1 files changed, 9 insertions, 6 deletions
diff --git a/synapse/handlers/directory.py b/synapse/handlers/directory.py
index 2ea52257cb..1fb23cc9bf 100644
--- a/synapse/handlers/directory.py
+++ b/synapse/handlers/directory.py
@@ -14,7 +14,7 @@
 
 import logging
 import string
-from typing import TYPE_CHECKING, Iterable, List, Optional
+from typing import TYPE_CHECKING, Iterable, List, Optional, Sequence
 
 from typing_extensions import Literal
 
@@ -485,7 +485,8 @@ class DirectoryHandler:
                 )
             )
             if canonical_alias:
-                room_aliases.append(canonical_alias)
+                # Ensure we do not mutate room_aliases.
+                room_aliases = list(room_aliases) + [canonical_alias]
 
             if not self.config.roomdirectory.is_publishing_room_allowed(
                 user_id, room_id, room_aliases
@@ -496,9 +497,11 @@ class DirectoryHandler:
                 raise SynapseError(403, "Not allowed to publish room")
 
             # Check if publishing is blocked by a third party module
-            allowed_by_third_party_rules = await (
-                self.third_party_event_rules.check_visibility_can_be_modified(
-                    room_id, visibility
+            allowed_by_third_party_rules = (
+                await (
+                    self.third_party_event_rules.check_visibility_can_be_modified(
+                        room_id, visibility
+                    )
                 )
             )
             if not allowed_by_third_party_rules:
@@ -528,7 +531,7 @@ class DirectoryHandler:
 
     async def get_aliases_for_room(
         self, requester: Requester, room_id: str
-    ) -> List[str]:
+    ) -> Sequence[str]:
         """
         Get a list of the aliases that currently point to this room on this server
         """