copy list for __iadd__

Fixed issue where a list mapped with :func:`_orm.relationship` would go
into an endless loop if in-place added to itself, i.e. the ``+=`` operator
were used, as well as if ``.extend()`` were given the same list.

Fixes: #7389
Change-Id: Idd5118420f8bc684d1ee36b2b6d4c5812f36cc4c

1 files changed, 13 insertions, 0 deletions

diff --git a/test/orm/test_collection.py b/test/orm/test_collection.py
index 9babe6366..806d98a69 100644
--- a/test/orm/test_collection.py
+++ b/test/orm/test_collection.py
@@ -2500,6 +2500,19 @@ class CustomCollectionsTest(fixtures.MappedTest):
         assert control == p.children
         assert control == list(p.children)
 
+        # test #7389
+        if hasattr(p.children, "__iadd__"):
+            control += control
+            p.children += p.children
+            assert control == list(p.children)
+
+        control[:] = [o]
+        p.children[:] = [o]
+        if hasattr(p.children, "extend"):
+            control.extend(control)
+            p.children.extend(p.children)
+            assert control == list(p.children)
+
     def test_custom(self):
         someothertable, sometable = (
             self.tables.someothertable,