Add get_canonical_room_alias to module API (#15450)

Co-authored-by: Boxdot <d@zerovolt.org>

Author: gferon

changelog.d/15450.feature

@@ -0,0 +1 @@
+Support resolving a room's [canonical alias](https://spec.matrix.org/v1.7/client-server-api/#mroomcanonical_alias) via the module API.

synapse/module_api/__init__.py

@@ -122,6 +122,7 @@
     JsonMapping,
     Requester,
     RoomAlias,
+    RoomID,
     StateMap,
     UserID,
     UserInfo,
@@ -1570,6 +1571,32 @@ async def get_monthly_active_users_by_service(
             start_timestamp, end_timestamp
         )
 
+    async def get_canonical_room_alias(self, room_id: RoomID) -> Optional[RoomAlias]:
+        """
+        Retrieve the given room's current canonical alias.
+
+        A room may declare an alias as "canonical", meaning that it is the
+        preferred alias to use when referring to the room. This function
+        retrieves that alias from the room's state.
+
+        Added in Synapse v1.86.0.
+
+        Args:
+            room_id: The Room ID to find the alias of.
+
+        Returns:
+            None if the room ID does not exist, or if the room exists but has no canonical alias.
+            Otherwise, the parsed room alias.
+        """
+        room_alias_str = (
+            await self._storage_controllers.state.get_canonical_alias_for_room(
+                room_id.to_string()
+            )
+        )
+        if room_alias_str:
+            return RoomAlias.from_string(room_alias_str)
+        return None
+
     async def lookup_room_alias(self, room_alias: str) -> Tuple[str, List[str]]:
         """
         Get the room ID associated with a room alias.

synapse/storage/controllers/state.py

@@ -485,7 +485,7 @@ async def get_canonical_alias_for_room(self, room_id: str) -> Optional[str]:
         if not event:
             return None
 
-        return event.content.get("canonical_alias")
+        return event.content.get("alias")
 
     @trace
     @tag_args