[3.12] GH-124639: add back loop param to staggered_race (GH-124700) (#124744)

GH-124639: add back loop param to staggered_race (GH-124700)
(cherry picked from commit e0a41a5dd1)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Lib/asyncio/staggered.py

@@ -11,7 +11,7 @@
 class _Done(Exception):
     pass
 
-async def staggered_race(coro_fns, delay):
+async def staggered_race(coro_fns, delay, *, loop=None):
     """Run coroutines with staggered start times and take the first to finish.
 
     This method takes an iterable of coroutine functions. The first one is
@@ -82,7 +82,13 @@ async def run_one_coro(this_index, coro_fn, this_failed):
             raise _Done
 
     try:
-        async with taskgroups.TaskGroup() as tg:
+        tg = taskgroups.TaskGroup()
+        # Intentionally override the loop in the TaskGroup to avoid
+        # using the running loop, preserving backwards compatibility
+        # TaskGroup only starts using `_loop` after `__aenter__`
+        # so overriding it here is safe.
+        tg._loop = loop
+        async with tg:
             for this_index, coro_fn in enumerate(coro_fns):
                 this_failed = locks.Event()
                 exceptions.append(None)

Lib/test/test_asyncio/test_staggered.py

@@ -121,6 +121,25 @@ async def coro(index):
         self.assertIsInstance(excs[0], ValueError)
         self.assertIsNone(excs[1])
 
+    def test_loop_argument(self):
+        loop = asyncio.new_event_loop()
+        async def coro():
+            self.assertEqual(loop, asyncio.get_running_loop())
+            return 'coro'
+
+        async def main():
+            winner, index, excs = await staggered_race(
+                [coro],
+                delay=0.1,
+                loop=loop
+            )
+
+            self.assertEqual(winner, 'coro')
+            self.assertEqual(index, 0)
+
+        loop.run_until_complete(main())
+        loop.close()
+
 
 if __name__ == "__main__":
     unittest.main()