If caching is globally disabled force dogpile to use the null backend

Due to the way caching is disabled the SHOULD_CACHE_FN() is used
to determine if new values are stored in the cache backend, and is
only called when the regeneration is required. If dogpile is
configured to connect to a memcache, redis, etc to store data
it is possible with caching disabled to still pull values from
the cache (SHOULD_CACHE_FN has no bearing on reads).

The issue described only impacts the use of the memoization
decorator.

This change forces dogpile to use the null backend if caching is
globally disabled to ensure no data is read from the external
cache. This will not affect subsystem disabling of cache.

Even with cache disabled but reads coming from the external cache,
there stale data is not a concern as invalidates will still be
processed and the data from the backend will eventually timeout
in most cases.

Change-Id: I845b6cc18faa2be516676eeacc574473ca84c995
Closes-Bug: #1567413

oslo_cache/_opts.py

@@ -15,6 +15,8 @@
 from oslo_config import cfg
 
 
+_DEFAULT_BACKEND = 'dogpile.cache.null'
+
 FILE_OPTIONS = {
     'cache': [
         cfg.StrOpt('config_prefix', default='cache.oslo',
@@ -35,7 +37,7 @@ FILE_OPTIONS = {
         # prevent issues with the memory cache ending up in "production"
         # unintentionally, we register a no-op as the keystone default caching
         # backend.
-        cfg.StrOpt('backend', default='dogpile.cache.null',
+        cfg.StrOpt('backend', default=_DEFAULT_BACKEND,
                    help='Dogpile.cache backend module. It is recommended '
                         'that Memcache with pooling '
                         '(oslo_cache.memcache_pool) or Redis '

oslo_cache/core.py

@@ -114,7 +114,9 @@ def _build_cache_config(conf):
     """
     prefix = conf.cache.config_prefix
     conf_dict = {}
-    conf_dict['%s.backend' % prefix] = conf.cache.backend
+    conf_dict['%s.backend' % prefix] = _opts._DEFAULT_BACKEND
+    if conf.cache.enabled is True:
+        conf_dict['%s.backend' % prefix] = conf.cache.backend
     conf_dict['%s.expiration_time' % prefix] = conf.cache.expiration_time
     for argument in conf.cache.backend_argument:
         try:

oslo_cache/tests/test_cache.py

@@ -23,6 +23,7 @@ from oslo_config import cfg
 from oslo_config import fixture as config_fixture
 from oslotest import base
 
+from oslo_cache import _opts
 from oslo_cache import core as cache
 from oslo_cache import exception
 
@@ -266,6 +267,19 @@ class CacheRegionTest(BaseTestCase):
                          config_dict['test_prefix.arguments.arg2'])
         self.assertNotIn('test_prefix.arguments.arg3', config_dict)
 
+    def test_cache_dictionary_config_builder_global_disabled(self):
+        """Validate the backend is reset to default if caching is disabled."""
+        self.config_fixture.config(group='cache',
+                                   enabled=False,
+                                   config_prefix='test_prefix',
+                                   backend='some_test_backend')
+
+        self.assertFalse(self.config_fixture.conf.cache.enabled)
+        config_dict = cache._build_cache_config(self.config_fixture.conf)
+        self.assertEqual(
+            _opts._DEFAULT_BACKEND,
+            config_dict['test_prefix.backend'])
+
     def test_cache_debug_proxy(self):
         single_value = 'Test Value'
         single_key = 'testkey'