pythongh-128552: fix refcycles in eager task creation (python#128553)

(cherry picked from commit 61b9811)

Author: graingert

Lib/asyncio/base_events.py

@@ -466,7 +466,12 @@ def create_task(self, coro, *, name=None, context=None):
 
             tasks._set_task_name(task, name)
 
-        return task
+        try:
+            return task
+        finally:
+            # gh-128552: prevent a refcycle of
+            # task.exception().__traceback__->BaseEventLoop.create_task->task
+            del task
 
     def set_task_factory(self, factory):
         """Set a task factory that will be used by loop.create_task().

Lib/asyncio/taskgroups.py

@@ -193,7 +193,12 @@ def create_task(self, coro, *, name=None, context=None):
         else:
             self._tasks.add(task)
             task.add_done_callback(self._on_task_done)
-        return task
+        try:
+            return task
+        finally:
+            # gh-128552: prevent a refcycle of
+            # task.exception().__traceback__->TaskGroup.create_task->task
+            del task
 
     # Since Python 3.8 Tasks propagate all exceptions correctly,
     # except for KeyboardInterrupt and SystemExit which are

Lib/test/test_asyncio/test_taskgroups.py

@@ -1,6 +1,8 @@
 # Adapted with permission from the EdgeDB project;
 # license: PSFL.
 
+import weakref
+import sys
 import gc
 import asyncio
 import contextvars
@@ -27,7 +29,25 @@ def get_error_types(eg):
     return {type(exc) for exc in eg.exceptions}
 
 
-class TestTaskGroup(unittest.IsolatedAsyncioTestCase):
+def set_gc_state(enabled):
+    was_enabled = gc.isenabled()
+    if enabled:
+        gc.enable()
+    else:
+        gc.disable()
+    return was_enabled
+
+
+@contextlib.contextmanager
+def disable_gc():
+    was_enabled = set_gc_state(enabled=False)
+    try:
+        yield
+    finally:
+        set_gc_state(enabled=was_enabled)
+
+
+class BaseTestTaskGroup:
 
     async def test_taskgroup_01(self):
 
@@ -820,8 +840,82 @@ async def test_taskgroup_without_parent_task(self):
         coro = asyncio.sleep(0)
         with self.assertRaisesRegex(RuntimeError, "has not been entered"):
             tg.create_task(coro)
-        # We still have to await coro to avoid a warning
-        await coro
+
+    async def test_coro_closed_when_tg_closed(self):
+        async def run_coro_after_tg_closes():
+            async with taskgroups.TaskGroup() as tg:
+                pass
+            coro = asyncio.sleep(0)
+            with self.assertRaisesRegex(RuntimeError, "is finished"):
+                tg.create_task(coro)
+
+        await run_coro_after_tg_closes()
+
+    async def test_cancelling_level_preserved(self):
+        async def raise_after(t, e):
+            await asyncio.sleep(t)
+            raise e()
+
+        try:
+            async with asyncio.TaskGroup() as tg:
+                tg.create_task(raise_after(0.0, RuntimeError))
+        except* RuntimeError:
+            pass
+        self.assertEqual(asyncio.current_task().cancelling(), 0)
+
+    async def test_nested_groups_both_cancelled(self):
+        async def raise_after(t, e):
+            await asyncio.sleep(t)
+            raise e()
+
+        try:
+            async with asyncio.TaskGroup() as outer_tg:
+                try:
+                    async with asyncio.TaskGroup() as inner_tg:
+                        inner_tg.create_task(raise_after(0, RuntimeError))
+                        outer_tg.create_task(raise_after(0, ValueError))
+                except* RuntimeError:
+                    pass
+                else:
+                    self.fail("RuntimeError not raised")
+            self.assertEqual(asyncio.current_task().cancelling(), 1)
+        except* ValueError:
+            pass
+        else:
+            self.fail("ValueError not raised")
+        self.assertEqual(asyncio.current_task().cancelling(), 0)
+
+    async def test_error_and_cancel(self):
+        event = asyncio.Event()
+
+        async def raise_error():
+            event.set()
+            await asyncio.sleep(0)
+            raise RuntimeError()
+
+        async def inner():
+            try:
+                async with taskgroups.TaskGroup() as tg:
+                    tg.create_task(raise_error())
+                    await asyncio.sleep(1)
+                    self.fail("Sleep in group should have been cancelled")
+            except* RuntimeError:
+                self.assertEqual(asyncio.current_task().cancelling(), 1)
+            self.assertEqual(asyncio.current_task().cancelling(), 1)
+            await asyncio.sleep(1)
+            self.fail("Sleep after group should have been cancelled")
+
+        async def outer():
+            t = asyncio.create_task(inner())
+            await event.wait()
+            self.assertEqual(t.cancelling(), 0)
+            t.cancel()
+            self.assertEqual(t.cancelling(), 1)
+            with self.assertRaises(asyncio.CancelledError):
+                await t
+            self.assertTrue(t.cancelled())
+
+        await outer()
 
     async def test_exception_refcycles_direct(self):
         """Test that TaskGroup doesn't keep a reference to the raised ExceptionGroup"""
@@ -880,6 +974,30 @@ async def coro_fn():
         self.assertIsInstance(exc, _Done)
         self.assertListEqual(gc.get_referrers(exc), [])
 
+
+    async def test_exception_refcycles_parent_task_wr(self):
+        """Test that TaskGroup deletes self._parent_task and create_task() deletes task"""
+        tg = asyncio.TaskGroup()
+        exc = None
+
+        class _Done(Exception):
+            pass
+
+        async def coro_fn():
+            async with tg:
+                raise _Done
+
+        with disable_gc():
+            try:
+                async with asyncio.TaskGroup() as tg2:
+                    task_wr = weakref.ref(tg2.create_task(coro_fn()))
+            except* _Done as excs:
+                exc = excs.exceptions[0].exceptions[0]
+
+        self.assertIsNone(task_wr())
+        self.assertIsInstance(exc, _Done)
+        self.assertListEqual(gc.get_referrers(exc), [])
+
     async def test_exception_refcycles_propagate_cancellation_error(self):
         """Test that TaskGroup deletes propagate_cancellation_error"""
         tg = asyncio.TaskGroup()
@@ -912,6 +1030,32 @@ class MyKeyboardInterrupt(KeyboardInterrupt):
         self.assertIsNotNone(exc)
         self.assertListEqual(gc.get_referrers(exc), [])
 
+if sys.platform == "win32":
+    EventLoop = asyncio.ProactorEventLoop
+else:
+    EventLoop = asyncio.SelectorEventLoop
+
+
+class IsolatedAsyncioTestCase(unittest.IsolatedAsyncioTestCase):
+    loop_factory = None
+
+    def _setupAsyncioRunner(self):
+        assert self._asyncioRunner is None, 'asyncio runner is already initialized'
+        runner = asyncio.Runner(debug=True, loop_factory=self.loop_factory)
+        self._asyncioRunner = runner
+
+
+class TestTaskGroup(BaseTestTaskGroup, IsolatedAsyncioTestCase):
+    loop_factory = EventLoop
+
+
+class TestEagerTaskTaskGroup(BaseTestTaskGroup, IsolatedAsyncioTestCase):
+    @staticmethod
+    def loop_factory():
+        loop = EventLoop()
+        loop.set_task_factory(asyncio.eager_task_factory)
+        return loop
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2025-01-06-18-41-08.gh-issue-128552.fV-f8j.rst

@@ -0,0 +1 @@
+Fix cyclic garbage introduced by :meth:`asyncio.loop.create_task` and :meth:`asyncio.TaskGroup.create_task` holding a reference to the created task if it is eager.