[3.7] bpo-32751: Wait for task cancellation in asyncio.wait_for() (GH-7216) #7223
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…-7216) Currently, asyncio.wait_for(fut), upon reaching the timeout deadline, cancels the future and returns immediately. This is problematic for when *fut* is a Task, because it will be left running for an arbitrary amount of time. This behavior is iself surprising and may lead to related bugs such as the one described in bpo-33638: condition = asyncio.Condition() async with condition: await asyncio.wait_for(condition.wait(), timeout=0.5) Currently, instead of raising a TimeoutError, the above code will fail with `RuntimeError: cannot wait on un-acquired lock`, because `__aexit__` is reached _before_ `condition.wait()` finishes its cancellation and re-acquires the condition lock. To resolve this, make `wait_for` await for the task cancellation. The tradeoff here is that the `timeout` promise may be broken if the task decides to handle its cancellation in a slow way. This represents a behavior change and should probably not be back-patched to 3.6 and earlier. (cherry picked from commit e2b340a) Co-authored-by: Elvis Pranskevichus <elvis@magic.io>
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Currently, asyncio.wait_for(fut), upon reaching the timeout deadline,
cancels the future and returns immediately. This is problematic for
when fut is a Task, because it will be left running for an arbitrary
amount of time. This behavior is iself surprising and may lead to
related bugs such as the one described in bpo-33638:
Currently, instead of raising a TimeoutError, the above code will fail
with
RuntimeError: cannot wait on un-acquired lock, because__aexit__is reached beforecondition.wait()finishes itscancellation and re-acquires the condition lock.
To resolve this, make
wait_forawait for the task cancellation.The tradeoff here is that the
timeoutpromise may be broken if thetask decides to handle its cancellation in a slow way. This represents
a behavior change and should probably not be back-patched to 3.6 and
earlier.
(cherry picked from commit e2b340a)
Co-authored-by: Elvis Pranskevichus elvis@magic.io
https://bugs.python.org/issue32751