gh-157157: Make test_taskgroup_cancel_keeps_outer_cancellation deterministic

The test cancelled the parent task from outside after a 10 ms sleep,
expecting the group to still be waiting for its child. If the event loop
stalled for longer than that before the body task first ran, the outer
cancel() landed while the parent's awaited future was already cancelled
by the group, and its message was dropped, failing the test on slow
builds such as the TSan CI job.

Synchronize on events instead: the child signals from its finally block
that the group is cancelling and waits to be released, and only then is
the parent task cancelled from outside.

Author: iamsharduld

Lib/test/test_asyncio/test_taskgroups.py

@@ -1200,11 +1200,17 @@ async def child(tg):
 
     async def test_taskgroup_cancel_keeps_outer_cancellation(self):
         # gh-155433: any cancellation from outside the group must propagate.
+        cancelling = asyncio.Event()
+        release = asyncio.Event()
+
         async def child():
             try:
                 await asyncio.sleep(10)
             finally:
-                await asyncio.sleep(0.1)
+                # The group is cancelling: it has cancelled its parent task
+                # and is waiting for this task to finish.
+                cancelling.set()
+                await release.wait()
 
         async def body():
             async with asyncio.TaskGroup() as tg:
@@ -1213,8 +1219,9 @@ async def body():
                 tg.cancel()
 
         task = asyncio.create_task(body())
-        await asyncio.sleep(0.01)
+        await cancelling.wait()
         task.cancel('message')
+        release.set()
         with self.assertRaises(asyncio.CancelledError) as cm:
             await task
         self.assertEqual('message', cm.exception.args[0])

Misc/NEWS.d/next/Tests/2026-09-08-02-40-00.gh-issue-157157.5X2ypw.rst

@@ -0,0 +1,2 @@
+Fix a timing dependence in ``test_taskgroup_cancel_keeps_outer_cancellation``
+in ``test_asyncio``, which failed on slow builds.