[3.15] gh-155418: Fix TaskGroup hang when a task cancels it before suspending (#156760)

Author: deadlovelll

Lib/asyncio/taskgroups.py

@@ -222,6 +222,9 @@ def create_task(self, coro, **kwargs):
         # the current task too early. gh-128550, gh-128588
         self._tasks.add(task)
         task.add_done_callback(self._on_task_done)
+        # gh-155418: an eager task can cancel the group before joining _tasks
+        if self._aborting and not task.done():
+            task.cancel()
         try:
             return task
         finally:

Lib/test/test_asyncio/test_taskgroups.py

@@ -1154,6 +1154,17 @@ async def test_taskgroup_cancel_before_create_task(self):
             with self.assertRaises(RuntimeError):
                 tg.create_task(asyncio.sleep(1))
 
+    async def test_taskgroup_cancel_from_child_before_first_suspension(self):
+        # gh-155418: an eager task can cancel the group before joining _tasks
+        async def child(tg):
+            tg.cancel()
+            await asyncio.sleep(10)
+            self.fail("the child was not cancelled")
+
+        async with asyncio.TaskGroup() as tg:
+            task = tg.create_task(child(tg))
+        self.assertTrue(task.cancelled())
+
     async def test_taskgroup_cancel_before_exception(self):
         async def raise_exc(parent_tg: asyncio.TaskGroup):
             parent_tg.cancel()

Misc/NEWS.d/next/Library/2026-08-09-14-27-23.gh-issue-155418.kCXUIG.rst

@@ -0,0 +1,2 @@
+Fix :class:`asyncio.TaskGroup` hang when a task created by
+:func:`asyncio.eager_task_factory` cancels the group before suspending.