Warn when using null cache backend

The default backend for oslo_cache is dogpile.cache.null, leading to a
broken setup when following the current release notes or the deprecation
message verbatim. So we should at least emit a warning when the config
does not specify a different backend.

Note: I'm not sure whether it is possible to amend the release note like
this or whether there needs a new note to be added, please advise.

Change-Id: I16647e424d8382dae98f13cb1f73a7e0c0aebaf5
Closes-Bug: 1572062

nova/cache_utils.py

@@ -14,24 +14,32 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-"""Super simple fake memcache client."""
+"""Simple wrapper for oslo_cache."""
 
 from oslo_cache import core as cache
+from oslo_log import log as logging
 
 import nova.conf
-from nova.i18n import _
-
+from nova.i18n import _, _LW
 
 CONF = nova.conf.CONF
 
+LOG = logging.getLogger(__name__)
+
 WEEK = 604800
 
 
+def _warn_if_null_backend():
+    if CONF.cache.backend == 'dogpile.cache.null':
+        LOG.warning(_LW("Cache enabled with backend dogpile.cache.null."))
+
+
 def get_memcached_client(expiration_time=0):
     """Used ONLY when memcached is explicitly needed."""
     # If the operator has [cache]/enabled flag on then we let oslo_cache
     # configure the region from the configuration settings
     if CONF.cache.enabled and CONF.cache.memcache_servers:
+        _warn_if_null_backend()
         return CacheClient(
                 _get_default_cache_region(expiration_time=expiration_time))
 
@@ -41,6 +49,7 @@ def get_client(expiration_time=0):
     # If the operator has [cache]/enabled flag on then we let oslo_cache
     # configure the region from configuration settings.
     if CONF.cache.enabled:
+        _warn_if_null_backend()
         return CacheClient(
                 _get_default_cache_region(expiration_time=expiration_time))
     # If [cache]/enabled flag is off, we use the dictionary backend

releasenotes/notes/switch-to-oslo-cache-7114a0ab2dea52df.yaml

@@ -5,5 +5,6 @@ deprecations:
 
   - Option ``memcached_servers`` is deprecated in Mitaka. Operators should
     use oslo.cache configuration instead. Specifically ``enabled`` option
-    under [cache] section should be set to True and the url(s) for the
-    memcached servers should be in [cache]/memcache_servers option.
+    under [cache] section should be set to True, the ``backend`` option to
+    ``oslo_cache.memcache_pool`` and the location(s) of the
+    memcached servers should be in [cache]/memcache_servers option.