Fix bug where we didn't correctly serialize the redacted_because key over federation

synapse/events/__init__.py

@@ -90,7 +90,7 @@ class EventBase(object):
         d = dict(self._event_dict)
         d.update({
             "signatures": self.signatures,
-            "unsigned": self.unsigned,
+            "unsigned": dict(self.unsigned),
         })
 
         return d
@@ -109,6 +109,9 @@ class EventBase(object):
             pdu_json.setdefault("unsigned", {})["age"] = int(age)
             del pdu_json["unsigned"]["age_ts"]
 
+        # This may be a frozen event
+        pdu_json["unsigned"].pop("redacted_because", None)
+
         return pdu_json
 
     def __set__(self, instance, value):

synapse/storage/events.py

@@ -811,6 +811,8 @@ class EventsStore(SQLBaseStore):
             )
 
             if because:
+                # It's fine to do add the event directly, since get_pdu_json
+                # will serialise this field correctly
                 ev.unsigned["redacted_because"] = because
 
         if get_prev_content and "replaces_state" in ev.unsigned: