From 69232e8b9e4d232493889c659bf40f905c3f8402 Mon Sep 17 00:00:00 2001
From: Andrew Morgan <andrew@amorgan.xyz>
Date: Thu, 14 Aug 2025 12:22:24 +0100
Subject: [PATCH] Only send one power levels event in upgraded rooms

The second attempt to send a power levels event was not MSC4289-aware - it would drop the request back down to PL0 when copying the old room's power levels a second time.

The second attempt to send power levels didn't appear to do anything useful anyways. So simply removing it fixes both the bug, and saves a state event.

Fixes https://github.com/element-hq/synapse/issues/18810
---
 synapse/handlers/room.py | 38 ++++----------------------------------
 1 file changed, 4 insertions(+), 34 deletions(-)

diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 47bd139ca7..fe2d348ee3 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -419,34 +419,27 @@ class RoomCreationHandler:
             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 @@ class RoomCreationHandler:
             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,
@@ -686,7 +657,6 @@ class RoomCreationHandler:
             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,