Merge "Fix WeakSequence circular reference"

3 files changed, 28 insertions, 4 deletions

diff --git a/doc/build/changelog/unreleased_13/5050.rst b/doc/build/changelog/unreleased_13/5050.rst
new file mode 100644
index 000000000..9831406b8
--- /dev/null
+++ b/doc/build/changelog/unreleased_13/5050.rst
@@ -0,0 +1,9 @@
+.. change::
+    :tags: bug, orm
+    :tickets: 5050
+
+    Fixed a reference cycle which could impact the GC behavior of the
+    :class:`.WeakSequence` object, currently used within one place in certain
+    mapper configurations.  The issue only affects configuration-time
+    structures. Pull request courtesy Carson Ip.
+
diff --git a/lib/sqlalchemy/util/_collections.py b/lib/sqlalchemy/util/_collections.py
index 75e1727df..04409cfd9 100644
--- a/lib/sqlalchemy/util/_collections.py
+++ b/lib/sqlalchemy/util/_collections.py
@@ -676,16 +676,18 @@ class IdentitySet(object):
 
 class WeakSequence(object):
     def __init__(self, __elements=()):
+        def _remove(item, selfref=weakref.ref(self)):
+            self = selfref()
+            if self is not None:
+                self._storage.remove(item)
+        self._remove = _remove
         self._storage = [
-            weakref.ref(element, self._remove) for element in __elements
+            weakref.ref(element, _remove) for element in __elements
         ]
 
     def append(self, item):
         self._storage.append(weakref.ref(item, self._remove))
 
-    def _remove(self, ref):
-        self._storage.remove(ref)
-
     def __len__(self):
         return len(self._storage)
 
diff --git a/test/base/test_utils.py b/test/base/test_utils.py
index e4d5a4d5f..7cdda0c23 100644
--- a/test/base/test_utils.py
+++ b/test/base/test_utils.py
@@ -188,6 +188,19 @@ class WeakSequenceTest(fixtures.TestBase):
         eq_(len(w), 2)
         eq_(len(w._storage), 2)
 
+    @testing.requires.predictable_gc
+    def test_cleanout_container(self):
+        import weakref
+
+        class Foo(object):
+            pass
+
+        f = Foo()
+        w = WeakSequence([f])
+        w_wref = weakref.ref(w)
+        del w
+        eq_(w_wref(), None)
+
 
 class OrderedDictTest(fixtures.TestBase):
     def test_odict(self):