8000 bpo-37658: Actually return result in race condition by Dreamsorcerer · Pull Request #29202 · python/cpython · GitHub
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bpo-37658: Actually return result in race condition #29202

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Merged
merged 6 commits into from
Nov 29, 2021
Merged

bpo-37658: Actually return result in race condition #29202

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019ed9d
Actually fix issue37658
Dreamsorcerer Oct 24, 2021
ffe5106
Correct test
Dreamsorcerer Oct 24, 2021
722fe3a
Update test_tasks.py
Dreamsorcerer Oct 24, 2021
54d0118
Remove redundant else condition.
Dreamsorcerer Oct 24, 2021
169294e
Avoid timing issues with test.
Dreamsorcerer Oct 25, 2021
5cfc8b7
Create 2021-11-28-15-30-34.bpo-37658.8Hno7d.rst
Dreamsorcerer Nov 28, 2021
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Avoid timing issues with test.
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@Dreamsorcerer
Dreamsorcerer authored Oct 25, 2021
commit 169294e95f8f373317d048bcd89ab3516352314f
5 changes: 3 additions & 2 deletions Lib/test/test_asyncio/test_tasks.py
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Expand Up @@ -1010,11 +1010,12 @@ def gen():

def test_wait_for_cancellation_race_condition(self):
async def inner():
await asyncio.wait_for(asyncio.sleep(1), timeout=2)
with contextlib.suppress(asyncio.CancelledError):
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Initially I just emulated the race condition in the bug report, but because it's a race condition, it seems that the test occasionally failed due to actually hitting the timeout.

Instead, I've just suppressed the cancellation, which makes the behaviour 100% reproducible.

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This is not an equivalent test. As I understand, the whole point of that test was to inject the cancellation at the exact point to cause the race on the return to ‘await waiter’ once fut is done. The new test is not checking this race at all but instead checking that a cancellation ignoring task can skip the timeout.

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It's not an equivalent test, because the previous test is completely wrong. It does not emulate a race condition, and it does not reproduce the issue in the bug report.

I wrote a test which actually reproduces the race condition as given in the bug report, and the issue was not fixed. As mentioned above, I had to use this suppress cancellation trick to make the test reproducible (it will still run things in the same order as the race condition), the original test worked correctly everytime I ran it, but failed in one of the CI runs, and we can't have flaky tests.

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You can see the old version which was timing dependent in the commit history:
169294e

await asyncio.sleep(1)
return 1

async def main():
result = await asyncio.wait_for(inner(), timeout=1)
result = await asyncio.wait_for(inner(), timeout=.01)
assert result == 1

asyncio.run(main())
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