[3.12] gh-128588: fix refcycles in eager task creation and remove eager tasks optimization that missed and introduced incorrect cancellations (#129063) (#128586)

Co-authored-by: Kumar Aditya <[email protected]>

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

@@ -185,15 +185,20 @@ def create_task(self, coro, *, name=None, context=None):
         else:
             task = self._loop.create_task(coro, context=context)
         tasks._set_task_name(task, name)
-        # optimization: Immediately call the done callback if the task is
+
+        # Always schedule the done callback even if the task is
         # already done (e.g. if the coro was able to complete eagerly),
-        # and skip scheduling a done callback
-        if task.done():
-            self._on_task_done(task)
-        else:
-            self._tasks.add(task)
-            task.add_done_callback(self._on_task_done)
-        return task
+        # otherwise if the task completes with an exception then it will cancel
+        # the current task too early. gh-128550, gh-128588
+
+        self._tasks.add(task)
+        task.add_done_callback(self._on_task_done)
+        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):
 
@@ -880,6 +900,30 @@ 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 +956,81 @@ class MyKeyboardInterrupt(KeyboardInterrupt):
         self.assertIsNotNone(exc)
         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)
+
+
+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.

Misc/NEWS.d/next/Library/2025-01-20-13-12-39.gh-issue-128550.AJ5TOL.rst

@@ -0,0 +1 @@
+Removed an incorrect optimization relating to eager tasks in :class:`asyncio.TaskGroup` that resulted in cancellations being missed.