bpo-27585: Cancelled lock waiter wakes up the next one

[msornay]

Avoid a deadlock when the waiter who is about to take the lock is cancelled

Issue #27585: http://bugs.python.org/issue27585
Original PR on asyncio repository: python/asyncio#467

[mention-bot]

@msornay, thanks for your PR! By analyzing the history of the files in this pull request, we identified @1st1, @asvetlov and @serhiy-storchaka to be potential reviewers.

[popravich]

Guys! @1st1, @asvetlov, @serhiy-storchaka,
Can anyone take a look at this patch, the issue is real PITA.

@msornay, probably, the same fix should be applied to asyncio.Condition.wait() method
as it uses the same logic as asyncio.Lock.acquire():
if await cond.wait() is resolved but owning task is cancelled -- other waiters could end up in a deadlock

[asvetlov]

lgtm

[popravich]

Any news?

[jettify]

ping

[1st1]

I'll take a look tomorrow/the day after.

[Mariatta]

Does this still need backport? If not please remove the label.

[miss-islington]

🐍🍒⛏🤖 Thanks @msornay for the PR, and @1st1 for merging it 🌮🎉.I'm working now to backport this PR to: 3.6.

[miss-islington]

Sorry, @msornay and @1st1, I could not cleanly backport this to 3.6 due to a conflict.
Please backport using cherry_picker on command line.