Automatically delete empty groups/communities (#6453)

Signed-off-by: Werner Sembach <werner.sembach@fau.de>

AUTHORS.rst

@@ -46,3 +46,6 @@ Joseph Weston <joseph at weston.cloud>
 
 Benjamin Saunders <ben.e.saunders at gmail dot com>
  * Documentation improvements
+
+Werner Sembach <werner.sembach at fau dot de>
+ * Automatically remove a group/community when it is empty

changelog.d/6453.feature

+Automatically delete empty groups/communities.

synapse/groups/groups_server.py

@@ -773,6 +773,11 @@ class GroupsServerHandler(object):
         if not self.hs.is_mine_id(user_id):
             yield self.store.maybe_delete_remote_profile_cache(user_id)
 
+        # Delete group if the last user has left
+        users = yield self.store.get_users_in_group(group_id, include_private=True)
+        if not users:
+            yield self.store.delete_group(group_id)
+
         return {}
 
     @defer.inlineCallbacks

synapse/storage/data_stores/main/schema/delta/56/nuke_empty_communities_from_db.sql

+/* Copyright 2019 Werner Sembach
+ *
+ * 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.
+ */
+
+-- Groups/communities now get deleted when the last member leaves. This is a one time cleanup to remove old groups/communities that were already empty before that change was made.
+DELETE FROM group_attestations_remote WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM group_attestations_renewals WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM group_invites WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM group_roles WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM group_room_categories WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM group_rooms WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM group_summary_roles WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM group_summary_room_categories WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM group_summary_rooms WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM group_summary_users WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM local_group_membership WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM local_group_updates WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));
+DELETE FROM groups WHERE group_id IN (SELECT group_id FROM groups WHERE NOT EXISTS (SELECT group_id FROM group_users WHERE group_id = groups.group_id));