Test for short term reference cycles and resolve as many as possible

Added test support and repaired a wide variety of unnecessary reference
cycles created for short-lived objects, mostly in the area of ORM queries.

Fixes: #5056
Change-Id: Ifd93856eba550483f95f9ae63d49f36ab068b85a

2 files changed, 20 insertions, 2 deletions

diff --git a/lib/sqlalchemy/util/__init__.py b/lib/sqlalchemy/util/__init__.py
index 78155b08a..a0821a3cc 100644
--- a/lib/sqlalchemy/util/__init__.py
+++ b/lib/sqlalchemy/util/__init__.py
@@ -31,7 +31,6 @@ from ._collections import OrderedDict  # noqa
 from ._collections import OrderedIdentitySet  # noqa
 from ._collections import OrderedProperties  # noqa
 from ._collections import OrderedSet  # noqa
-from ._collections import populate_column_dict  # noqa
 from ._collections import PopulateDict  # noqa
 from ._collections import Properties  # noqa
 from ._collections import ScopedRegistry  # noqa
@@ -42,6 +41,7 @@ from ._collections import to_set  # noqa
 from ._collections import unique_list  # noqa
 from ._collections import UniqueAppender  # noqa
 from ._collections import update_copy  # noqa
+from ._collections import WeakPopulateDict  # noqa
 from ._collections import WeakSequence  # noqa
 from .compat import b  # noqa
 from .compat import b64decode  # noqa
diff --git a/lib/sqlalchemy/util/_collections.py b/lib/sqlalchemy/util/_collections.py
index 04409cfd9..7b46bc8e6 100644
--- a/lib/sqlalchemy/util/_collections.py
+++ b/lib/sqlalchemy/util/_collections.py
@@ -676,10 +676,13 @@ class IdentitySet(object):
 
 class WeakSequence(object):
     def __init__(self, __elements=()):
+        # adapted from weakref.WeakKeyDictionary, prevent reference
+        # cycles in the collection itself
         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, _remove) for element in __elements
@@ -737,6 +740,22 @@ class PopulateDict(dict):
         return val
 
 
+class WeakPopulateDict(dict):
+    """Like PopulateDict, but assumes a self + a method and does not create
+    a reference cycle.
+
+    """
+
+    def __init__(self, creator_method):
+        self.creator = creator_method.__func__
+        weakself = creator_method.__self__
+        self.weakself = weakref.ref(weakself)
+
+    def __missing__(self, key):
+        self[key] = val = self.creator(self.weakself(), key)
+        return val
+
+
 # Define collections that are capable of storing
 # ColumnElement objects as hashable keys/elements.
 # At this point, these are mostly historical, things
@@ -744,7 +763,6 @@ class PopulateDict(dict):
 column_set = set
 column_dict = dict
 ordered_column_set = OrderedSet
-populate_column_dict = PopulateDict
 
 
 _getters = PopulateDict(operator.itemgetter)