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):