08:20.800327 I am in a thread
2021-10-27 17:08:21.801521 I am in a thread
2021-10-27 17:08:22.802819 I am in a thread
2021-10-27 17:08:23.804014 I am in a thread
2021-10-27 17:08:24.805434 I am in a thread
2021-10-27 17:08:25.806640 I am in a thread
2021-10-27 17:08:26.807797 I am in a thread
2021-10-27 17:08:27.808880 I am in a thread
2021-10-27 17:08:28.810305 I am in a thread
2021-10-27 17:08:29.811247 I am in a thread
2021-10-27 17:08:30.811901 I am in a thread
2021-10-27 17:08:31.813083 I am in a thread
2021-10-27 17:08:32.814301 I am in a thread
2021-10-27 17:08:33.815499 I am in a thread
2021-10-27 17:08:34.816565 I am in a thread
2021-10-27 17:08:35.817920 I am in a thread
2021-10-27 17:08:36.818478 I am in a thread
2021-10-27 17:08:37.819603 I am in a thread
2021-10-27 17:08:38.820844 I am in a thread
2021-10-27 17:08:39.822042 I am in a thread
2021-10-27 17:08:40.822491 I am in a thread
2021-10-27 17:08:41.823690 I am in a thread
2021-10-27 17:08:42.824871 I am in a thread
2021-10-27 17:08:43.826012 I am in a thread
2021-10-27 17:08:44.826516 I am in a thread
2021-10-27 17:08:45.827698 I am in a thread
2021-10-27 17:08:46.829813 I am an async watchdog

Answer 1

长话短说：.shutdown()只取消尚未运行的挂起的执行器任务。

这是因为threading.Thread没有.cancel()或.abort()方法。如果您想要停止正在运行的线程中的循环，请创建一个标志，在每次迭代中检查它，如果需要停止，则提升该标志。