[3.13] gh-128588: gh-128550: remove eager tasks optimization that missed and introduced incorrect cancellations (GH-129063) (#129089)

gh-128588: gh-128550: remove eager tasks optimization that missed and introduced incorrect cancellations (GH-129063)
(cherry picked from commit ed6934e71e)

Co-authored-by: Thomas Grainger <tagrain@gmail.com>
Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Lib/test/test_asyncio/test_taskgroups.py

@@ -1032,6 +1032,56 @@ class BaseTestTaskGroup:
         self.assertListEqual(gc.get_referrers(exc), [])
 
 
+    async def test_cancels_task_if_created_during_creation(self):
+        # regression test for gh-128550
+        ran = False
+        class MyError(Exception):
+            pass
+
+        exc = None
+        try:
+            async with asyncio.TaskGroup() as tg:
+                async def third_task():
+                    raise MyError("third task failed")
+
+                async def second_task():
+                    nonlocal ran
+                    tg.create_task(third_task())
+                    with self.assertRaises(asyncio.CancelledError):
+                        await asyncio.sleep(0)  # eager tasks cancel here
+                        await asyncio.sleep(0)  # lazy tasks cancel here
+                    ran = True
+
+                tg.create_task(second_task())
+        except* MyError as excs:
+            exc = excs.exceptions[0]
+
+        self.assertTrue(ran)
+        self.assertIsInstance(exc, MyError)
+
+
+    async def test_cancellation_does_not_leak_out_of_tg(self):
+        class MyError(Exception):
+            pass
+
+        async def throw_error():
+            raise MyError
+
+        try:
+            async with asyncio.TaskGroup() as tg:
+                tg.create_task(throw_error())
+        except* MyError:
+            pass
+        else:
+            self.fail("should have raised one MyError in group")
+
+        # if this test fails this current task will be cancelled
+        # outside the task group and inside unittest internals
+        # we yield to the event loop with sleep(0) so that
+        # cancellation happens here and error is more understandable
+        await asyncio.sleep(0)
+
+
 class TestTaskGroup(BaseTestTaskGroup, unittest.IsolatedAsyncioTestCase):
     loop_factory = asyncio.EventLoop