python - 将带有 websockets 的 Quart 项目从 asyncio 迁移到 trio

Question

我正在尝试将我的 asyncio 项目转换为 trio。我知道我必须使用内存通道而不是队列，但由于某种原因，我没有得到我期望的结果。

我的主要问题是，当我运行两个客户端时，如果第二个客户端离开，第一个客户端不会收到通知（从服务器广播“部分”消息会引发错误）。另一个问题是，有时客户端在打开 websocket 时会立即退出。当我使用 asyncio 时，一切正常。

这是我在第二个客户端断开连接时得到的堆栈跟踪：

[2021-07-30 18:39:51,899] ERROR in app: Exception on websocket /ws
Traceback (most recent call last):
  File "/tmp/debug/venv/lib/python3.9/site-packages/quart_trio/app.py", line 175, in handle_websocket
    return await self.full_dispatch_websocket(websocket_context)
  File "/tmp/debug/venv/lib/python3.9/site-packages/quart_trio/app.py", line 197, in full_dispatch_websocket
    result = await self.handle_user_exception(error)
  File "/tmp/debug/venv/lib/python3.9/site-packages/quart_trio/app.py", line 166, in handle_user_exception
    raise error
  File "/tmp/debug/venv/lib/python3.9/site-packages/quart_trio/app.py", line 195, in full_dispatch_websocket
    result = await self.dispatch_websocket(websocket_context)
  File "/tmp/debug/venv/lib/python3.9/site-packages/quart/app.py", line 1651, in dispatch_websocket
    return await self.ensure_async(handler)(**websocket_.view_args)
  File "/tmp/debug/server.py", line 103, in wsocket
    nursery.start_soon(receiving, u)
  File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_run.py", line 815, in __aexit__
    raise combined_error_from_nursery
trio.MultiError: Cancelled(), Cancelled(), Cancelled()

Details of embedded exception 1:

  Traceback (most recent call last):
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_run.py", line 1172, in raise_cancel
      raise Cancelled._create()
  trio.Cancelled: Cancelled

Details of embedded exception 2:

  Traceback (most recent call last):
    File "/tmp/debug/server.py", line 68, in receiving
      data = await websocket.receive_json()
    File "/tmp/debug/venv/lib/python3.9/site-packages/quart/wrappers/websocket.py", line 68, in receive_json
      data = await self.receive()
    File "/tmp/debug/venv/lib/python3.9/site-packages/quart/wrappers/websocket.py", line 57, in receive
      return await self._receive()
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_channel.py", line 314, in receive
      return await trio.lowlevel.wait_task_rescheduled(abort_fn)
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_traps.py", line 166, in wait_task_rescheduled
      return (await _async_yield(WaitTaskRescheduled(abort_func))).unwrap()
    File "/tmp/debug/venv/lib/python3.9/site-packages/outcome/_impl.py", line 138, in unwrap
      raise captured_error
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_run.py", line 1172, in raise_cancel
      raise Cancelled._create()
  trio.Cancelled: Cancelled

Details of embedded exception 3:

  Traceback (most recent call last):
    File "/tmp/debug/server.py", line 54, in sending
      data = await u.queue_recv.receive()
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_channel.py", line 314, in receive
      return await trio.lowlevel.wait_task_rescheduled(abort_fn)
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_traps.py", line 166, in wait_task_rescheduled
      return (await _async_yield(WaitTaskRescheduled(abort_func))).unwrap()
    File "/tmp/debug/venv/lib/python3.9/site-packages/outcome/_impl.py", line 138, in unwrap
      raise captured_error
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_run.py", line 1172, in raise_cancel
      raise Cancelled._create()
  trio.Cancelled: Cancelled

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
    File "/tmp/debug/server.py", line 63, in sending
      await broadcast({'type': 'part', 'data': u.name})
    File "/tmp/debug/server.py", line 75, in broadcast
      await user.queue_send.send(message)
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_channel.py", line 159, in send
      await trio.lowlevel.checkpoint_if_cancelled()
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_run.py", line 2361, in checkpoint_if_cancelled
      await _core.checkpoint()
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_run.py", line 2339, in checkpoint
      await _core.wait_task_rescheduled(lambda _: _core.Abort.SUCCEEDED)
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_traps.py", line 166, in wait_task_rescheduled
      return (await _async_yield(WaitTaskRescheduled(abort_func))).unwrap()
    File "/tmp/debug/venv/lib/python3.9/site-packages/outcome/_impl.py", line 138, in unwrap
      raise captured_error
    File "/tmp/debug/venv/lib/python3.9/site-packages/trio/_core/_run.py", line 1172, in raise_cancel
      raise Cancelled._create()
  trio.Cancelled: Cancelled

