gh-155854: Fix asyncio.shield() leaking cancelled waiters

Author: deadlovelll

Lib/asyncio/tasks.py

@@ -994,6 +994,10 @@ def _inner_done_callback(inner):
     def _outer_done_callback(outer):
         if not inner.done():
             inner.remove_done_callback(_inner_done_callback)
+            # gh-155854: waiter is gone but inner lives on, clean up here
+            if cur_task is not None:
+                inner.remove_done_callback(_clear_awaited_by_callback)
+                futures.future_discard_from_awaited_by(inner, cur_task)
             # Keep only one callback to log on cancel
             inner.remove_done_callback(_log_on_exception)
             inner.add_done_callback(_log_on_exception)

Lib/test/test_asyncio/test_tasks.py

@@ -2150,6 +2150,19 @@ def test_shield_duplicate_log_once(self):
         test_utils.run_briefly(self.loop)
         mock_handler.assert_called_once()
 
+    def test_shield_discards_awaited_by_on_outer_cancel(self):
+        # gh-155854: a cancelled waiter must not stay in inner's await-graph
+        async def coro():
+            inner = self.new_future(self.loop)
+            for _ in range(3):
+                asyncio.shield(inner).cancel()
+                await asyncio.sleep(0)
+            self.assertFalse(inner._asyncio_awaited_by)
+            self.assertEqual(1, len(inner._callbacks))
+            inner.cancel()
+
+        self.loop.run_until_complete(self.new_task(self.loop, coro()))
+
     def test_shield_shortcut(self):
         fut = self.new_future(self.loop)
         fut.set_result(42)

Misc/NEWS.d/next/Library/2026-08-15-18-30-39.gh-issue-155854.jQYOhY.rst

@@ -0,0 +1,2 @@
+Fix :func:`asyncio.shield` keeping cancelled waiters alive via
+``awaited_by``.