Properly handle redactions of creation events (#15973)

changelog.d/15973.bugfix

@@ -0,0 +1 @@
+Properly handle redactions of creation events.

synapse/events/utils.py

@@ -136,11 +136,13 @@ def prune_event_dict(room_version: RoomVersion, event_dict: JsonDict) -> JsonDic
                     ]
 
     elif event_type == EventTypes.Create:
-        # MSC2176 rules state that create events cannot be redacted.
         if room_version.updated_redaction_rules:
-            return event_dict
-
+            # MSC2176 rules state that create events cannot have their `content` redacted.
+            new_content = event_dict["content"]
+        elif not room_version.implicit_room_creator:
+            # Some room versions give meaning to `creator`
             add_fields("creator")
+
     elif event_type == EventTypes.JoinRules:
         add_fields("join_rule")
         if room_version.restricted_join_rule:

tests/events/test_utils.py

@@ -225,9 +225,14 @@ class PruneEventTestCase(stdlib_unittest.TestCase):
             },
         )
 
-        # After MSC2176, create events get nothing redacted.
+        # After MSC2176, create events should preserve field `content`
         self.run_test(
-            {"type": "m.room.create", "content": {"not_a_real_key": True}},
+            {
+                "type": "m.room.create",
+                "content": {"not_a_real_key": True},
+                "origin": "some_homeserver",
+                "nonsense_field": "some_random_garbage",
+            },
             {
                 "type": "m.room.create",
                 "content": {"not_a_real_key": True},