Refactor compute_utils to avoid db lookup

This simple change passes bdms into the following compute_utils function
to avoid it doing a database lookup:

  get_device_name_for_instance()

Related to bp/no-db-compute

Change-Id: I1475b184d8e88b700d606aaf77cc7cd21e83efbb

nova/compute/manager.py

@@ -2464,8 +2464,11 @@ class ComputeManager(manager.SchedulerDependentManager):
 
         @lockutils.synchronized(instance['uuid'], 'nova-')
         def do_reserve():
+            bdms = self.conductor_api.block_device_mapping_get_all_by_instance(
+                context, instance)
             result = compute_utils.get_device_name_for_instance(context,
                                                                 instance,
+                                                                bdms,
                                                                 device)
             # NOTE(vish): create bdm here to avoid race condition
             values = {'instance_uuid': instance['uuid'],

nova/compute/utils.py

@@ -71,7 +71,7 @@ def add_instance_fault_from_exc(context, instance, fault, exc_info=None):
     db.instance_fault_create(context, values)
 
 
-def get_device_name_for_instance(context, instance, device):
+def get_device_name_for_instance(context, instance, bdms, device):
     """Validates (or generates) a device name for instance.
 
     If device is not set, it will generate a unique device appropriate
@@ -88,8 +88,6 @@ def get_device_name_for_instance(context, instance, device):
             req_prefix, req_letters = block_device.match_device(device)
         except (TypeError, AttributeError, ValueError):
             raise exception.InvalidDevicePath(path=device)
-    bdms = db.block_device_mapping_get_all_by_instance(context,
-                instance['uuid'])
     mappings = block_device.instance_block_mapping(instance, bdms)
     try:
         prefix = block_device.match_device(mappings['root'])[0]

nova/tests/compute/test_compute_utils.py

@@ -69,8 +69,11 @@ class ComputeValidateDeviceTestCase(test.TestCase):
                        lambda context, instance: self.data)
 
     def _validate_device(self, device=None):
+        bdms = db.block_device_mapping_get_all_by_instance(
+            self.context, self.instance['uuid'])
         return compute_utils.get_device_name_for_instance(self.context,
                                                           self.instance,
+                                                          bdms,
                                                           device)
 
     @staticmethod