[3.15] gh-155143: Fix asyncio.shield() leaking tasks via await-graph (GH-155144) (#157320)

Co-authored-by: Andrew Geng <pteromys@gmail.com>

Author: miss-islington

Lib/asyncio/tasks.py

@@ -1013,6 +1013,9 @@ def _outer_done_callback(outer):
             # Keep only one callback to log on cancel
             inner.remove_done_callback(_log_on_exception)
             inner.add_done_callback(_log_on_exception)
+            if cur_task is not None:
+                inner.remove_done_callback(_clear_awaited_by_callback)
+                futures.future_discard_from_awaited_by(inner, cur_task)
 
     if cur_task is not None:
         inner.add_done_callback(_clear_awaited_by_callback)

Lib/test/test_asyncio/test_tasks.py

@@ -2147,6 +2147,8 @@ def test_shield_cancel_outer(self):
         test_utils.run_briefly(self.loop)
         self.assertTrue(outer.cancelled())
         self.assertEqual(0, 0 if outer._callbacks is None else len(outer._callbacks))
+        self.assertFalse(inner._asyncio_awaited_by)
+        self.assertTrue({f for f, _ctx in inner._callbacks or []} <= {asyncio.tasks._log_on_exception})
 
     def test_shield_cancel_outer_result(self):
         mock_handler = mock.Mock()
@@ -2172,6 +2174,21 @@ def test_shield_cancel_outer_exception(self):
         test_utils.run_briefly(self.loop)
         mock_handler.assert_called_once()
 
+    def test_shield_cancel_outer_in_task(self):
+        inner = self.new_future(self.loop)
+
+        async def coro():
+            outer = asyncio.shield(inner)
+            self.assertNotEqual(0, len(inner._callbacks))
+            outer.cancel()
+            await asyncio.sleep(0)
+            self.assertTrue(outer.cancelled())
+
+        task = self.new_task(self.loop, coro())
+        self.loop.run_until_complete(task)
+        self.assertFalse(inner._asyncio_awaited_by)
+        self.assertTrue({f for f, _ctx in inner._callbacks or []} <= {asyncio.tasks._log_on_exception})
+
     def test_shield_duplicate_log_once(self):
         mock_handler = mock.Mock()
         self.loop.set_exception_handler(mock_handler)

Misc/NEWS.d/next/Library/2026-08-03-22-46-07.gh-issue-155143.9-byZp.rst

@@ -0,0 +1,2 @@
+Fix :func:`asyncio.shield` leaking the calling task via the await-graph and
+callbacks when called on a future that never resolves.