Aiohttp: 当连接意外关闭时，Websocket 读取消息循环会引发低级 CanceledError

创建于 2017-07-06 · 4评论 · 资料来源: aio-libs/aiohttp

实际行为

当连接意外关闭时，读取消息循环async for msg in ws:会引发低级concurrent.futures._base.CancelledError 。

预期行为

预计会收到类型aiohtto.http_websocket.WSMsgType.ERROR ，或者静默停止循环，或者至少aiohtto.http_websocket.WebSocketError 。

重现步骤

运行以下两个脚本server.py和client.py ，然后通过Ctrl+C停止client.py Ctrl+C 。

服务器.py

import logging

from aiohttp import web


logger = logging.getLogger(__name__)


async def index(request):
    ws = web.WebSocketResponse()
    request.app['websockets'].add(ws)

    try:
        await ws.prepare(request)
        logger.debug('Connected')
        async for msg in ws:
            logger.info('Received: %r', msg.data)
    except Exception:
        logger.exception('Error')
    logger.debug('Disconnected')

    request.app['websockets'].discard(ws)
    return ws


async def on_shutdown(app):
    for ws in app['websockets']:
        await ws.close()
    app['websockets'].clear()


def main():
    logging.basicConfig(level=logging.DEBUG)

    app = web.Application()
    app['websockets'] = set()
    app.router.add_get('/', index)
    app.on_shutdown.append(on_shutdown)

    web.run_app(app, host='127.0.0.1', port=9000)


if __name__ == '__main__':
    main()

客户端.py

import asyncio

import aiohttp


async def communicate(loop):
    async with aiohttp.ClientSession(loop=loop) as session:
        async with session.ws_connect('http://127.0.0.1:9000') as ws:
            while True:
                await ws.send_str('Hello')
                await asyncio.sleep(1, loop=loop)


def main():
    loop = asyncio.get_event_loop()
    loop.run_until_complete(communicate(loop))


if __name__ == '__main__':
    main()

`server.py`日志输出

$ python server.py 
DEBUG:asyncio:Using selector: EpollSelector
======== Running on http://127.0.0.1:9000 ========
(Press CTRL+C to quit)
DEBUG:__main__:Connected
INFO:__main__:Received: 'Hello'
INFO:__main__:Received: 'Hello'
INFO:__main__:Received: 'Hello'
INFO:__main__:Received: 'Hello'
INFO:__main__:Received: 'Hello'
INFO:__main__:Received: 'Hello'
INFO:__main__:Received: 'Hello'
INFO:__main__:Received: 'Hello'
INFO:__main__:Received: 'Hello'
ERROR:__main__:Error
Traceback (most recent call last):
  File "server.py", line 16, in index
    async for msg in ws:
  File "/home/vagrant/project/workspace/pyenv_dev/lib64/python3.5/site-packages/aiohttp/web_ws.py", line 343, in __anext__
    msg = yield from self.receive()
  File "/home/vagrant/project/workspace/pyenv_dev/lib64/python3.5/site-packages/aiohttp/web_ws.py", line 273, in receive
    msg = yield from self._reader.read()
  File "/home/vagrant/project/workspace/pyenv_dev/lib64/python3.5/site-packages/aiohttp/streams.py", line 627, in read
    return (yield from super().read())
  File "/home/vagrant/project/workspace/pyenv_dev/lib64/python3.5/site-packages/aiohttp/streams.py", line 509, in read
    yield from self._waiter
  File "/usr/lib64/python3.5/asyncio/futures.py", line 380, in __iter__
    yield self  # This tells Task to wait for completion.
  File "/usr/lib64/python3.5/asyncio/tasks.py", line 304, in _wakeup
    future.result()
  File "/usr/lib64/python3.5/asyncio/futures.py", line 285, in result
    raise CancelledError
concurrent.futures._base.CancelledError
DEBUG:__main__:Disconnected
INFO:aiohttp.access:- - - [06/Jul/2017:11:41:25 +0000] "GET / HTTP/1.1" 101 0 "-" "Python/3.5 aiohttp/2.2.3"

