从同步代码安排任务到运行事件循环

Question

考虑这个程序，其中主循环和停止它的协程实际上是由我正在使用的库实现的。

import asyncio
import signal

running = True

async def stop():
    global running
    print("setting false")
    running = False
    await asyncio.sleep(3)
    print("reached end")

async def mainloop():
    while running:
        print("loop")
        await asyncio.sleep(1)

def handle_signal():
    loop.create_task(stop())

loop = asyncio.get_event_loop()
loop.add_signal_handler(signal.SIGINT, handle_signal)
loop.run_until_complete(mainloop())
loop.close()

当程序收到信号时，我需要调用停止协程来停止主循环。虽然在使用asyncio.BaseEventLoop.create_task 调度停止协程时，它首先停止了停止事件循环的主循环，并且停止协程无法完成：

$ ./test.py 
loop
loop
loop
^Csetting false
Task was destroyed but it is pending!
task: <Task pending coro=<stop() done, defined at ./test.py:7> wait_for=<Future pending cb=[Task._wakeup()]>>

如何将协程添加到正在运行的事件循环中，同时让事件循环等待完成？

Answer 1

正如您所发现的，问题在于事件循环只等待mainloop() 完成，而留下stop() 待处理，asyncio 正确地抱怨了这一点。

如果handle_signal 和顶层代码在您的控制之下，您可以轻松地将循环直到mainloop 替换为循环直到自定义协程完成。这个协程会调用mainloop，然后等待清理代码完成：

# ... omitted definition of mainloop() and stop()

# list of tasks that must be waited for before we can actually exit
_cleanup = []

async def run():
    await mainloop()
    # wait for all _cleanup tasks to finish
    await asyncio.wait(_cleanup)

def handle_signal():
    # schedule stop() to run, and also add it to the list of
    # tasks run() must wait for before it is done
    _cleanup.append(loop.create_task(stop()))

loop = asyncio.get_event_loop()
loop.add_signal_handler(signal.SIGINT, handle_signal)
loop.run_until_complete(run())
loop.close()

另一个选项，不需要新的run() 协程（但仍需要修改后的handle_signal），是在mainloop 完成后发出第二个run_until_complete()：

# handle_signal and _cleanup defined as above

loop = asyncio.get_event_loop()
loop.add_signal_handler(signal.SIGINT, handle_signal)
loop.run_until_complete(mainloop())
if _cleanup:
    loop.run_until_complete(asyncio.wait(_cleanup))
loop.close()