Copy events to not mutate cache

Signed-off-by: Šimon Brandner <[email protected]>

synapse/handlers/receipts.py

@@ -173,6 +173,7 @@ def filter_out_private(events: List[JsonDict], our_user_id: str) -> List[JsonDic
         current user.
         """
 
+        events = events.copy()
         # filter out private receipts the user shouldn't see
         for index, event in enumerate(events):
             content = event.get("content", {})