From ce20e5ec8f3fcf7c658597dd845f0b663ede8711 Mon Sep 17 00:00:00 2001
From: Shay <hillerys@element.io>
Date: Sun, 23 Jul 2023 16:32:01 -0700
Subject: [PATCH] Properly handle redactions of creation events (#15973)

---
 changelog.d/15973.bugfix   | 1 +
 synapse/events/utils.py    | 8 +++++---
 tests/events/test_utils.py | 9 +++++++--
 3 files changed, 13 insertions(+), 5 deletions(-)
 create mode 100644 changelog.d/15973.bugfix

diff --git a/changelog.d/15973.bugfix b/changelog.d/15973.bugfix
new file mode 100644
index 000000000000..c9280d003733
--- /dev/null
+++ b/changelog.d/15973.bugfix
@@ -0,0 +1 @@
+Properly handle redactions of creation events.
diff --git a/synapse/events/utils.py b/synapse/events/utils.py
index 0123aa661d6f..fe05a74b4b51 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -136,11 +136,13 @@ def add_fields(*fields: str) -> None:
                     ]
 
     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")
 
-        add_fields("creator")
     elif event_type == EventTypes.JoinRules:
         add_fields("join_rule")
         if room_version.restricted_join_rule:
diff --git a/tests/events/test_utils.py b/tests/events/test_utils.py
index 6a52af4d82af..978612e4322f 100644
--- a/tests/events/test_utils.py
+++ b/tests/events/test_utils.py
@@ -225,9 +225,14 @@ def test_create(self) -> None:
             },
         )
 
-        # 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},