For backwards compat, populate the top-level redacts field.

Author: clokep

synapse/events/utils.py

@@ -473,6 +473,15 @@ 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
+
     only_event_fields = config.only_event_fields
     if only_event_fields:
         if not isinstance(only_event_fields, list) or not all(

tests/rest/client/test_redactions.py

@@ -20,6 +20,8 @@
 from synapse.rest import admin
 from synapse.rest.client import login, room, sync
 from synapse.server import HomeServer
+from synapse.storage._base import db_to_json
+from synapse.storage.database import LoggingTransaction
 from synapse.types import JsonDict
 from synapse.util import Clock
 
@@ -597,5 +599,20 @@ def test_content_redaction(self) -> None:
         redact_event = timeline[-1]
         self.assertEqual(redact_event["type"], EventTypes.Redaction)
         # The redacts key should be in the content.
-        self.assertNotIn("redacts", redact_event)
         self.assertEquals(redact_event["content"]["redacts"], event_id)
+
+        # It should also be copied as the top-level redacts field for backwards
+        # compatibility.
+        self.assertEquals(redact_event["redacts"], event_id)
+
+        # But it isn't actually part of the event.
+        def get_event(txn: LoggingTransaction) -> JsonDict:
+            return db_to_json(
+                main_datastore._fetch_event_rows(txn, [event_id])[event_id].json
+            )
+
+        main_datastore = self.hs.get_datastores().main
+        event_json = self.get_success(
+            main_datastore.db_pool.runInteraction("get_event", get_event)
+        )
+        self.assertNotIn("redacts", event_json)