Add support for Option Groups in LazyPluggable

Use @markmc's suggestion to enhance LazyPluggable with an
optional config group. Also fix the baremetal database
backend option to use the "baremetal" config group.

Fixes LP #1093043

Change-Id: I28cf51a2962f516fcef4ced19e30c985220e86dc

nova/utils.py

@@ -507,14 +507,18 @@ def str_dict_replace(s, mapping):
 class LazyPluggable(object):
     """A pluggable backend loaded lazily based on some value."""
 
-    def __init__(self, pivot, **backends):
+    def __init__(self, pivot, config_group=None, **backends):
         self.__backends = backends
         self.__pivot = pivot
         self.__backend = None
+        self.__config_group = config_group
 
     def __get_backend(self):
         if not self.__backend:
-            backend_name = CONF[self.__pivot]
+            if self.__config_group is None:
+                backend_name = CONF[self.__pivot]
+            else:
+                backend_name = CONF[self.__config_group][self.__pivot]
             if backend_name not in self.__backends:
                 msg = _('Invalid backend: %s') % backend_name
                 raise exception.NovaException(msg)

nova/virt/baremetal/db/api.py

@@ -50,16 +50,21 @@ from nova import utils
 #             because utils.LazyPluggable doesn't support reading from
 #             option groups. See bug #1093043.
 db_opts = [
-    cfg.StrOpt('baremetal_db_backend',
+    cfg.StrOpt('db_backend',
                default='sqlalchemy',
-               help='The backend to use for db'),
+               help='The backend to use for bare-metal database'),
     ]
 
+baremetal_group = cfg.OptGroup(name='baremetal',
+                               title='Baremetal Options')
+
 CONF = cfg.CONF
-CONF.register_opts(db_opts)
+CONF.register_group(baremetal_group)
+CONF.register_opts(db_opts, baremetal_group)
 
 IMPL = utils.LazyPluggable(
-        'baremetal_db_backend',
+        'db_backend',
+        config_group='baremetal',
         sqlalchemy='nova.virt.baremetal.db.sqlalchemy.api')
 
 

nova/virt/baremetal/db/migration.py

@@ -22,7 +22,8 @@ from nova import utils
 
 
 IMPL = utils.LazyPluggable(
-        'baremetal_db_backend',
+        'db_backend',
+        config_group='baremetal',
         sqlalchemy='nova.virt.baremetal.db.sqlalchemy.migration')
 
 INIT_VERSION = 0