[issue42526] Exceptions in asyncio.Server callbacks are not retrievable

[Andrew Svetlov]

Andrew Svetlov  added the comment:

You can use try/except in handler() and dispatch the exception whatever you 
want.
It doesn't require a new asyncio version, debug-only flag, etc.

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[issue42526] Exceptions in asyncio.Server callbacks are not retrievable

[Tom]

Tom  added the comment:

How do you suggest one might test code in a Server callback with asyncio?

Of course, I don't want any old exception to affect another client connection,
only an exception which is uncaught up to the handler coro. And I'm not
suggesting that it happen by default, only that it be possible.

With this, the behaviour would perfectly align with the asyncio.gather
functionality, and its 'return_exceptions' kwarg.

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[issue42526] Exceptions in asyncio.Server callbacks are not retrievable

[Andrew Svetlov]

Andrew Svetlov  added the comment:

This is a deliberate decision.
An exception in handling one client connection should not break another 
connected client.

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[issue42526] Exceptions in asyncio.Server callbacks are not retrievable

[Tom]

New submission from Tom :

Consider this program:

import asyncio

async def handler(r, w):
raise RuntimeError

async def main():
server = await asyncio.start_server(handler, host='localhost', 
port=1234)
r, w = await asyncio.open_connection(host='localhost', port=1234)
await server.serve_forever()
server.close()

asyncio.run(main())

The RuntimeError is not retrievable via the serve_forever coroutine. To my
knowledge, there is no feature of the asyncio API which causes the server to
stop on an exception and retrieve it. I have also tried wrapping serve_forever
in a Task, and waiting on the coro with FIRST_EXCEPTION.

This severely complicates testing asyncio servers, since failing tests hang
forever if the failure occurs in a callback.

It should be possible to configure the server to end if a callback fails, e.g.
by a 'stop_on_error' kwarg to start_server (defaulting to False for
compatibility).

I know this isn't a technical problem, since AnyIO, which uses asyncio, does
this by default. This equivalent program ends after the exception:

import anyio

async def handler(client):
raise RuntimeError

async def main():
async with anyio.create_task_group() as tg:
listener = await anyio.create_tcp_listener(local_host='localhost', 
local_port=1234)
await tg.spawn(listener.serve, handler)
async with await anyio.connect_tcp('localhost', 1234) as client:
pass

anyio.run(main)

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components: asyncio
messages: 382265
nosy: asvetlov, tmewett, yselivanov
priority: normal
severity: normal
status: open
title: Exceptions in asyncio.Server callbacks are not retrievable
versions: Python 3.7

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