Avoid creating reference cycles in case of nursery cancel

trio/_core/_multierror.py

@@ -114,7 +114,7 @@ def push_tb_down(tb, exc, preserved):
     preserved = set()
     new_root_exc = filter_tree(root_exc, preserved)
     push_tb_down(None, root_exc, preserved)
-    # Delete the local functions avoid a reference cycle (see
+    # Delete the local functions to avoid a reference cycle (see
     # test_simple_cancel_scope_usage_doesnt_create_cyclic_garbage)
     del filter_tree, push_tb_down
     return new_root_exc

trio/_core/_run.py

@@ -937,7 +937,12 @@ def aborted(raise_cancel):
         popped = self._parent_task._child_nurseries.pop()
         assert popped is self
         if self._pending_excs:
-            return MultiError(self._pending_excs)
+            try:
+                return MultiError(self._pending_excs)
+            finally:
+                # avoid a garbage cycle
+                # (see test_nursery_cancel_doesnt_create_cyclic_garbage)
+                del self._pending_excs
 
     def start_soon(self, async_fn, *args, name=None):
         """Creates a child task, scheduling ``await async_fn(*args)``.

trio/_core/tests/test_run.py

@@ -2226,3 +2226,26 @@ async def do_a_cancel():
     finally:
         gc.set_debug(old_flags)
         gc.garbage.clear()
+
+
+@pytest.mark.skipif(
+    sys.implementation.name != "cpython", reason="Only makes sense with refcounting GC"
+)
+async def test_nursery_cancel_doesnt_create_cyclic_garbage():
+    # https://github.com/python-trio/trio/issues/1770#issuecomment-730229423
+    gc.collect()
+
+    old_flags = gc.get_debug()
+    try:
+        for i in range(3):
+            async with _core.open_nursery() as nursery:
+                gc.collect()
+                gc.set_debug(gc.DEBUG_LEAK)
+                nursery.cancel_scope.cancel()
+
+            gc.collect()
+            gc.set_debug(0)
+            assert not gc.garbage
+    finally:
+        gc.set_debug(old_flags)
+        gc.garbage.clear()