这是代码（设置TRIO为False使用 asyncio）：

服务器.py

#!/usr/bin/env python
from quart import Quart, websocket, request, jsonify, json
from quart_trio import QuartTrio
from functools import wraps
import uuid
import trio
import asyncio
from quart_auth import AuthUser, AuthManager, login_user, _AuthSerializer

TRIO = True

if TRIO:
    app = QuartTrio(__name__)
else:
    app = Quart(__name__)
app.secret_key = '**changeme**'

authorized_users = set()

class User(AuthUser):
    @staticmethod
    def current():
        token = websocket.cookies['QUART_AUTH']
        serializer = _AuthSerializer('**changeme**', 'quart auth salt')
        user_id = serializer.loads(token)
        for u in authorized_users:
            if u.auth_id == user_id:
                return u
        return None

    def __init__(self, auth_id):
        super().__init__(auth_id)
        self.name = None
        self.queue = None # asyncio
        self.queue_send = None #trio
        self.queue_recv = None #trio
        self.connected = False
        self.websockets = set()    

    def to_dict(self):
        return {
            'id': self.auth_id,
            'name': self.name
        }

auth_manager = AuthManager()
auth_manager.user_class = User

async def sending(u: User):
    await broadcast({'type': 'join', 'data': u.name})
    try:
        while True:
            if TRIO:
                data = await u.queue_recv.receive()
            else:
                data = await u.queue.get()
            for s in u.websockets:
                await s.send_json(data)
    finally:
        u.websockets.remove(websocket._get_current_object())
        if len(u.websockets) == 0:
            u.connected = False
            await broadcast({'type': 'part', 'data': u.name})


async def receiving(u: User):
    while True:
        data = await websocket.receive_json()
        if data['type'] == 'msg':
            await broadcast({'type': 'msg', 'user': u.name, 'data': data['data']})

async def broadcast(message):
    for user in [u for u in authorized_users if u.connected]:
        if TRIO:
            await user.queue_send.send(message)
        else:
            await user.queue.put(message)

@app.route('/api/v1/auth', methods=['POST'])
async def auth_login():
    data = await request.json
    user_id = str(uuid.uuid4())[:8]
    u = User(user_id)
    u.name = data['login'] or 'Anonymous'+user_id
    if TRIO:
        u.queue_send, u.queue_recv = trio.open_memory_channel(float('inf'))
    else:
        u.queue = asyncio.Queue()
    login_user(u, True)
    authorized_users.add(u)
    return jsonify({'id': user_id, 'name': u.name}), 200

@app.websocket('/ws')
async def wsocket():
    u = User.current()
    if u is None:
        return
    u.websockets.add(websocket._get_current_object())
    u.connected = True
    if TRIO:
        async with trio.open_nursery() as nursery:
            nursery.start_soon(sending, u)
            nursery.start_soon(receiving, u)
    else:
        producer = asyncio.create_task(sending(u))
        consumer = asyncio.create_task(receiving(u))
        await asyncio.gather(producer, consumer)


auth_manager.init_app(app)

if __name__ == "__main__":
    app.run(host='localhost', port=8080)

客户端.py

#!/usr/bin/env python

import asks
import trio
import trio_websocket
import json

asks.init(trio)

class User:
    def __init__(self, name: str="") -> None:
        self.name = name

class Client(User):
    def __init__(self) -> None:
        super(Client, self).__init__()
        self.web_url = 'http://localhost:8080/api/v1'
        self.ws_url = 'ws://localhost:8080/ws'
        self.ws = None
        self.nursery = None
        self.cookiejar = {}
    
    async def send(self, msg: dict) -> None:
        if self.ws is not None:
            await self.ws.send_message(json.dumps(msg))

    async def reader(self, websocket) -> None:
        while True:
            try:
                message_raw = await websocket.get_message()
                msg = json.loads(message_raw)
                if msg['type'] == 'msg':
                    print(f"<{msg['user']}> {msg['data']}")
                elif msg['type'] == 'join':
                    print(f"* {msg['data']} joined")
                elif msg['type'] == 'part':
                    print(f"* {msg['data']} left")
            except trio_websocket.ConnectionClosed:
                break

    async def login(self) -> None:
        rlogin = await asks.post(self.web_url + '/auth', json={'login': self.name, 'password': 'password'})
        for c in rlogin.cookies:
            if c.name == 'QUART_AUTH':
                self.cookiejar = {'QUART_AUTH': c.value}

    async def connect(self) -> None:
        await self.login()
        async with trio_websocket.open_websocket_url(self.ws_url, extra_headers=[('Cookie', 'QUART_AUTH'+'='+self.cookiejar['QUART_AUTH'])]) as websocket:
            self.ws = websocket
            await self.send({'type': 'msg', 'data': 'hello'})
            async with trio.open_nursery() as nursery:
                self.nursery = nursery
                nursery.start_soon(self.reader, websocket)

    def run(self) -> None:
        trio.run(self.connect)

c = Client()
c.name = 'clientA'
c.run()

