Make mergeDetails a no-op when both arguments are the same object

Fix buffer flushing when switching between audio-only from main and alt (variant and media playlists)
Add additional test coverage

src/controller/audio-stream-controller.ts

@@ -14,6 +14,7 @@ import {
   alignMediaPlaylistByPDT,
 } from '../utils/discontinuities';
 import { mediaAttributesIdentical } from '../utils/media-option-attributes';
+import { useAlternateAudio } from '../utils/rendition-helper';
 import type { FragmentTracker } from './fragment-tracker';
 import type Hls from '../hls';
 import type { Fragment, MediaFragment, Part } from '../loader/fragment';
@@ -536,8 +537,8 @@ class AudioStreamController
     const mainDetails = this.mainDetails;
     if (
       !mainDetails ||
-      mainDetails.expired ||
-      newDetails.endCC > mainDetails.endCC
+      newDetails.endCC > mainDetails.endCC ||
+      mainDetails.expired
     ) {
       this.cachedTrackLoadedData = data;
       if (this.state !== State.STOPPED) {
@@ -1027,9 +1028,14 @@ class AudioStreamController
         bufferedTrack.name !== switchingTrack.name ||
         bufferedTrack.lang !== switchingTrack.lang)
     ) {
-      this.log('Switching audio track : flushing all audio');
-      super.flushMainBuffer(0, Number.POSITIVE_INFINITY, 'audio');
-      this.bufferedTrack = null;
+      if (useAlternateAudio(switchingTrack.url, this.hls)) {
+        this.log('Switching audio track : flushing all audio');
+        super.flushMainBuffer(0, Number.POSITIVE_INFINITY, 'audio');
+        this.bufferedTrack = null;
+      } else {
+        // Main is being buffered. Set bufferedTrack so that it is flushed when switching back to alt-audio
+        this.bufferedTrack = switchingTrack;
+      }
     }
   }
 

src/controller/base-stream-controller.ts

@@ -1308,7 +1308,7 @@ export default class BaseStreamController
         const mainStart = this.hls.startPosition;
         const liveSyncPosition = this.hls.liveSyncPosition;
         const startPosition = frag
-          ? (mainStart !== -1 && mainStart >= pos
+          ? (mainStart !== -1 && mainStart >= start
               ? mainStart
               : liveSyncPosition) || frag.start
           : pos;

src/loader/level-details.ts

@@ -179,7 +179,7 @@ export class LevelDetails {
   }
 
   get expired(): boolean {
-    if (this.live && this.age) {
+    if (this.live && this.age && this.misses < 3) {
       const playlistWindowDuration = this.partEnd - this.fragmentStart;
       return (
         this.age >

src/utils/level-helper.ts

@@ -136,7 +136,10 @@ export function updateFragPTSDTS(
 export function mergeDetails(
   oldDetails: LevelDetails,
   newDetails: LevelDetails,
-): void {
+) {
+  if (oldDetails === newDetails) {
+    return;
+  }
   // Track the last initSegment processed. Initialize it to the last one on the timeline.
   let currentInitSegment: Fragment | null = null;
   const oldFragments = oldDetails.fragments;

src/utils/rendition-helper.ts

@@ -502,6 +502,9 @@ function searchDownAndUpList(
   return -1;
 }
 
-export function useAlternateAudio(audioTrackUrl: string, hls: Hls): boolean {
+export function useAlternateAudio(
+  audioTrackUrl: string | undefined,
+  hls: Hls,
+): boolean {
   return !!audioTrackUrl && audioTrackUrl !== hls.levels[hls.loadLevel]?.uri;
 }