bpo-32751: Wait for task cancellation in asyncio.wait_for() (GH-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 e2b340ab4196e1beb902327f503574b5d7369185)
Co-authored-by: Elvis Pranskevichus <elvis@magic.io>diff --git a/Doc/library/asyncio-task.rst b/Doc/library/asyncio-task.rst
index dc450c3..3121b47 100644
--- a/Doc/library/asyncio-task.rst
+++ b/Doc/library/asyncio-task.rst
@@ -790,7 +790,9 @@
Returns result of the Future or coroutine. When a timeout occurs, it
cancels the task and raises :exc:`asyncio.TimeoutError`. To avoid the task
- cancellation, wrap it in :func:`shield`.
+ cancellation, wrap it in :func:`shield`. The function will wait until
+ the future is actually cancelled, so the total wait time may exceed
+ the *timeout*.
If the wait is cancelled, the future *fut* is also cancelled.
@@ -800,3 +802,8 @@
.. versionchanged:: 3.4.3
If the wait is cancelled, the future *fut* is now also cancelled.
+
+ .. versionchanged:: 3.7
+ When *fut* is cancelled due to a timeout, ``wait_for`` now waits
+ for *fut* to be cancelled. Previously,
+ it raised :exc:`~asyncio.TimeoutError` immediately.