gh-128550: TaskGroup: cancel tasks added to set after aborting

Lib/asyncio/taskgroups.py

@@ -205,6 +205,11 @@ def create_task(self, coro, *, name=None, context=None):
         else:
             self._tasks.add(task)
             task.add_done_callback(self._on_task_done)
+            if self._aborting:
+                # gh-128550: if this task is eager it might have started
+                # another eager task that aborts us, if so we must cancel
+                # this task.
+                task.cancel()
         return task
 
     # Since Python 3.8 Tasks propagate all exceptions correctly,

Lib/test/test_asyncio/test_taskgroups.py

@@ -997,6 +997,30 @@ class MyKeyboardInterrupt(KeyboardInterrupt):
         self.assertIsNotNone(exc)
         self.assertListEqual(gc.get_referrers(exc), no_other_refs())
 
+    async def test_cancels_task_if_created_during_creation(self):
+        ran = False
+        class MyError(Exception):
+            pass
+
+        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.assertIsInstance(exc, MyError)
+        self.assertTrue(ran)
 
 if __name__ == "__main__":
     unittest.main()