fix(backend): fallback if `sinceId` is older than the oldest in cache when using FTT (#14061)

* fix(backend): fallback if `sinceId` is older than the oldest in cache when using FTT

* Update CHANGELOG.md

* chore: fix description of test

1 files changed, 4 insertions, 8 deletions

diff --git a/packages/backend/src/core/FanoutTimelineEndpointService.ts b/packages/backend/src/core/FanoutTimelineEndpointService.ts
index d5058f37c2..b05af99c5e 100644
--- a/packages/backend/src/core/FanoutTimelineEndpointService.ts
+++ b/packages/backend/src/core/FanoutTimelineEndpointService.ts
@@ -55,9 +55,6 @@ export class FanoutTimelineEndpointService {
 
 	@bindThis
 	private async getMiNotes(ps: TimelineOptions): Promise<MiNote[]> {
-		let noteIds: string[];
-		let shouldFallbackToDb = false;
-
 		// 呼び出し元と以下の処理をシンプルにするためにdbFallbackを置き換える
 		if (!ps.useDbFallback) ps.dbFallback = () => Promise.resolve([]);
 
@@ -67,12 +64,11 @@ export class FanoutTimelineEndpointService {
 		const redisResult = await this.fanoutTimelineService.getMulti(ps.redisTimelines, ps.untilId, ps.sinceId);
 
 		// TODO: いい感じにgetMulti内でソート済だからuniqするときにredisResultが全てソート済なのを利用して再ソートを避けたい
-		const redisResultIds = Array.from(new Set(redisResult.flat(1)));
-
-		redisResultIds.sort(idCompare);
-		noteIds = redisResultIds.slice(0, ps.limit);
+		const redisResultIds = Array.from(new Set(redisResult.flat(1))).sort(idCompare);
 
-		shouldFallbackToDb = shouldFallbackToDb || (noteIds.length === 0);
+		let noteIds = redisResultIds.slice(0, ps.limit);
+		const oldestNoteId = ascending ? redisResultIds[0] : redisResultIds[redisResultIds.length - 1];
+		const shouldFallbackToDb = noteIds.length === 0 || ps.sinceId != null && ps.sinceId < oldestNoteId;
 
 		if (!shouldFallbackToDb) {
 			let filter = ps.noteFilter ?? (_note => true);