cache: new module for utility methods with cache

First function is get_shared_cache(), it returns an instance of a shared
if it can or raise ImproperlyConfigured.

authentic2/cache.py

@@ -0,0 +1,25 @@
+from django.core.exceptions import ImproperlyConfigured
+
+from django.core.cache import InvalidCacheBackendError, cache, get_cache
+from django.core.cache.backends.locmem import LocMemCache
+from django.core.cache.backends.dummy import DummyCache
+
+
+def get_shared_cache(name=None):
+    '''Try to return a cache backend shared between requests. Fail by raising
+       ImproperlyConfigured.
+    '''
+    candidate = cache
+    try:
+        candidate = get_cache('persistent')
+    except InvalidCacheBackendError:
+        pass
+    if name is not None:
+        try:
+            candidate = get_cache(name)
+        except InvalidCacheBackendError:
+            pass
+    if type(cache) in (DummyCache, LocMemCache):
+        raise ImproperlyConfigured('no shared cache backend is configured for %r' % name)
+    return cache
+