From 5dbb5b015c2aa3ac142bb77a86ac2b6a4e749444 Mon Sep 17 00:00:00 2001
From: Tom Foster <tom@tcpip.uk>
Date: Sun, 10 Aug 2025 12:40:29 +0100
Subject: [PATCH] fix(relations): Fix duplicate messages in thread pagination

The /relations endpoint was incorrectly modifying the pagination starting
point with from.saturating_inc(dir), causing duplicate events to appear
when scrolling up in message threads. This was particularly noticeable
when viewing replies in Element.

The fix removes the incorrect increment/decrement logic and adds proper
filtering to exclude the starting event itself from results. The endpoint
now uses ShortEventId directly as the pagination token, avoiding issues
with PduCount in federated environments where events can arrive out of
order.

Fixes: #814
---
 src/api/client/relations.rs            | 14 ++++++--------
 src/service/rooms/pdu_metadata/data.rs | 14 +++++++++++++-
 2 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/src/api/client/relations.rs b/src/api/client/relations.rs
index 1aa34ada..1376609f 100644
--- a/src/api/client/relations.rs
+++ b/src/api/client/relations.rs
@@ -109,13 +109,13 @@ async fn paginate_relations_with_filter(
 	recurse: bool,
 	dir: Direction,
 ) -> Result<get_relating_events::v1::Response> {
-	let start: PduCount = from
-		.map(str::parse)
-		.transpose()?
-		.unwrap_or_else(|| match dir {
+	let start: PduCount = from.and_then(|s| s.parse::<u64>().ok()).map_or_else(
+		|| match dir {
 			| Direction::Forward => PduCount::min(),
 			| Direction::Backward => PduCount::max(),
-		});
+		},
+		PduCount::Normal,
+	);
 
 	let to: Option<PduCount> = to.map(str::parse).flat_ok();
 
@@ -156,9 +156,7 @@ async fn paginate_relations_with_filter(
 		| Direction::Forward => events.last(),
 		| Direction::Backward => events.first(),
 	}
-	.map(at!(0))
-	.as_ref()
-	.map(ToString::to_string);
+	.map(|(count, _)| count.into_unsigned().to_string());
 
 	Ok(get_relating_events::v1::Response {
 		next_batch,
diff --git a/src/service/rooms/pdu_metadata/data.rs b/src/service/rooms/pdu_metadata/data.rs
index c1376cb0..f9cc80a0 100644
--- a/src/service/rooms/pdu_metadata/data.rs
+++ b/src/service/rooms/pdu_metadata/data.rs
@@ -61,9 +61,10 @@ impl Data {
 		from: PduCount,
 		dir: Direction,
 	) -> impl Stream<Item = (PduCount, impl Event)> + Send + '_ {
+		let from_unsigned = from.into_unsigned();
 		let mut current = ArrayVec::<u8, 16>::new();
 		current.extend(target.to_be_bytes());
-		current.extend(from.saturating_inc(dir).into_unsigned().to_be_bytes());
+		current.extend(from_unsigned.to_be_bytes());
 		let current = current.as_slice();
 		match dir {
 			| Direction::Forward => self.tofrom_relation.raw_keys_from(current).boxed(),
@@ -73,6 +74,17 @@ impl Data {
 		.ready_take_while(move |key| key.starts_with(&target.to_be_bytes()))
 		.map(|to_from| u64_from_u8(&to_from[8..16]))
 		.map(PduCount::from_unsigned)
+		.ready_filter(move |count| {
+			if from == PduCount::min() || from == PduCount::max() {
+				true
+			} else {
+				let count_unsigned = count.into_unsigned();
+				match dir {
+					| Direction::Forward => count_unsigned > from_unsigned,
+					| Direction::Backward => count_unsigned < from_unsigned,
+				}
+			}
+		})
 		.wide_filter_map(move |shorteventid| async move {
 			let pdu_id: RawPduId = PduId { shortroomid, shorteventid }.into();