From f92032f6da36ae6a2d8e43ed3ef5da67a2c831e2 Mon Sep 17 00:00:00 2001
From: Erwan Leboucher <erwanleboucher@gmail.com>
Date: Thu, 27 Aug 2026 15:53:58 +0200
Subject: [PATCH] fix(sync): Retain overlapping sync positions

---
 ...iding-sync-overlapping-responses.bugfix.md |  1 +
 src/service/sync/mod.rs                       | 50 +++++++++----------
 2 files changed, 25 insertions(+), 26 deletions(-)
 create mode 100644 changelog.d/sliding-sync-overlapping-responses.bugfix.md

diff --git a/changelog.d/sliding-sync-overlapping-responses.bugfix.md b/changelog.d/sliding-sync-overlapping-responses.bugfix.md
new file mode 100644
index 000000000..cd14e029b
--- /dev/null
+++ b/changelog.d/sliding-sync-overlapping-responses.bugfix.md
@@ -0,0 +1 @@
+Fixed simplified Sliding Sync restarting when overlapping sync responses arrive out of order. Contributed by @eleboucher.
diff --git a/src/service/sync/mod.rs b/src/service/sync/mod.rs
index 6a97377df..9f67ad9a1 100644
--- a/src/service/sync/mod.rs
+++ b/src/service/sync/mod.rs
@@ -37,32 +37,18 @@ struct SnakeSyncCache {
 	subscriptions: BTreeMap<OwnedRoomId, v5::request::RoomSubscription>,
 	known_rooms: BTreeMap<String, BTreeMap<OwnedRoomId, u64>>,
 	acknowledged_pos: u64,
-	pending: Option<PendingSnakeSync>,
+	pending: BTreeMap<u64, BTreeMap<String, BTreeMap<OwnedRoomId, u64>>>,
 	extensions: v5::request::Extensions,
 }
 
-struct PendingSnakeSync {
-	pos: u64,
-	known_rooms: BTreeMap<String, BTreeMap<OwnedRoomId, u64>>,
-}
-
 impl SnakeSyncCache {
 	fn accepts_pos(&self, pos: u64) -> bool {
-		self.acknowledged_pos == pos
-			|| self
-				.pending
-				.as_ref()
-				.is_some_and(|pending| pending.pos == pos)
+		self.acknowledged_pos == pos || self.pending.contains_key(&pos)
 	}
 
 	fn acknowledge(&mut self, pos: u64) {
-		if self
-			.pending
-			.as_ref()
-			.is_some_and(|pending| pending.pos == pos)
-		{
-			let pending = self.pending.take().expect("pending sync checked above");
-			self.known_rooms = pending.known_rooms;
+		if let Some(known_rooms) = self.pending.remove(&pos) {
+			self.known_rooms = known_rooms;
 			self.acknowledged_pos = pos;
 		}
 	}
@@ -308,7 +294,7 @@ pub fn store_snake_sync_pending(
 				list.insert(room_id, pos);
 			}
 		}
-		cached.pending = Some(PendingSnakeSync { pos, known_rooms: pending_rooms });
+		cached.pending.insert(pos, pending_rooms);
 	}
 
 	pub fn update_snake_sync_subscriptions(
@@ -425,13 +411,10 @@ async fn subscriber_still_woken_after_a_sibling_disconnects() {
 	fn pending_rooms_are_committed_only_when_the_response_pos_is_acknowledged() {
 		let room_id = OwnedRoomId::try_from("!room:example.com").unwrap();
 		let mut cache = SnakeSyncCache {
-			pending: Some(PendingSnakeSync {
-				pos: 42,
-				known_rooms: BTreeMap::from([(
-					"list".to_owned(),
-					BTreeMap::from([(room_id, 42)]),
-				)]),
-			}),
+			pending: BTreeMap::from([(
+				42,
+				BTreeMap::from([("list".to_owned(), BTreeMap::from([(room_id, 42)]))]),
+			)]),
 			..Default::default()
 		};
 
@@ -447,4 +430,19 @@ fn pending_rooms_are_committed_only_when_the_response_pos_is_acknowledged() {
 		assert_eq!(cache.acknowledged_pos, 42);
 		assert_eq!(cache.known_rooms["list"].len(), 1);
 	}
+
+	#[test]
+	fn overlapping_responses_accept_either_response_position() {
+		let mut cache = SnakeSyncCache {
+			acknowledged_pos: 100,
+			pending: BTreeMap::from([(200, BTreeMap::new()), (201, BTreeMap::new())]),
+			..Default::default()
+		};
+
+		assert!(cache.accepts_pos(200));
+		assert!(cache.accepts_pos(201));
+
+		cache.acknowledge(200);
+		assert!(cache.accepts_pos(201));
+	}
 }