Allow plugins to be specified as a cloud_backend

Currently, the cloud_backend configuration option can only use modules
imported in-tree before clients.py is imported. This does not work for
plugins because they are dynamically imported in
heat/engine/resources/__init__.py after clients.py is imported. Change
the cloud_backend module to be specified when the client class needs
to be initialized rather than when clients.py is imported. This change
requires a class to be specified for the cloud_backend rather than a
module name.

Change-Id: I8f2b93bfb6a9585d1e7dc52d70092105b6e1bfaa
Closes-Bug: 1242870

etc/heat/heat.conf.sample

@@ -107,9 +107,9 @@
 # Options defined in heat.engine.clients
 #
 
-# Cloud module to use as a backend. Defaults to OpenStack.
+# Fully qualified class name to use as a client backend.
 # (string value)
-#cloud_backend=<None>
+#cloud_backend=heat.engine.clients.OpenStackClients
 
 
 #

heat/engine/clients.py

@@ -46,11 +46,12 @@ except ImportError:
     ceilometerclient = None
     logger.info('ceilometerclient not available')
 
+_default_backend = "heat.engine.clients.OpenStackClients"
 
 cloud_opts = [
     cfg.StrOpt('cloud_backend',
-               default=None,
-               help="Cloud module to use as a backend. Defaults to OpenStack.")
+               default=_default_backend,
+               help="Fully qualified class name to use as a client backend.")
 ]
 cfg.CONF.register_opts(cloud_opts)
 
@@ -59,7 +60,6 @@ class OpenStackClients(object):
     '''
     Convenience class to create and cache client instances.
     '''
-
     def __init__(self, context):
         self.context = context
         self._nova = {}
@@ -210,10 +210,18 @@ class OpenStackClients(object):
         return self._ceilometer
 
 
-if cfg.CONF.cloud_backend:
-    cloud_backend_module = importutils.import_module(cfg.CONF.cloud_backend)
-    Clients = cloud_backend_module.Clients
-else:
-    Clients = OpenStackClients
+class ClientBackend(object):
+    '''Delay choosing the backend client module until the client's class needs
+    to be initialized.
+    '''
+    def __new__(cls, context):
+        if cfg.CONF.cloud_backend == _default_backend:
+            return OpenStackClients(context)
+        else:
+            return importutils.import_object(
+                cfg.CONF.cloud_backend,
+                context
+            )
 
-logger.debug('Using backend %s' % Clients)
+
+Clients = ClientBackend

heat/tests/test_clients.py

@@ -0,0 +1,23 @@
+# vim: tabstop=4 shiftwidth=4 softtabstop=4
+
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from heat.engine import clients
+from heat.tests.common import HeatTestCase
+
+
+class ClientsTest(HeatTestCase):
+
+    def test_clients_chosen_at_module_initilization(self):
+        self.assertFalse(hasattr(clients.Clients, 'nova'))
+        self.assertTrue(hasattr(clients.Clients('fakecontext'), 'nova'))