Add Loki service plugin for optional DB havoc

This adds a service plugin to introduce random delays
and deadlocks to DB operations to make it easier for
us to see that retry decorators are correctly applied
and race conditions are handled.

Change-Id: I8e283c1b53165faee548d26b3560a2c883dfb977

doc/source/devref/db_transient_failure_injection.rst

@@ -0,0 +1,38 @@
+..
+      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.
+
+
+      Convention for heading levels in Neutron devref:
+      =======  Heading 0 (reserved for the title in a document)
+      -------  Heading 1
+      ~~~~~~~  Heading 2
+      +++++++  Heading 3
+      '''''''  Heading 4
+      (Avoid deeper levels because they do not render well.)
+
+
+Transient DB Failure Injection
+==============================
+
+Neutron has a service plugin to inject random delays and Deadlock exceptions
+into normal Neutron operations. The service plugin is called 'Loki' and is
+located under neutron.services.loki.loki_plugin.
+
+To enable the plugin, just add 'loki' to the list of service_plugins in your
+neutron-server neutron.conf file.
+
+The plugin will inject a Deadlock exception on database flushes with a 1/50
+probability and a delay of 1 second with a 1/200 probability when SQLAlchemy
+objects are loaded into the persistent state from the DB. The goal is to ensure
+the code is tolerant of these transient delays/failures that will be experienced
+in busy production (and Galera) systems.

doc/source/devref/index.rst

@@ -91,6 +91,7 @@ Testing
    fullstack_testing
    testing_coverage
    template_model_sync_test
+   db_transient_failure_injection
 
 Module Reference
 ----------------

neutron/services/loki/loki_plugin.py

@@ -0,0 +1,49 @@
+#
+#    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.
+
+import random
+import time
+
+from neutron.db import api as db_api
+from neutron.services import service_base
+
+from oslo_db import exception as db_exc
+from oslo_log import log as logging
+from sqlalchemy.orm import session as se
+
+LOG = logging.getLogger(__name__)
+
+
+class LokiPlugin(service_base.ServicePluginBase):
+    """Loki brings us the gift of sporadic database failures and delays."""
+
+    def __init__(self):
+        super(LokiPlugin, self).__init__()
+        db_api.sqla_listen(se.Session, 'before_flush', self.random_deadlock)
+        db_api.sqla_listen(se.Session, 'loaded_as_persistent',
+                           self.random_delay)
+
+    def random_deadlock(self, session, flush_context, instances):
+        if random.randrange(0, 51) > 49:  # 1/50 probability
+            raise db_exc.DBDeadlock()
+
+    def random_delay(self, session, instance):
+        if random.randrange(0, 201) > 199:  # 1/200 probability
+            LOG.debug("Loki has delayed loading of instance %s", instance)
+            time.sleep(1)
+
+    def get_plugin_type(self):
+        return "loki"
+
+    def get_plugin_description(self):
+        return "Injects deadlocks and delays into database operations."

setup.cfg

@@ -81,6 +81,7 @@ neutron.service_plugins =
     revisions = neutron.services.revisions.revision_plugin:RevisionPlugin
     timestamp = neutron.services.timestamp.timestamp_plugin:TimeStampPlugin
     trunk = neutron.services.trunk.plugin:TrunkPlugin
+    loki = neutron.services.loki.loki_plugin:LokiPlugin
 neutron.qos.notification_drivers =
     message_queue = neutron.services.qos.notification_drivers.message_queue:RpcQosServiceNotificationDriver
 neutron.ml2.type_drivers =