From 13f08a034f63733f85659abc98ca7fb1a3eb538f Mon Sep 17 00:00:00 2001
From: mertalev <101130780+mertalev@users.noreply.github.com>
Date: Sat, 9 May 2026 02:01:42 -0400
Subject: [PATCH] reject on variant switch

---
 server/src/services/hls.service.spec.ts | 10 ++++++++
 server/src/services/hls.service.ts      | 34 +++++++++++++++++--------
 2 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/server/src/services/hls.service.spec.ts b/server/src/services/hls.service.spec.ts
index 7a39c2905e..6a6b998df5 100644
--- a/server/src/services/hls.service.spec.ts
+++ b/server/src/services/hls.service.spec.ts
@@ -303,6 +303,16 @@ describe(HlsService.name, () => {
       });
     });
 
+    it('rejects pending waiters for the previous variant on variant change', async () => {
+      mocks.storage.checkFileExists.mockResolvedValueOnce(false);
+
+      const pending = sut.getSegment(auth, assetId, sessionId, 0, 'seg_1.m4s');
+      await new Promise((resolve) => setImmediate(resolve));
+      await sut.getSegment(auth, assetId, sessionId, 1, 'seg_1.m4s');
+
+      await expect(pending).rejects.toThrow('Variant changed');
+    });
+
     it('throws NotFoundException when the session does not exist', async () => {
       mocks.videoStream.getSession.mockReset();
       await expect(sut.getSegment(auth, assetId, sessionId, variantIndex, 'init.mp4')).rejects.toBeInstanceOf(
diff --git a/server/src/services/hls.service.ts b/server/src/services/hls.service.ts
index 488dc1f845..fba8b8e060 100644
--- a/server/src/services/hls.service.ts
+++ b/server/src/services/hls.service.ts
@@ -21,12 +21,13 @@ import { ImmichFileResponse } from 'src/utils/file';
 import { getOutputSize } from 'src/utils/media';
 
 type AssetWithStreamInfo = { videoStream: VideoStreamInfo & { timeBase: number }; packets: VideoPacketInfo };
+type ApiSession = { lastRequestedSegment: number | null; lastVariantIndex: number | null };
 
 @Injectable()
 export class HlsService extends BaseService {
   private pendingSegments = new PendingEvents<'HlsSegmentResult'>({ timeoutMs: 15_000 });
   private pendingSessions = new PendingEvents<'HlsSessionResult'>({ timeoutMs: 5000 });
-  private sessions = new Map<string, { lastRequestedSegment: number | null }>();
+  private sessions = new Map<string, ApiSession>();
 
   @OnEvent({ name: 'HlsSessionResult', server: true, workers: [ImmichWorker.Api] })
   onSessionResult(event: ArgOf<'HlsSessionResult'>) {
@@ -65,7 +66,7 @@ export class HlsService extends BaseService {
     const sessionId = this.cryptoRepository.randomUUID();
     this.websocketRepository.serverSend('HlsSessionRequest', { sessionId, assetId, ownerId: auth.user.id });
     await this.pendingSessions.wait(sessionId);
-    this.sessions.set(sessionId, { lastRequestedSegment: null });
+    this.trackSession(sessionId);
 
     return this.generateMainPlaylist(sessionId, ffmpeg, asset);
   }
@@ -97,7 +98,8 @@ export class HlsService extends BaseService {
       cacheControl: CacheControl.PrivateWithCache,
     });
 
-    const segmentIndex = this.getSegmentIndex(sessionId, filename);
+    const apiSession = this.trackSession(sessionId, variantIndex);
+    const segmentIndex = this.getSegmentIndex(apiSession, filename);
     this.websocketRepository.serverSend('HlsHeartbeat', { sessionId, variantIndex, segmentIndex });
 
     if (await this.storageRepository.checkFileExists(path, constants.R_OK)) {
@@ -170,17 +172,27 @@ export class HlsService extends BaseService {
     return `${sessionId}:${variantIndex}:${segmentIndex}`;
   }
 
-  private getSegmentIndex(sessionId: string, filename: string) {
-    const existing = this.sessions.get(sessionId);
+  private getSegmentIndex(session: ApiSession, filename: string) {
     if (filename.endsWith('.mp4')) {
-      return (existing?.lastRequestedSegment ?? -1) + 1;
+      return (session.lastRequestedSegment ?? -1) + 1;
     }
     const segmentIndex = Number.parseInt(HLS_SEGMENT_FILENAME_REGEX.exec(filename)![1]);
-    if (existing) {
-      existing.lastRequestedSegment = segmentIndex;
-    } else {
-      this.sessions.set(sessionId, { lastRequestedSegment: segmentIndex });
-    }
+    session.lastRequestedSegment = segmentIndex;
     return segmentIndex;
   }
+
+  private trackSession(id: string, variantIndex: number | null = null) {
+    const session = this.sessions.get(id);
+    if (!session) {
+      const newSession = { lastRequestedSegment: null, lastVariantIndex: variantIndex };
+      this.sessions.set(id, newSession);
+      return newSession;
+    }
+
+    if (session.lastVariantIndex !== null && session.lastVariantIndex !== variantIndex) {
+      this.pendingSegments.rejectByPrefix(`${id}:${session.lastVariantIndex}:`, 'Variant changed');
+    }
+    session.lastVariantIndex = variantIndex;
+    return session;
+  }
 }