IsolatedAsyncioTestCase freezes when exception is raised

[fornellas]

BPO	39101
Nosy	@asvetlov, @1st1, @lisroach, @miss-islington, @fornellas, @r-owen
PRs	bpo-39101: Fixes BaseException hang in IsolatedAsyncioTestCase. #22654 [3.9] bpo-39101: Fixes BaseException hang in IsolatedAsyncioTestCase. (GH-22654) #22988 [3.8] bpo-39101: Fixes BaseException hang in IsolatedAsyncioTestCase. (GH-22654) #22989

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-10-26.16:29:40.338>
created_at = <Date 2019-12-19.19:44:59.009>
labels = ['3.8', 'type-crash', 'expert-asyncio']
title = 'IsolatedAsyncioTestCase freezes when exception is raised'
updated_at = <Date 2021-03-02.00:30:08.714>
user = 'https://github.com/fornellas'

bugs.python.org fields:

activity = <Date 2021-03-02.00:30:08.714>
actor = 'r3owen'
assignee = 'none'
closed = True
closed_date = <Date 2020-10-26.16:29:40.338>
closer = 'lisroach'
components = ['asyncio']
creation = <Date 2019-12-19.19:44:59.009>
creator = 'fornellas'
dependencies = []
files = []
hgrepos = []
issue_num = 39101
keywords = ['patch']
message_count = 5.0
messages = ['358690', '379670', '383195', '383196', '387887']
nosy_count = 6.0
nosy_names = ['asvetlov', 'yselivanov', 'lisroach', 'miss-islington', 'fornellas', 'r3owen']
pr_nums = ['22654', '22988', '22989']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue39101'
versions = ['Python 3.8']

[fornellas]

IsolatedAsyncioTestCase freezes whenever an exception that inherits from BaseException is raised:

import unittest
class TestHangsForever(unittest.IsolatedAsyncioTestCase):
    async def test_hangs_forever(self):
        raise BaseException("Hangs forever")
if __name__ == "__main__":
    import unittest
    unittest.main()

A kind of similar issue present on 3.7 was fixed on 3.8 here https://github.com/python/cpython/blob/3.8/Lib/asyncio/events.py#L84, where BaseExceptions would not be correctly handled by the event loop, this seems somewhat related.

I had a look at IsolatedAsyncioTestCase implementation, did not spot any obvious broken thing there, I could use some light here. Thanks.

[lisroach]

New changeset 8374d2e by Lisa Roach in branch 'master':
bpo-39101: Fixes BaseException hang in IsolatedAsyncioTestCase. (GH-22654)
8374d2e

[miss-islington]

New changeset 9d409d6 by Miss Islington (bot) in branch '3.9':
bpo-39101: Fixes BaseException hang in IsolatedAsyncioTestCase. (GH-22654)
9d409d6

[miss-islington]

New changeset d549d0b by Miss Islington (bot) in branch '3.8':
bpo-39101: Fixes BaseException hang in IsolatedAsyncioTestCase. (GH-22654)
d549d0b

[r-owen]

Here is a variant that also hangs, I suspect for the same reason. The task raises asyncio.CancelledError which inherits from BaseException and my guess is it somehow kills the event loop.

import asyncio
import unittest


class DemonstrateHang(unittest.IsolatedAsyncioTestCase):
    async def test_hang(self):
        task = asyncio.create_task(asyncio.sleep(2))
        await asyncio.sleep(0.1)
        task.cancel()
        await task
        # this also hangs: await asyncio.wait_for(task, 5)