"Freeze" a room when the last admin of that room leaves

synapse/third_party_rules/access_rules.py

@@ -472,21 +472,27 @@ async def _freeze_room_if_last_admin_is_leaving(
         if not power_level_state_event:
             return
         power_level_content = power_level_state_event.content.copy()
-        if not isinstance(power_level_content, dict):
-            # The power level content has been set to something other than a dict...
-            # bail out.
+
+        # Do some validation checks on the power level state event
+        if (
+            not isinstance(power_level_content, dict)
+            or "users" not in power_level_content
+            or not isinstance(power_level_content["users"], dict)
+        ):
+            # We can't use this power level event to determine whether the room should be
+            # frozen. Bail out.
             return
 
         user_id = event.get("sender")
         if not user_id:
             return
 
         # Get every admin user defined in the room's state
-        admin_users = [
+        admin_users = {
             user
             for user, power_level in power_level_content["users"].items()
             if power_level >= 100
-        ]
+        }
 
         if user_id not in admin_users:
             # This user is not an admin, ignore them
@@ -499,12 +505,14 @@ async def _freeze_room_if_last_admin_is_leaving(
             if event_type == EventTypes.Member
             and event.membership in [Membership.JOIN, Membership.INVITE]
         ]
-        admin_users = [user for user in admin_users if user in joined_members]
+        admin_users = {user for user in admin_users if user in joined_members}
 
         if len(admin_users) > 1:
             # There's another admin user in, or invited to, the room
             return
 
+        logger.info("Freezing room '%s'", event.room_id)
+
         # Modify the existing power levels to raise all required types to 100
         #
         # This changes a power level state event's content from something like:
@@ -561,8 +569,6 @@ async def _freeze_room_if_last_admin_is_leaving(
         power_level_content["events"] = {}
 
         # Freeze the room by raising the required power level to send events to 100
-        logger.info("Freezing room '%s'", event.room_id)
-
         await self.module_api.create_and_send_event_into_room(
             {
                 "room_id": event.room_id,