From f74c5910e186ed9a5f642325ce74f0e5564d1a3a Mon Sep 17 00:00:00 2001
From: abhinav-phi <abhinav-phi@users.noreply.github.com>
Date: Wed, 30 Sep 2026 07:36:34 +0530
Subject: [PATCH] gh-158097: Make test_staggered_race_with_eager_tasks
 deterministic

staggered_race() awaits the coroutines it is given inline, so the
"await asyncio.sleep(0)" in the test's fail() helper suspended the
run_one_coro() task running it and left a scheduled resumption behind.
If the asyncio.sleep(1) winner (started 0.25s late by the stagger delay)
completes before that resumption runs, the winner's cancel() sets
_must_cancel and the resumption throws CancelledError into fail(), so
excs[2] reports a CancelledError instead of a ValueError.

Pending timers are only moved onto the ready queue at the top of
BaseEventLoop._run_once() and are appended after the handles already
queued, so the t=0.50s stagger timer and the t=1.25s sleep(1) timer run
in due-time order.  When both are due in the same _run_once() - which is
what a busy CI machine causes - the stagger timer starts fail(), which
immediately suspends again, and the very next handle is the winner.

Raise straight away instead, so the ValueError is stored in excs[2] in
the same step that starts the coroutine and there is no longer a
scheduled resumption for a cancellation to overtake.  The outcome no
longer depends on the event loop getting another iteration.
---
 Lib/test/test_asyncio/test_eager_task_factory.py           | 7 ++++++-
 .../Tests/2026-09-30-00-00-00.gh-issue-158097.pcB3mO.rst   | 2 ++
 2 files changed, 8 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Tests/2026-09-30-00-00-00.gh-issue-158097.pcB3mO.rst

diff --git a/Lib/test/test_asyncio/test_eager_task_factory.py b/Lib/test/test_asyncio/test_eager_task_factory.py
index 594a48baa32e522..fd9ce08d43ee340 100644
--- a/Lib/test/test_asyncio/test_eager_task_factory.py
+++ b/Lib/test/test_asyncio/test_eager_task_factory.py
@@ -217,7 +217,12 @@ def test_staggered_race_with_eager_tasks(self):
         # See https://github.com/python/cpython/issues/124309
 
         async def fail():
-            await asyncio.sleep(0)
+            # Fail without suspending first.  staggered_race() awaits the
+            # coroutines inline and cancels every one of them that has not
+            # finished once another one wins, so a coroutine that yields before
+            # raising may be cancelled instead, and excs[2] would report a
+            # CancelledError.  Raising right away keeps the outcome
+            # independent of how busy the event loop is.
             raise ValueError("no good")
 
         async def blocked():
diff --git a/Misc/NEWS.d/next/Tests/2026-09-30-00-00-00.gh-issue-158097.pcB3mO.rst b/Misc/NEWS.d/next/Tests/2026-09-30-00-00-00.gh-issue-158097.pcB3mO.rst
new file mode 100644
index 000000000000000..ccf4ecfcfdc1901
--- /dev/null
+++ b/Misc/NEWS.d/next/Tests/2026-09-30-00-00-00.gh-issue-158097.pcB3mO.rst
@@ -0,0 +1,2 @@
+Fix a rare flakiness in ``test_staggered_race_with_eager_tasks`` in
+``test_asyncio``.