Fix users being set to PL0 when downgrading v12 rooms

changelog.d/18818.bugfix

@@ -0,0 +1 @@
+Fix a bug where room creators would be set to the default power level when downgrading v12 rooms.

synapse/events/utils.py

@@ -763,8 +763,8 @@ def copy_and_fixup_power_levels_contents(
     integer, e.g. `"`100"` instead of 100. Such strings are converted to integers
     in the returned dictionary (hence "fixup" in the function name).
 
-    Note that future room versions will outlaw such stringy power levels (see
-    https://github.com/matrix-org/matrix-spec/issues/853).
+    Note that room versions 10+ outlaws such stringy power levels (see
+    https://github.com/matrix-org/matrix-spec-proposals/pull/3667).
 
     Raises:
         TypeError if the input does not look like a valid power levels event content

synapse/handlers/room.py

@@ -419,34 +419,27 @@ async def _upgrade_room(
             old_room_id, new_room_id
         )
 
-        # finally, shut down the PLs in the old room, and update them in the new
-        # room.
-        await self._update_upgraded_room_pls(
+        # finally, shut down the PLs in the old room
+        await self._shutdown_power_levels_in_upgraded_room(
             requester,
             old_room_id,
-            new_room_id,
             old_room_state,
-            additional_creators,
         )
 
         return new_room_id
 
-    async def _update_upgraded_room_pls(
+    async def _shutdown_power_levels_in_upgraded_room(
         self,
         requester: Requester,
         old_room_id: str,
-        new_room_id: str,
         old_room_state: StateMap[str],
-        additional_creators: Optional[List[str]],
     ) -> None:
-        """Send updated power levels in both rooms after an upgrade
+        """Send updated power levels in the old room after it has been upgraded
 
         Args:
             requester: the user requesting the upgrade
             old_room_id: the id of the room to be replaced
-            new_room_id: the id of the replacement room
             old_room_state: the state map for the old room
-            additional_creators: Additional creators in the new room.
         Raises:
             ShadowBanError if the requester is shadow-banned.
         """
@@ -502,28 +495,6 @@ async def _update_upgraded_room_pls(
             except AuthError as e:
                 logger.warning("Unable to update PLs in old room: %s", e)
 
-        new_room_version = await self.store.get_room_version(new_room_id)
-        if new_room_version.msc4289_creator_power_enabled:
-            self._remove_creators_from_pl_users_map(
-                old_room_pl_state.content.get("users", {}),
-                requester.user.to_string(),
-                additional_creators,
-            )
-
-        await self.event_creation_handler.create_and_send_nonmember_event(
-            requester,
-            {
-                "type": EventTypes.PowerLevels,
-                "state_key": "",
-                "room_id": new_room_id,
-                "sender": requester.user.to_string(),
-                "content": copy_and_fixup_power_levels_contents(
-                    old_room_pl_state.content
-                ),
-            },
-            ratelimit=False,
-        )
-
     def _calculate_upgraded_room_creation_content(
         self,
         old_room_create_event: EventBase,
@@ -685,8 +656,28 @@ async def clone_existing_room(
         if current_power_level_int < needed_power_level:
             user_power_levels[user_id] = needed_power_level
 
+        # If downgrading from a room that supports MSC4289 to one that doesn't...
+        if (
+            old_room_create_event.room_version.msc4289_creator_power_enabled
+            and not new_room_version.msc4289_creator_power_enabled
+        ):
+            # Additionally raise the power levels of all `additional_creators` in the old
+            # room to the `needed_power_level` in the new room.
+            old_room_additional_creators = old_room_create_event.content.get(
+                "additional_creators", []
+            )
+            for user_id in old_room_additional_creators:
+                old_power_level = user_power_levels.get(user_id, users_default)
+                try:
+                    old_power_level = int(old_power_level)  # type: ignore[arg-type]
+                except (TypeError, ValueError):
+                    old_power_level = 0
+
+                # Raise the requester's power level in the new room if necessary
+                if old_power_level < needed_power_level:
+                    user_power_levels[user_id] = needed_power_level
+
         if new_room_version.msc4289_creator_power_enabled:
-            # the creator(s) cannot be in the users map
             self._remove_creators_from_pl_users_map(
                 user_power_levels,
                 user_id,