fix: Do not make late-arriving events forward extremities

changelog.d/2115.bugfix.md

@@ -0,0 +1 @@
+Events arriving after one of their children has already been backfilled are no longer used as forward extremities. Contributed by @eleboucher.

src/service/rooms/event_handler/fetch_and_handle_outliers.rs

@@ -154,9 +154,9 @@ impl super::Service {
 				.stream()
 				.fold(0_u8, |i, event_id| async move {
 					if self.services.timeline.pdu_exists(event_id).await {
-						i.expected_add(1)
-					} else {
 						i
+					} else {
+						i.expected_add(1)
 					}
 				})
 				.await;

src/service/rooms/timeline/append.rs

@@ -194,7 +194,24 @@ impl super::Service {
 			})
 			.collect::<Vec<_>>()
 			.await;
-		forward_extremities.push(incoming_pdu.event_id().to_owned());
+
+		// An event with a child is not a leaf, but a room with no extremities cannot be
+		// written to, so it is added anyway when nothing else remains.
+		let has_child = self
+			.services
+			.pdu_metadata
+			.is_event_referenced(&incoming_pdu.room_id_or_hash(), incoming_pdu.event_id())
+			.await;
+		if !has_child || forward_extremities.is_empty() {
+			if has_child {
+				warn!(
+					"Adding {} as an extremity despite its child, the room has no other leaves",
+					incoming_pdu.event_id()
+				);
+			}
+			forward_extremities.push(incoming_pdu.event_id().to_owned());
+		}
+
 		debug!(
 			"Retained {} extremities checked against {} prev_events",
 			forward_extremities.len(),

src/service/rooms/timeline/backfill.rs

@@ -213,6 +213,12 @@ impl super::Service {
 
 		drop(insert_lock);
 
+		// Backfilled events skip append_pdu, so this is the only place their
+		// prev_events can be marked as referenced. See issue #2115.
+		self.services
+			.pdu_metadata
+			.mark_as_referenced(&room_id, pdu.prev_events().map(AsRef::as_ref));
+
 		if pdu.kind == TimelineEventType::RoomMessage {
 			let content: ExtractBody = pdu.get_content()?;
 			if let Some(body) = content.body {