From fa655837a368ee4ea0a859436a08f927a921b4a4 Mon Sep 17 00:00:00 2001
From: Till Faelligen <2353100+S7evinK@users.noreply.github.com>
Date: Tue, 11 Jul 2023 12:58:56 +0200
Subject: [PATCH] Don't try to backfill if we reached the beginning of the room

---
 syncapi/routing/messages.go | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/syncapi/routing/messages.go b/syncapi/routing/messages.go
index ac337f60d..c80658f52 100644
--- a/syncapi/routing/messages.go
+++ b/syncapi/routing/messages.go
@@ -253,8 +253,8 @@ func OnIncomingMessagesRequest(
 	}
 
 	// If start and end are equal, we either reached the beginning or something else
-	// is wrong. To avoid endless loops from clients, set end to 0 an empty string
-	if start == end {
+	// is wrong. If we have nothing to return set end to 0.
+	if start == end || len(clientEvents) == 0 {
 		end = types.TopologyToken{}
 	}
 
@@ -286,11 +286,6 @@ func OnIncomingMessagesRequest(
 		})...)
 	}
 
-	// If we didn't return any events, set the end to an empty string, so it will be omitted
-	// in the response JSON.
-	if len(res.Chunk) == 0 {
-		res.End = ""
-	}
 	if fromStream != nil {
 		res.StartStream = fromStream.String()
 	}
@@ -489,6 +484,12 @@ func (r *messagesReq) handleEmptyEventsSlice() (
 func (r *messagesReq) handleNonEmptyEventsSlice(streamEvents []types.StreamEvent) (
 	events []*rstypes.HeaderedEvent, err error,
 ) {
+	// We've reached the beginning of the room, nothing more to do.
+	if r.backwardOrdering && streamEvents[len(streamEvents)-1].Type() == spec.MRoomCreate && streamEvents[len(streamEvents)-1].StateKeyEquals("") {
+		events = append(events, r.snapshot.StreamEventsToEvents(r.ctx, nil, streamEvents, r.rsAPI)...)
+		sort.Sort(eventsByDepth(events))
+		return events, nil
+	}
 	// Check if we have enough events.
 	isSetLargeEnough := len(streamEvents) >= r.filter.Limit
 	if !isSetLargeEnough {