python asyncio cancel run_in_executor 阻塞主线程

Question

通过 asyncio run_in_executor 启动同步功能然后取消它时，我看到了不想要的行为。 ThreadPoolExecutor（以及 ProcessPoolExecutor）上下文管理器将在退出时调用 executor.shutdown 并等待所有待处理的工作完成（在其线程上使用 join()）。

我原以为连接是异步管理的，所以其他任务可能不会被阻塞，但我发现连接是同步调用的。

run_in_executor 的实现应该考虑这个吗？还是应该处理一些事情（比如在线程上调用 executor.shutdown）？

此示例代码启动了2个任务，并取消了在子线程上运行的一个，从日志中可以看出，子线程是取消后唯一正在执行的代码：

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() 仅取消 待处理 执行程序任务，尚未运行。

那是因为threading.Thread 没有.cancel() 或.abort() 方法。如果您想在正在运行的线程中停止循环，请创建一个标志，在每次迭代中检查它，如果您需要停止，请提高标志。