您的环境

操作系统：CentOS Linux 7
Linux 内核：3.10.0-514.16.1.el7.x86_64
蟒蛇：3.5.3
aiohttp: 2.2.3

资料来源

kr41

👍2

最有用的评论

也许最好引入一个单独的 ConnectionClosed 异常，就像在websockets库中所做的那样？

s-kostyuk 于 2018-05-07

👍2

所有4条评论

从技术上讲，aiohttp 根据客户端请求创建一个任务。
在客户端断开连接时，系统会尽快停止任务。
唯一的方法是取消任务（假设 web 处理程序正在等待来自 DB 或其他服务的响应，我们也想取消它而不等待通过连接到 websocket 客户端的显式操作）。

Task.cancel()是通过发送asyncio.CancelledError异常来完成的，异常类派生自标准Exception 。这是 asyncio 行为，没有特定于 aiohttp 本身。

我唯一能建议的是在您的处理程序中明确捕获CancelledError ：

try:
    ...
except asyncio.CancelledError:
    pass
except Exception as exc:
    log(exc)

或者你不能像Exception这样广泛的类型。

asvetlov 于 2017-07-11

👍1

我看到两个选项：

不要改变任何东西， CancelledError在异步世界中是正常的。
捕获CancelledError并返回closed消息。我认为，这对于 webocket 处理程序来说是更好的解决方案。

fafhrd91 于 2017-07-17

也许最好引入一个单独的 ConnectionClosed 异常，就像在websockets库中所做的那样？

s-kostyuk 于 2018-05-07

👍2

从技术上讲，aiohttp 根据客户端请求创建一个任务。
在客户端断开连接时，系统会尽快停止任务。
唯一的方法是取消任务（假设 web 处理程序正在等待来自 DB 或其他服务的响应，我们也想取消它而不等待通过连接到 websocket 客户端的显式操作）。

我想我刚刚被这种行为咬了。我有一些这样的代码：

async def handler(request):
    ws = web.WebSocketResponse()
    await ws.prepare(request)

    async with contextlib.AsyncExitStack() as stack:
        # acquire_resource_X are async context managers
        await stack.enter_async_context(acquire_resource_1())
        await stack.enter_async_context(acquire_resource_2())
        await stack.enter_async_context(acquire_resource_3())

        async for msg in ws:
            # do stuff

    await ws.close()

    return ws

将其投入生产后，我发现acquire_resource_3()的退出部分将被悄悄跳过。更多的日志显示CancelledError在acquire_resource_3内被引发。这是我认为发生的事情：

客户端关闭 WebSocket
async for msg in ws循环退出， AsyncExitStack开始展开， acquire_resource_3的退出部分开始执行，遇到await
aiohttp取消处理程序任务
CancelledError是在handler内在当前await内引发的，它在acquire_resource_3 ，因此acquire_resource_3的其余部分被跳过
acquire_resource_2和acquire_resource_1的退出部分仍然正常执行，因为从他们的角度来看，他们只是在发生异常时退出异步上下文

这是一个非常奇怪的问题，特别是因为它如何打破上下文管理器的退出部分将始终运行的期望。我必须基本上屏蔽所有异步上下文免于取消，如下所示：

async def handler(request):
    ws = web.WebSocketResponse()
    await ws.prepare(request)

    await asyncio.shield(asyncio.ensure_future(actually_do_stuff(ws)))

    return ws

async def actually_do_stuff(ws):
    async with contextlib.AsyncExitStack() as stack:
        # acquire_resource_X are async context managers
        await stack.enter_async_context(acquire_resource_1())
        await stack.enter_async_context(acquire_resource_2())
        await stack.enter_async_context(acquire_resource_3())

        async for msg in ws:
            # do stuff

    await ws.close()

有一个更好的方法吗？

twisteroidambassador 于 2019-02-13

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