Question

I adapted this code for using Google Cloud PubSub in Async Python: https://github.com/cloudfind/google-pubsub-asyncio

import asyncio
import datetime
import functools
import os

from google.cloud import pubsub
from google.gax.errors import RetryError
from grpc import StatusCode

async def message_producer():
    """ Publish messages which consist of the current datetime """
    while True:
        await asyncio.sleep(0.1)


async def proc_message(message):
    await asyncio.sleep(0.1)
    print(message)
    message.ack()


def main():
    """ Main program """
    loop = asyncio.get_event_loop()

    topic = "projects/{project_id}/topics/{topic}".format(
        project_id=PROJECT, topic=TOPIC)
    subscription_name = "projects/{project_id}/subscriptions/{subscription}".format(
        project_id=PROJECT, subscription=SUBSCRIPTION)

    subscription = make_subscription(
        topic, subscription_name)

    def create_proc_message_task(message):
        """ Callback handler for the subscription; schedule a task on the event loop """
        print("Task created!")
        task = loop.create_task(proc_message(message))

    subscription.open(create_proc_message_task)
    # Produce some messages to consume

    loop.create_task(message_producer())

    print("Subscribed, let's do this!")
    loop.run_forever()


def make_subscription(topic, subscription_name):
    """ Make a publisher and subscriber client, and create the necessary resources """
    subscriber = pubsub.SubscriberClient()
    try:
        subscriber.create_subscription(subscription_name, topic)
    except:
        pass
    subscription = subscriber.subscribe(subscription_name)

    return subscription


if __name__ == "__main__":
    main()

I basically removed the publishing code and only use the subscription code. However, initially I did not include the loop.create_task(message_producer()) line. I figured that tasks were created as they were supposed to however they never actually run themselves. Only if I add said line the code properly executes and all created Tasks run. What causes this behaviour?

Answer 1

PubSub正在从另一个线程调用create_proc_message_task回调。由于create_task是not thread-safe，因此只能从运行事件循环的线程（通常是主线程）调用它。要解决此问题，请将loop.create_task(proc_message(message))替换为asyncio.run_coroutine_threadsafe(proc_message(message), loop)，将不再需要message_producer。

至于为什么message_producer似乎修复了代码，请考虑run_coroutine_threadsafe与create_task相比还有两件事：

它以线程安全的方式运行，因此当同时完成时，事件循环数据结构不会被破坏。
确保事件循环尽快唤醒，以便它可以处理新任务。

在你的情况下create_task将任务添加到循环的可运行队列（没有任何锁定），但未能确保唤醒，因为在事件循环线程中运行时不需要。然后message_producer用于强制循环以固定间隔唤醒，这时它也检查并执行可运行的任务。

Python Asyncio run_forever() and Tasks

1 个答案: