Ensure we use a copy of the event content dict before modifying it in serialize_event

changelog.d/9585.bugfix

@@ -0,0 +1 @@
+Fix a longstanding bug that could cause issues when editing a reply to a message.

synapse/events/utils.py

@@ -22,6 +22,7 @@
 from synapse.api.errors import Codes, SynapseError
 from synapse.api.room_versions import RoomVersion
 from synapse.util.async_helpers import yieldable_gather_results
+from synapse.util.frozenutils import unfreeze
 
 from . import EventBase
 
@@ -400,10 +401,19 @@ async def serialize_event(
                 # If there is an edit replace the content, preserving existing
                 # relations.
 
+                # Ensure we take copies of the edit content, otherwise we risk modifying
+                # the original event.
+                edit_content = edit.content.copy()
+
+                # Unfreeze the event content if necessary, so that we may modify it below
+                edit_content = unfreeze(edit_content)
+                serialized_event["content"] = edit_content.get("m.new_content", {})
+
+                # Check for existing relations
                 relations = event.content.get("m.relates_to")
-                serialized_event["content"] = edit.content.get("m.new_content", {})
                 if relations:
-                    serialized_event["content"]["m.relates_to"] = relations
+                    # Keep the relations, ensuring we use a dict copy of the original
+                    serialized_event["content"]["m.relates_to"] = relations.copy()
                 else:
                     serialized_event["content"].pop("m.relates_to", None)