From 4b673692f6fc9dfffc9214f29098d1c9cc48c40e Mon Sep 17 00:00:00 2001
From: timedout <git@nexy7574.co.uk>
Date: Wed, 27 May 2026 00:43:37 +0100
Subject: [PATCH] fix: Be smarter when re-receiving already-seen PDUs

---
 src/service/rooms/event_handler/fetch_prev.rs |  4 +-
 .../event_handler/handle_incoming_pdu.rs      |  3 +-
 .../rooms/event_handler/handle_outlier_pdu.rs | 54 +++++++++++--------
 3 files changed, 36 insertions(+), 25 deletions(-)

diff --git a/src/service/rooms/event_handler/fetch_prev.rs b/src/service/rooms/event_handler/fetch_prev.rs
index f25b90085..25fdf9554 100644
--- a/src/service/rooms/event_handler/fetch_prev.rs
+++ b/src/service/rooms/event_handler/fetch_prev.rs
@@ -2,11 +2,11 @@
 
 use conduwuit::{
 	Event, PduEvent, debug, debug_info,
-	utils::{BoolExt, IterStream, math::try_into, stream::BroadbandExt},
+	utils::{BoolExt, IterStream, stream::BroadbandExt},
 	warn,
 };
 use futures::StreamExt;
-use ruma::{RoomId, ServerName, UInt};
+use ruma::{RoomId, ServerName};
 
 use crate::rooms::event_handler::build_local_dag;
 
diff --git a/src/service/rooms/event_handler/handle_incoming_pdu.rs b/src/service/rooms/event_handler/handle_incoming_pdu.rs
index 216a989d4..163779814 100644
--- a/src/service/rooms/event_handler/handle_incoming_pdu.rs
+++ b/src/service/rooms/event_handler/handle_incoming_pdu.rs
@@ -1,8 +1,7 @@
 use std::{collections::BTreeMap, time::Instant};
 
 use conduwuit::{
-	Err, Event, PduEvent, Result, debug::INFO_SPAN_LEVEL, debug_error, debug_info, defer, err,
-	info, trace, warn,
+	Err, Event, PduEvent, Result, debug_error, debug_info, defer, err, info, trace, warn,
 };
 use futures::{
 	FutureExt,
diff --git a/src/service/rooms/event_handler/handle_outlier_pdu.rs b/src/service/rooms/event_handler/handle_outlier_pdu.rs
index 5431b490c..c4d912cd9 100644
--- a/src/service/rooms/event_handler/handle_outlier_pdu.rs
+++ b/src/service/rooms/event_handler/handle_outlier_pdu.rs
@@ -7,7 +7,8 @@
 use futures::future::ready;
 use ruma::{
 	CanonicalJsonObject, CanonicalJsonValue, EventId, OwnedEventId, RoomId, ServerName,
-	api::federation::authorization::get_event_authorization, events::StateEventType,
+	api::federation::authorization::get_event_authorization, canonical_json::redact,
+	events::StateEventType,
 };
 
 use super::{check_room_id, get_room_version_rules};
@@ -48,27 +49,46 @@ pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 			.verify_event(&value, &room_version_rules)
 			.await
 		{
-			| Ok(ruma::signatures::Verified::All) => value,
+			| Ok(ruma::signatures::Verified::All) => {
+				if let Ok(pdu_event) = self.services.timeline.get_pdu(event_id).await {
+					debug!(
+						"Already have event {event_id} as an outlier or timeline event, not \
+						 re-processing"
+					);
+					value.insert(
+						"event_id".to_owned(),
+						CanonicalJsonValue::String(event_id.as_str().to_owned()),
+					);
+					check_room_id(room_id, &pdu_event)?;
+					return Ok((pdu_event, value));
+				}
+				value
+			},
 			| Ok(ruma::signatures::Verified::Signatures) => {
-				// Redact
 				debug_info!("Calculated hash does not match (redaction): {event_id}");
-				let Ok(obj) =
-					ruma::canonical_json::redact(value, &room_version_rules.redaction, None)
-				else {
-					return Err!(Request(InvalidParam("Redaction failed")));
+				let mut obj = match redact(value, &room_version_rules.redaction, None) {
+					| Ok(obj) => obj,
+					| Err(e) =>
+						return Err!(Request(BadJson("Failed to redact {event_id}: {e}"))),
 				};
 
-				// Skip the PDU if it is redacted and we already have it as an outlier event
-				if self.services.timeline.pdu_exists(event_id).await {
-					return Err!(Request(InvalidParam(
-						"Event was redacted and we already knew about it"
-					)));
+				if let Ok(pdu_event) = self.services.timeline.get_pdu(event_id).await {
+					debug!(
+						"Received a redacted copy of {event_id}, but we already knew about it. \
+						 Re-using known content instead."
+					);
+					obj.insert(
+						"event_id".to_owned(),
+						CanonicalJsonValue::String(event_id.as_str().to_owned()),
+					);
+					check_room_id(room_id, &pdu_event)?;
+					return Ok((pdu_event, obj));
 				}
 
 				obj
 			},
 			| Err(e) => {
-				return Err!(Request(InvalidParam(debug_error!(
+				return Err!(Request(Forbidden(debug_error!(
 					"Signature verification failed for {event_id}: {e}"
 				))));
 			},
@@ -81,14 +101,6 @@ pub(super) async fn handle_outlier_pdu<'a, Pdu>(
 			CanonicalJsonValue::String(event_id.as_str().to_owned()),
 		);
 
-		if let Ok(pdu_event) = self.services.timeline.get_pdu(event_id).await {
-			debug!(
-				"Already have event {event_id} as an outlier or timeline event, not \
-				 re-processing"
-			);
-			return Ok((pdu_event, incoming_pdu));
-		}
-
 		let pdu_event = serde_json::from_value::<PduEvent>(
 			serde_json::to_value(&incoming_pdu).expect("CanonicalJsonObj is a valid JsonValue"),
 		)