Improve sliding-sync robustness to deeper replays.

Signed-off-by: Jason Volk <jason@zemos.net>

src/api/client/sync/v5.rs

@@ -130,25 +130,22 @@ pub(crate) async fn sync_events_v5_route(
 
 	let (mut conn, _) = join(conn, ping_presence).await;
 
-	// The client must either use the last returned next_batch or replay the
-	// next_batch from the penultimate request: it's either up-to-date or
-	// one-behind. If we receive anything else we can boot them.
 	let advancing = since == conn.next_batch;
-	let replaying = since == conn.globalsince;
-	if !advancing && !replaying {
+	let retarding = since <= conn.globalsince;
+	if !advancing && !retarding {
 		return Err!(Request(UnknownPos("Requesting unknown or stale stream position.")));
 	}
 
 	debug_assert!(
-		advancing || replaying,
-		"Request should either be advancing or replaying the last request."
+		advancing || retarding,
+		"Request should either be advancing or replaying the since token."
 	);
 
 	// Update parameters regardless of replay or advance
 	conn.next_batch = services.globals.wait_pending().await?;
 	conn.globalsince = since.min(conn.next_batch);
 	conn.update_cache(request);
-	conn.update_rooms_prologue(advancing);
+	conn.update_rooms_prologue(retarding.then_some(since));
 
 	let mut response = Response {
 		txn_id: request.txn_id.clone(),

src/service/sync/mod.rs

@@ -53,8 +53,6 @@ pub struct Connection {
 #[derive(Clone, Copy, Debug, Default)]
 pub struct Room {
 	pub roomsince: u64,
-	pub last_batch: u64,
-	pub next_batch: u64,
 }
 
 type Connections = StdMutex<BTreeMap<ConnectionKey, ConnectionVal>>;
@@ -94,14 +92,12 @@ impl crate::Service for Service {
 }
 
 #[implement(Connection)]
-pub fn update_rooms_prologue(&mut self, advance: bool) {
+pub fn update_rooms_prologue(&mut self, retard_since: Option<u64>) {
 	self.rooms.values_mut().for_each(|room| {
-		if advance {
-			room.roomsince = room.next_batch;
-			room.last_batch = room.next_batch;
-		} else {
-			room.roomsince = room.last_batch;
-			room.next_batch = room.last_batch;
+		if let Some(retard_since) = retard_since {
+			if room.roomsince > retard_since {
+				room.roomsince = retard_since;
+			}
 		}
 	});
 }
@@ -115,7 +111,6 @@ where
 		let room = self.rooms.entry(room_id.into()).or_default();
 
 		room.roomsince = self.next_batch;
-		room.next_batch = self.next_batch;
 	});
 }