Add metadata plugin mechanism

Until now we only supported storing resources in memory, and that
mechanism was tightly coupled with everything else.

This patch decouples the metadata storing into a plugin mechanism using
'Python entrypoints.  The default storing mechanism is still in memory,
and for now it's the only mechanism available.

The JSON serialization mechanism has not yet been adapted to the
metadata plugin mechanism, so it will only work with the default plugin.

cinderlib/persistence/base.py

@@ -0,0 +1,118 @@
+# Copyright (c) 2018, Red Hat, Inc.
+# All Rights Reserved.
+#
+#    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.
+
+# NOTE(geguileo): Probably a good idea not to depend on cinder.cmd.volume
+# having all the other imports as they could change.
+from cinder.cmd import volume as volume_cmd
+from cinder.objects import base as cinder_base_ovo
+from oslo_utils import timeutils
+import six
+
+
+class PersistenceDriverBase(object):
+    """Provide Metadata Persistency for our resources.
+
+    This class will be used to store new resources as they are created,
+    updated, and removed, as well as provide a mechanism for users to retrieve
+    volumes, snapshots, and connections.
+    """
+    @property
+    def db(self):
+        raise NotImplemented()
+
+    def get_volumes(self, volume_id=None, volume_name=None, backend_name=None):
+        raise NotImplemented()
+
+    def get_snapshots(self, snapshot_id=None, snapshot_name=None,
+                      volume_id=None):
+        raise NotImplemented()
+
+    def get_connections(self, connection_id=None, volume_id=None):
+        raise NotImplemented()
+
+    def set_volume(self, volume):
+        self.reset_change_tracker(volume)
+
+    def set_snapshot(self, snapshot):
+        self.reset_change_tracker(snapshot)
+
+    def set_connection(self, connection):
+        self.reset_change_tracker(connection)
+
+    def delete_volume(self, volume):
+        self._set_deleted(volume)
+        self.reset_change_tracker(volume)
+
+    def delete_snapshot(self, snapshot):
+        self._set_deleted(snapshot)
+        self.reset_change_tracker(snapshot)
+
+    def delete_connection(self, connection):
+        self._set_deleted(connection)
+        self.reset_change_tracker(connection)
+
+    def _set_deleted(self, resource):
+        resource._ovo.deleted = True
+        resource._ovo.deleted_at = timeutils.utcnow()
+        if hasattr(resource._ovo, 'status'):
+            resource._ovo.status = 'deleted'
+
+    def reset_change_tracker(self, resource, fields=None):
+        if isinstance(fields, six.string_types):
+            fields = (fields,)
+        resource._ovo.obj_reset_changes(fields)
+
+    def get_changed_fields(self, resource):
+        return resource._ovo.cinder_obj_get_changes()
+
+
+class DB(object):
+    """Replacement for DB access methods.
+
+    This will serve as replacement for methods used by:
+
+    - Drivers
+    - OVOs' get_by_id and save methods
+    - DB implementation
+
+    Data will be retrieved using the persistence driver we setup.
+    """
+
+    def __init__(self, persistence_driver):
+        self.persistence = persistence_driver
+
+        # Replace the standard DB configuration for code that doesn't use the
+        # driver.db attribute (ie: OVOs).
+        volume_cmd.session.IMPL = self
+
+        # Replace get_by_id OVO methods with something that will return
+        # expected data
+        volume_cmd.objects.Volume.get_by_id = self.volume_get
+        volume_cmd.objects.Snapshot.get_by_id = self.snapshot_get
+
+        # Disable saving in OVOs
+        for ovo_name in cinder_base_ovo.CinderObjectRegistry.obj_classes():
+            ovo_cls = getattr(volume_cmd.objects, ovo_name)
+            ovo_cls.save = lambda *args, **kwargs: None
+
+    def volume_get(self, context, volume_id, *args, **kwargs):
+        return self.persistence.get_volumes(volume_id)._ovo
+
+    def snapshot_get(self, context, snapshot_id, *args, **kwargs):
+        return self.persistence.get_snapshots(snapshot_id)._ovo
+
+    @classmethod
+    def image_volume_cache_get_by_volume_id(cls, context, volume_id):
+        return None