Also makes it not a panic

@@ -454,3 +454,3 @@
 			sender_user,
 			replacement_room_id.as_deref(),
-			&new_room_state_lock,
+			Some(&new_room_state_lock),

@@ -1208,2 +1207,13 @@
 	}
 
+	pub(super) async fn room_extremities(&self, room_id: OwnedRoomId) -> Result {
+		let mut extremities = self
+			.services
+			.rooms
+			.state
+			.get_forward_extremities(&room_id)
+			.broad_filter_map(|event_id| async move {
+				self.services.rooms.timeline.get_pdu(&event_id).await.ok()
+			})
+			.collect::<Vec<_>>()
+			.await;

@@ -980,0 +1048,4 @@
+				.for_each(|(_, pdu)| {
+					let room_id = &room_id;
+					async move {
+						if pdu.origin_server_ts < epoch_ts {

@@ -431,0 +429,4 @@
+
+		for event_id in event_ids {
+			let key = (room_id, event_id);
+			self.db.roomid_pduleaves.put_raw(key, event_id);

@@ -173,3 +142,1 @@
-			self.derive_new_state(incoming_pdu, room_version_rules, state_before, state_lock)
-				.await?;
-		}
+	pub async fn progress_extremities(&self, incoming_pdu: &PduEvent) -> Result<()> {

@@ -215,0 +159,4 @@
+		// referenced extremities that we can now remove. Doing the walk
+		// from the prev events also means they will all automatically be
+		// removed from the extremity set.
+		let mut ancestors_todo = VecDeque::from_iter(incoming_pdu.prev_events.clone());

@@ -238,2 +188,3 @@
+						self.services.pdu_metadata.is_event_accepted(&prev_id),
 					);
-					(exists && !referenced && (is_outlier || is_soft_failed || is_rejected))
+					(exists && !matches!(count, PduCount::Backfilled(_)) && !is_accepted)