编辑：我使用 anyio 进行了测试，虽然 anyio+trio 的行为相同，但 anyio+asyncio 重现了问题（无任何例外）。所以我猜它来自队列替换。

标签： pythonwebsocketquartpython-trio

Answer 1

好的，@tibs，我想我找到了问题所在。问题在于 Trio 处理取消的方式。如需完整文档，请阅读此文档：

https://trio.readthedocs.io/en/stable/reference-core.html#cancellation-and-timeouts

然而，为了解释这里发生了什么，当用户断开连接时，Quart-Trio 所做的是Cancelled在该 websocket 下运行/等待的每个协程中引发异常。对于 websocket 用户，目前有两个位置正在等待：

在async def sending(u: User):

async def sending(u: User):
    await broadcast({'type': 'join', 'data': u.name})
    try:
        while True:
            if TRIO:
                data = await u.queue_recv.receive()  <--- Code is waiting here, Cancelled is raised here
            else:
                data = await u.queue.get()
            for s in u.websockets:
                await s.send_json(data)
    finally:
        u.websockets.remove(websocket._get_current_object())
        if len(u.websockets) == 0:
            u.connected = False
            await broadcast({'type': 'part', 'data': u.name})

在async def receiving(u: User):

async def receiving(u: User):
    while True:
        data = await websocket.receive_json()   <--- Code is waiting here, Cancelled is raised here
        if data['type'] == 'msg':
            await broadcast({'type': 'msg', 'user': u.name, 'data': data['data']})

好的，那么从这里会发生什么？好吧，在sending()函数中，我们向下移动到finally开始执行的块，但随后我们调用另一个等待函数：

    finally:
        u.websockets.remove(websocket._get_current_object())
        if len(u.websockets) == 0:
            u.connected = False
            await broadcast({'type': 'part', 'data': u.name})  <--- we call an awaitable here

来自 Trio 文档：

Trio 中的取消是“级别触发的”，这意味着一旦一个块被取消，该块中所有可取消的操作将不断提高 Cancelled。

所以当await broadcast(...)被调用时，它是立即Cancelled的，不像asyncio它的行为不同。这解释了为什么您的“部分”消息永远不会发送。所以当三重奏时，如果你想在被取消的时候做一些清理工作，你应该打开一个新的取消作用域，并屏蔽它不被取消，像这样：

async def sending(u: User):
    await broadcast({'type': 'join', 'data': u.name})
    try:
        while True:
            if TRIO:
                data = await u.queue_recv.receive()  <--- Code is waiting here, Cancelled is raised here
            else:
                data = await u.queue.get()
            for s in u.websockets:
                await s.send_json(data)
    finally:
        u.websockets.remove(websocket._get_current_object())
        if len(u.websockets) == 0:
            u.connected = False
            with trio.move_on_after(5) as leaving_cancel_scope:
                # Shield from the cancellation for 5s to run the broadcast of leaving
                leaving_cancel_scope.shield = True
                await broadcast({'type': 'part', 'data': u.name})

或者，您可以在应用程序 Nursery 上启动广播协程。请注意，如果broadcast(...)崩溃，您将使整个正在运行的应用程序崩溃，除非您在broadcast(...)函数中放置 try/except ：

async def sending(u: User):
    await broadcast({'type': 'join', 'data': u.name})
    try:
        while True:
            if TRIO:
                data = await u.queue_recv.receive()
            else:
                data = await u.queue.get()
            for s in u.websockets:
                await s.send_json(data)
    finally:
        u.websockets.remove(websocket._get_current_object())
        if len(u.websockets) == 0:
            u.connected = False
            app.nursery.start_soon(broadcast, {'type': 'part', 'data': u.name})

在此之后，您仍然会收到Cancelled流向您的 websocket 函数的异常，因此您可能希望在那里捕获它们。请注意，您需要通过 catchBaseException来捕获错误，例如：

@app.websocket('/ws')
async def wsocket():
    u = User.current()
    if u is None:
        return
    u.websockets.add(websocket._get_current_object())
    u.connected = True
    if TRIO:
        try:
            async with trio.open_nursery() as nursery:
                nursery.start_soon(sending, u)
                nursery.start_soon(receiving, u)
        except BaseException as e:
            print(f'websocket funcs crashed with exception: {e}')

特别是因为 trio 不允许您静默丢弃异常，您需要捕获它们或崩溃。我希望这足以让您开始解决您遇到的问题。