Fixed #25840 -- Fixed BaseCache.get_or_set() on the DummyCache backend.

This also fixes a possible data eviction race condition between
setting and getting a key. Another thread could remove the key
before get_and_set() accesses it again. In this case, now the
default value will be returned instead of None.

django/core/cache/backends/base.py

@@ -165,7 +165,7 @@ class BaseCache(object):
                 default = default()
             val = self.add(key, default, timeout=timeout, version=version)
             if val:
-                return self.get(key, version=version)
+                return self.get(key, default, version)
         return val
 
     def has_key(self, key, version=None):

docs/releases/1.9.1.txt

@@ -9,4 +9,5 @@ Django 1.9.1 fixes several bugs in 1.9.
 Bugfixes
 ========
 
-* ...
+* Fixed ``BaseCache.get_or_set()`` with the ``DummyCache`` backend
+  (:ticket:`25840`).

tests/cache/tests.py

@@ -202,6 +202,15 @@ class DummyCacheTests(SimpleTestCase):
         self.assertRaises(ValueError, cache.decr_version, 'answer')
         self.assertRaises(ValueError, cache.decr_version, 'does_not_exist')
 
+    def test_get_or_set(self):
+        self.assertEqual(cache.get_or_set('mykey', 'default'), 'default')
+
+    def test_get_or_set_callable(self):
+        def my_callable():
+            return 'default'
+
+        self.assertEqual(cache.get_or_set('mykey', my_callable), 'default')
+
 
 def custom_key_func(key, key_prefix, version):
     "A customized cache key function"