From 55a01e711320d49353af309c02beeb92d76c386e Mon Sep 17 00:00:00 2001
From: Olivia Lee <olivia@computer.surgery>
Date: Sat, 19 Jul 2025 23:19:41 -0700
Subject: [PATCH] don't strip unknown keys when copying redacts property
 to/from content

Servers are required to preserve unknown properties in event content,
since they may be added by a future version of the spec. Round-tripping
through RoomRedactionEventContent results in dropping all unknown
properties.
---
 book/changelog.md  |  3 +++
 src/service/pdu.rs | 52 +++++++++++++++++++++++++++++-----------------
 2 files changed, 36 insertions(+), 19 deletions(-)

diff --git a/book/changelog.md b/book/changelog.md
index 23ae40d6..cadf4c25 100644
--- a/book/changelog.md
+++ b/book/changelog.md
@@ -245,6 +245,9 @@ This will be the first release of Grapevine since it was forked from Conduit
 29. Fix bug where ban reasons would be ignored when the banned user already had
     a member event in the room.
     ([!185](https://gitlab.computer.surgery/matrix/grapevine/-/merge_requests/185))
+30. Stop stripping unknown properties from redaction events before sending them
+    to clients.
+    ([!191](https://gitlab.computer.surgery/matrix/grapevine/-/merge_requests/191))
 
 ### Added
 
diff --git a/src/service/pdu.rs b/src/service/pdu.rs
index 1131a5b8..fe13f1d2 100644
--- a/src/service/pdu.rs
+++ b/src/service/pdu.rs
@@ -1,16 +1,13 @@
-use std::{cmp::Ordering, collections::BTreeMap, sync::Arc};
+use std::{borrow::Cow, cmp::Ordering, collections::BTreeMap, sync::Arc};
 
 use ruma::{
     canonical_json::redact_content_in_place,
     events::{
-        room::{
-            member::RoomMemberEventContent,
-            redaction::RoomRedactionEventContent,
-        },
-        space::child::HierarchySpaceChildEvent,
-        AnyEphemeralRoomEvent, AnyMessageLikeEvent, AnyStateEvent,
-        AnyStrippedStateEvent, AnySyncStateEvent, AnySyncTimelineEvent,
-        AnyTimelineEvent, StateEvent, TimelineEventType,
+        room::member::RoomMemberEventContent,
+        space::child::HierarchySpaceChildEvent, AnyEphemeralRoomEvent,
+        AnyMessageLikeEvent, AnyStateEvent, AnyStrippedStateEvent,
+        AnySyncStateEvent, AnySyncTimelineEvent, AnyTimelineEvent, StateEvent,
+        TimelineEventType,
     },
     serde::Raw,
     state_res, CanonicalJsonObject, CanonicalJsonValue, EventId,
@@ -161,24 +158,41 @@ impl PduEvent {
     pub(crate) fn copy_redacts(
         &self,
     ) -> (Option<Arc<EventId>>, Box<RawJsonValue>) {
+        #[derive(Deserialize)]
+        struct ExtractRedacts<'a> {
+            #[serde(borrow)]
+            redacts: Option<Cow<'a, EventId>>,
+        }
+
         if self.kind != TimelineEventType::RoomRedaction {
             return (self.redacts.clone(), self.content.clone());
         }
-        let Ok(mut content) = serde_json::from_str::<RoomRedactionEventContent>(
-            self.content.get(),
-        ) else {
+        let Ok(extract) =
+            serde_json::from_str::<ExtractRedacts<'_>>(self.content.get())
+        else {
             return (self.redacts.clone(), self.content.clone());
         };
 
-        if let Some(redacts) = content.redacts {
+        if let Some(redacts) = extract.redacts {
             (Some(redacts.into()), self.content.clone())
         } else if let Some(redacts) = self.redacts.clone() {
-            content.redacts = Some(redacts.into());
-            (
-                self.redacts.clone(),
-                to_raw_value(&content)
-                    .expect("Must be valid, we only added redacts field"),
-            )
+            let content = serde_json::from_str::<BTreeMap<&str, &RawJsonValue>>(
+                self.content.get(),
+            );
+            let mut content = match content {
+                Ok(content) => content,
+                Err(error) => {
+                    warn!(%error, "PDU is not a valid json object");
+                    return (self.redacts.clone(), self.content.clone());
+                }
+            };
+
+            let redacts_json = to_raw_value(&redacts)
+                .expect("all strings should be representable as json");
+            content.insert("redacts", &redacts_json);
+            let content_json = to_raw_value(&content)
+                .expect("Must be valid, we only added redacts field");
+            (self.redacts.clone(), content_json)
         } else {
             (self.redacts.clone(), self.content.clone())
         }