立即从同步代码执行异步回调

Question

Problem

I have a library which currently has no async support and needs to be called from async code. The async code calls into the library through a handler ( handler function in the code below). While the handler executed, the library periodically calls a callback ( callback_wrapper ) to report progress.

The synchronous handler is executed in a ThreadPoolExecutor in order for the main event loop to be able to process further events while the handler is running.

What happens is that the synchronous callback is executed immediately, but the async callback is only executed after the main handler has executed. The desired result is the async callbacks to be executed immediately.

I guess the event loop is blocked at the run_in_executor call, but I am not sure how to resolve this.

Code

import asyncio
import time
from concurrent.futures.thread import ThreadPoolExecutor

loop = asyncio.get_event_loop()


def handler():
    print('handler started')
    callback_wrapper()
    time.sleep(1)
    print('handler stopped')


async def callback():
    print('callback')


def callback_wrapper():
    print('callback wrapper started')
    asyncio.ensure_future(callback(), loop=loop)
    print('callback wrapper stopped')


async def main():
    handler()


with ThreadPoolExecutor() as pool:
    async def thread_handler():
        await loop.run_in_executor(pool, handler)


    loop.run_until_complete(main())

Output

handler started
callback wrapper started
callback wrapper stopped
handler stopped
callback

Desired Output

handler started
callback wrapper started
callback
callback wrapper stopped
handler stopped

Answer 1

thanks to @user4815162342's input, I came up the the following solution:

import asyncio
import time
from concurrent.futures.thread import ThreadPoolExecutor

loop = asyncio.get_event_loop()


def handler():
    print('handler started')
    callback_wrapper()
    time.sleep(1)
    print('handler stopped')


async def callback():
    print('callback')


def callback_wrapper():
    print('callback wrapper started')
    asyncio.run_coroutine_threadsafe(callback(), loop).result()
    print('callback wrapper stopped')


async def main():
    await thread_handler()


with ThreadPoolExecutor() as pool:
    async def thread_handler():
        await loop.run_in_executor(pool, handler)


    loop.run_until_complete(main())

which produces the desired result:

handler started
callback wrapper started
callback
callback wrapper stopped
handler stopped