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.

1 files changed, 2 insertions, 0 deletions

diff --git a/Misc/NEWS.d/next/Library/2018-05-29-15-32-18.bpo-32751.oBTqr7.rst b/Misc/NEWS.d/next/Library/2018-05-29-15-32-18.bpo-32751.oBTqr7.rst
new file mode 100644
index 0000000..3e27cd4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-05-29-15-32-18.bpo-32751.oBTqr7.rst
@@ -0,0 +1,2 @@
+When cancelling the task due to a timeout, :meth:`asyncio.wait_for` will now
+wait until the cancellation is complete.