Protect tasks weak set manipulation in asyncio.all_tasks()

[asvetlov]

BPO	34970
Nosy	@ned-deily, @asvetlov, @1st1, @miss-islington
PRs	bpo-34970: Protect tasks weak set manipulation in asyncio.all_tasks() #9837 [3.7] bpo-34970: Protect tasks weak set manipulation in asyncio.all_tasks() (GH-9837) #9849

^{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 2018-10-20.17:07:37.062>
created_at = <Date 2018-10-13.08:11:44.726>
labels = ['3.7', '3.8', 'expert-asyncio']
title = 'Protect tasks weak set manipulation in asyncio.all_tasks()'
updated_at = <Date 2018-10-20.17:07:37.061>
user = 'https://github.com/asvetlov'

bugs.python.org fields:

activity = <Date 2018-10-20.17:07:37.061>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2018-10-20.17:07:37.062>
closer = 'ned.deily'
components = ['asyncio']
creation = <Date 2018-10-13.08:11:44.726>
creator = 'asvetlov'
dependencies = []
files = []
hgrepos = []
issue_num = 34970
keywords = ['patch']
message_count = 13.0
messages = ['327632', '327633', '327635', '327636', '327642', '327651', '327660', '327662', '327663', '327666', '327667', '328167', '328168']
nosy_count = 4.0
nosy_names = ['ned.deily', 'asvetlov', 'yselivanov', 'miss-islington']
pr_nums = ['9837', '9849']
priority = None
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue34970'
versions = ['Python 3.7', 'Python 3.8']

[asvetlov]

Traceback (most recent call last):
  File "C:\Users\max\AppData\Local\Programs\Python\Python37\lib\threading.py", line 917, in _bootstrap_inner
    self.run()
  File "D:\Proj\antwork\code\antwork_backend\server\data_handlers\order\playback_hd.py", line 147, in run
    send_to_ws(channel=self.room_name, data=json.loads(msg))
  File "D:\Proj\antwork\code\antwork_backend\server\data_exchange.py", line 26, in send_to_ws
    asyncio.run(channel_layer.group_send(channel, send_data))
  File "C:\Users\max\AppData\Local\Programs\Python\Python37\lib\asyncio\runners.py", line 46, in run
    _cancel_all_tasks(loop)
  File "C:\Users\max\AppData\Local\Programs\Python\Python37\lib\asyncio\runners.py", line 54, in _cancel_all_tasks
    to_cancel = tasks.all_tasks(loop)
  File "C:\Users\max\AppData\Local\Programs\Python\Python37\lib\asyncio\tasks.py", line 38, in all_tasks
    return {t for t in _all_tasks
  File "C:\Users\max\AppData\Local\Programs\Python\Python37\lib\asyncio\tasks.py", line 38, in <setcomp>
    return {t for t in _all_tasks
  File "C:\Users\max\AppData\Local\Programs\Python\Python37\lib\_weakrefset.py", line 61, in __iter__
    for itemref in self.data:
RuntimeError: Set changed size during iteration

416c1eb#commitcomment-30887909

[asvetlov]

The issue is hard to reproduce, the problem happens on an implicit garbage collector run during iteration over _all_tasks weakset.

[asvetlov]

Ned, would be nice to have this trivial bugfix in 3.7.1.

The problem happens if wakes up on weakset iteration.
The chance is very low, it leads to a flaky scary bug.

[ned-deily]

3.7.1rc2 is about to be released so to add this would require retagging and remanufacturing release bits. If the chances of it happening are "very low", I would prefer to not further delay rc2. If you and Yury agree that the risk is low and the benefit is high, we could consider risking cherry-picking it to 3.7.1 final. Sound OK?

[asvetlov]

Let's wait for Yuri opinion.

[1st1]

> we could consider risking cherry-picking it to 3.7.1 final. Sound OK?
Let's wait for Yuri opinion.

I agree it's a pretty serious bug; basically a time bomb that can crash a perfectly fine asyncio application. The PR itself seems to be safe to cherry-pick to 3.7.1 even after rc2. I suggest to merge it asap though to have some time for buildbots to test it.

[miss-islington]

New changeset 97cf082 by Miss Islington (bot) (Andrew Svetlov) in branch 'master':
bpo-34970: Protect tasks weak set manipulation in asyncio.all_tasks() (GH-9837)
97cf082

[asvetlov]

New changeset 5dbb1b7 by Andrew Svetlov (Miss Islington (bot)) in branch '3.7':
bpo-34970: Protect tasks weak set manipulation in asyncio.all_tasks() (GH-9837) (GH-9849)
5dbb1b7

[asvetlov]

Ned, the fix has landed on both master and 3.7

Your turn, please.

[ned-deily]

Thanks, Andrew and Yury! Just leave the issue open as a "release blocker" for now and I will take care of it for 3.7.1 final.