Fix Low-Latency HLS subtitle part loading (#7893)

Fixes #7885

api-extractor/report/hls.js.api.md

@@ -1998,6 +1998,26 @@ export class Fragment extends BaseSegment {
     urlId: number;
 }
 
+// Warning: (ae-missing-release-tag) "FragmentEntity" is part of the package's API, but it is missing a release tag (@alpha, @beta, @public, or @internal)
+//
+// @public (undocumented)
+export interface FragmentEntity {
+    // (undocumented)
+    appendedPTS: number | null;
+    // (undocumented)
+    body: MediaFragment;
+    // (undocumented)
+    buffered: boolean;
+    // (undocumented)
+    loaded: FragLoadedData | null;
+    // Warning: (ae-forgotten-export) The symbol "FragmentBufferedRange" needs to be exported by the entry point hls.d.ts
+    //
+    // (undocumented)
+    range: {
+        [key in SourceBufferName | 'subs']: FragmentBufferedRange;
+    };
+}
+
 // Warning: (ae-missing-release-tag) "FragmentLoader" is part of the package's API, but it is missing a release tag (@alpha, @beta, @public, or @internal)
 //
 // @public (undocumented)
@@ -2076,7 +2096,7 @@ export class FragmentTracker implements ComponentAPI {
     detectEvictedFragments(elementaryStream: SourceBufferName, timeRange: TimeRanges, playlistType: PlaylistLevelType, appendedFrag?: MediaFragment | null, appendedPart?: Part | null, removeAppending?: boolean): void;
     detectPartialFragments(data: FragBufferedData): void;
     // (undocumented)
-    fragBuffered(frag: MediaFragment, force?: true): void;
+    fragBuffered(frag: MediaFragment, force?: true): FragmentEntity | undefined;
     getAppendedFrag(position: number, levelType: PlaylistLevelType): MediaFragment | Part | null;
     getBackBufferEvictionEnd(beforePosition: number, levelType: PlaylistLevelType, bytesNeeded: number): number;
     getBufferedFrag(position: number, levelType: PlaylistLevelType): MediaFragment | null;

src/controller/fragment-tracker.ts

@@ -318,7 +318,10 @@ export class FragmentTracker implements ComponentAPI {
     );
   }
 
-  public fragBuffered(frag: MediaFragment, force?: true) {
+  public fragBuffered(
+    frag: MediaFragment,
+    force?: true,
+  ): FragmentEntity | undefined {
     const fragKey = getFragmentKey(frag);
     let fragmentEntity = this.fragments[fragKey];
     if (!fragmentEntity && force) {
@@ -337,6 +340,7 @@ export class FragmentTracker implements ComponentAPI {
       fragmentEntity.loaded = null;
       this.bufferedEnd(fragmentEntity, frag);
     }
+    return fragmentEntity;
   }
 
   private getBufferedTimes(
@@ -646,7 +650,8 @@ function isPartial(fragmentEntity: FragmentEntity): boolean {
       fragmentEntity.body.gap ||
       fragmentEntity.range.video?.partial ||
       fragmentEntity.range.audio?.partial ||
-      fragmentEntity.range.audiovideo?.partial
+      fragmentEntity.range.audiovideo?.partial ||
+      fragmentEntity.range.subs?.partial
     )
   );
 }

src/controller/subtitle-stream-controller.ts

@@ -156,7 +156,13 @@ export class SubtitleStreamController
       buffered.push(timeRange);
     }
     if (!part || end >= frag.end) {
-      this.fragmentTracker.fragBuffered(frag as MediaFragment);
+      const entity = this.fragmentTracker.fragBuffered(frag as MediaFragment);
+      if (part && entity) {
+        entity.range.subs = {
+          time: [{ startPTS: frag.start, endPTS: end }],
+          partial: true,
+        };
+      }
       if (isMediaFragment(frag) && !this.fragContextChanged(frag)) {
         this.fragPrevious = frag;
       }

src/hls.ts

@@ -1453,6 +1453,7 @@ export type {
   FragmentState,
   FragmentTracker,
 } from './controller/fragment-tracker';
+export type { FragmentEntity } from './types/fragment-tracker';
 export type {
   PathwayClone,
   SteeringManifest,

src/types/fragment-tracker.ts

@@ -9,7 +9,7 @@ export interface FragmentEntity {
   appendedPTS: number | null;
   loaded: FragLoadedData | null;
   buffered: boolean;
-  range: { [key in SourceBufferName]: FragmentBufferedRange };
+  range: { [key in SourceBufferName | 'subs']: FragmentBufferedRange };
 }
 
 export interface FragmentTimeRange {