Add forward-compatibility for the redacts property (MSC2174). (#16013)

The location of the redacts field changes in room version 11. Ensure
it is copied to the *new* location for *old* room versions for
forwards-compatibility with clients.

Note that copying it to the *old* location for the *new* room version
was previously handled.

1 files changed, 10 insertions, 8 deletions

diff --git a/synapse/events/utils.py b/synapse/events/utils.py

index c890833b1d..967a6c245b 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -475,14 +475,16 @@ def serialize_event(
     if config.as_client_event:
         d = config.event_format(d)
 
-    # If the event is a redaction, copy the redacts field from the content to
-    # top-level for backwards compatibility.
-    if (
-        e.type == EventTypes.Redaction
-        and e.room_version.updated_redaction_rules
-        and e.redacts is not None
-    ):
-        d["redacts"] = e.redacts
+    # If the event is a redaction, the field with the redacted event ID appears
+    # in a different location depending on the room version. e.redacts handles
+    # fetching from the proper location; copy it to the other location for forwards-
+    # and backwards-compatibility with clients.
+    if e.type == EventTypes.Redaction and e.redacts is not None:
+        if e.room_version.updated_redaction_rules:
+            d["redacts"] = e.redacts
+        else:
+            d["content"] = dict(d["content"])
+            d["content"]["redacts"] = e.redacts
 
     only_event_fields = config.only_event_fields
     if only_event_fields: