Raise correct error instead of class exist in Placement API

When create resource class using placement API, and at
the DB layer, we have a retry logic for 100 loop, and
error will raise when the retry limit exceed, currently
a log about limit exceed is generated but a
ResourceClassExists Error is raised, which is a little
bit weird and might confuse the users.

We'd better add a particular Exception for this kind of
scenario and it could be used in other places also.

Co-Authored-By: huangtianhua@huawei.com
Closes-Bug: #1658868

Change-Id: I89156be82ad520bdeacb12b31b44e7170b77ce3f

nova/api/openstack/placement/handlers/resource_class.py

@@ -83,6 +83,13 @@ def create_resource_class(req):
             _('Conflicting resource class already exists: %(name)s') %
             {'name': data['name']},
             json_formatter=util.json_error_formatter)
+    except exception.MaxDBRetriesExceeded:
+        raise webob.exc.HTTPConflict(
+            _('Max retries of DB transaction exceeded attempting '
+              'to create resource class: %(name)s, please'
+              'try again.') %
+            {'name': data['name']},
+            json_formatter=util.json_error_formatter)
 
     req.response.location = util.resource_class_url(req.environ, rc)
     req.response.status = 201

nova/exception.py

@@ -2067,6 +2067,11 @@ class LibguestfsCannotReadKernel(Invalid):
     msg_fmt = _("Libguestfs does not have permission to read host kernel.")
 
 
+class MaxDBRetriesExceeded(NovaException):
+    msg_fmt = _("Max retries of DB transaction exceeded attempting to "
+                "perform %(action)s.")
+
+
 class RealtimePolicyNotSupported(Invalid):
     msg_fmt = _("Realtime policy not supported by hypervisor")
 

nova/objects/resource_provider.py

@@ -28,7 +28,7 @@ from nova.db.sqlalchemy import api as db_api
 from nova.db.sqlalchemy import api_models as models
 from nova.db.sqlalchemy import resource_class_cache as rc_cache
 from nova import exception
-from nova.i18n import _LW
+from nova.i18n import _, _LW
 from nova import objects
 from nova.objects import base
 from nova.objects import fields
@@ -1366,7 +1366,8 @@ class ResourceClass(base.NovaObject):
             LOG.warning(_LW("Exceeded retry limit on ID generation while "
                             "creating ResourceClass %(name)s"),
                         {'name': self.name})
-            raise exception.ResourceClassExists(resource_class=self.name)
+            msg = _("creating resource class %s") % self.name
+            raise exception.MaxDBRetriesExceeded(action=msg)
 
     @staticmethod
     @db_api.api_context_manager.writer

nova/tests/functional/db/test_resource_provider.py

@@ -1402,7 +1402,7 @@ class ResourceClassTestCase(ResourceProviderBaseCase):
             name='CUSTOM_TWO',
         )
         rc2.RESOURCE_CREATE_RETRY_COUNT = 3
-        self.assertRaises(exception.ResourceClassExists, rc2.create)
+        self.assertRaises(exception.MaxDBRetriesExceeded, rc2.create)
 
     def test_create_duplicate_custom(self):
         rc = objects.ResourceClass(