asyncio.gather(..., return_exceptions=True) swallows cancellation

[socketpair]

BPO	32684
Nosy	@bitdancer, @asvetlov, @socketpair, @1st1
PRs	bpo-32684: Fix gather to propagate cancel of itself with return_exceptions #7209 [3.7] bpo-32684: Fix gather to propagate cancel of itself with return_exceptions (GH-7209) #7222 [3.6] bpo-32684: Fix gather to propagate cancel of itself with return_exceptions #7224 bpo-32684: Fix nits in tests #7225 [3.7] bpo-32684: Fix nits in tests (GH-7225) #7231

^{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 2018-05-30.00:59:24.287>
created_at = <Date 2018-01-27.05:38:11.141>
labels = ['3.8', 'type-bug', '3.7', 'expert-asyncio']
title = 'asyncio.gather(..., return_exceptions=True) swallows cancellation'
updated_at = <Date 2018-05-30.00:59:24.286>
user = 'https://github.com/socketpair'

bugs.python.org fields:

activity = <Date 2018-05-30.00:59:24.286>
actor = 'yselivanov'
assignee = 'none'
closed = True
closed_date = <Date 2018-05-30.00:59:24.287>
closer = 'yselivanov'
components = ['asyncio']
creation = <Date 2018-01-27.05:38:11.141>
creator = 'socketpair'
dependencies = []
files = []
hgrepos = []
issue_num = 32684
keywords = ['patch']
message_count = 8.0
messages = ['310843', '310846', '318059', '318093', '318127', '318138', '318139', '318142']
nosy_count = 4.0
nosy_names = ['r.david.murray', 'asvetlov', 'socketpair', 'yselivanov']
pr_nums = ['7209', '7222', '7224', '7225', '7231']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32684'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[socketpair]

Proof:

================

import asyncio

async def test():
    while True:
        print('sleeping 1')
        await asyncio.gather(asyncio.sleep(1), return_exceptions=True)

async def amain():
    print('creating task')
    qwe = asyncio.Task(test())
    print('sleeping 2')
    await asyncio.sleep(2)
    print('cancelling task')
    qwe.cancel()
    print('waiting task for completion')
    try:
        await qwe
    except Exception as e:
        print('task complete: %r', e)

loop = asyncio.get_event_loop()
loop.run_until_complete(amain())

================

This program will never complete.

This case should be either fixed, or documented.

[bitdancer]

See also bpo-26923, but I think this problem is different.

[1st1]

Thanks for reporting this. Agree, this is a bug and makes it hard (if not impossible) to use gather. I've created a PR to address the issue.

[1st1]

New changeset 863b674 by Yury Selivanov in branch 'master':
bpo-32684: Fix gather to propagate cancel of itself with return_exceptions (GH-7209)
863b674

[1st1]

New changeset 0364342 by Yury Selivanov (Miss Islington (bot)) in branch '3.7':
bpo-32684: Fix gather to propagate cancel of itself with return_exceptions (GH-7209) (bpo-7222)
0364342

[1st1]

New changeset 3b263e6 by Yury Selivanov in branch '3.6':
bpo-32684: Fix gather to propagate cancel of itself with return_exceptions (GH-7224)
3b263e6

[1st1]

New changeset 6f75bae by Yury Selivanov in branch 'master':
bpo-32684: Fix nits in tests (GH-7225)
6f75bae

[1st1]

New changeset 3ddee64 by Yury Selivanov (Miss Islington (bot)) in branch '3.7':
bpo-32684: Fix nits in tests (GH-7225) (bpo-7231)
3ddee64