Fix currentLevel and FRAG_CHANGED reporting when seeking back from ed…

…ge in Low-Latency streams

Related to #5085 and #5077

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

@@ -745,8 +745,6 @@ export class Fragment extends BaseSegment {
     // (undocumented)
     abortRequests(): void;
     // (undocumented)
-    appendedPTS?: number;
-    // (undocumented)
     bitrateTest: boolean;
     // (undocumented)
     cc: number;

src/controller/fragment-tracker.ts

@@ -23,7 +23,7 @@ export enum FragmentState {
 }
 
 export class FragmentTracker implements ComponentAPI {
-  private activeFragment: Fragment | null = null;
+  private mainFragEntity: FragmentEntity | null = null;
   private activeParts: Part[] | null = null;
   private endListFragments: { [key in PlaylistLevelType]?: FragmentEntity } =
     Object.create(null);
@@ -65,7 +65,7 @@ export class FragmentTracker implements ComponentAPI {
       // @ts-ignore
       this.endListFragments =
       this.timeRanges =
-      this.activeFragment =
+      this.mainFragEntity =
       this.activeParts =
         null;
   }
@@ -79,34 +79,33 @@ export class FragmentTracker implements ComponentAPI {
     levelType: PlaylistLevelType
   ): Fragment | Part | null {
     if (levelType === PlaylistLevelType.MAIN) {
-      const { activeFragment, activeParts } = this;
-      if (!activeFragment) {
-        return null;
-      }
-      if (activeParts) {
-        for (let i = activeParts.length; i--; ) {
-          const activePart = activeParts[i];
-          const appendedPTS = activePart
-            ? activePart.end
-            : activeFragment.appendedPTS;
-          if (
-            activePart.start <= position &&
-            appendedPTS !== undefined &&
-            position <= appendedPTS
-          ) {
-            // 9 is a magic number. remove parts from lookup after a match but keep some short seeks back.
-            if (i > 9) {
-              this.activeParts = activeParts.slice(i - 9);
+      const { mainFragEntity, activeParts } = this;
+      if (mainFragEntity) {
+        if (mainFragEntity && activeParts) {
+          for (let i = activeParts.length; i--; ) {
+            const activePart = activeParts[i];
+            const appendedPTS = activePart
+              ? activePart.end
+              : mainFragEntity.appendedPTS;
+            if (
+              activePart.start <= position &&
+              appendedPTS !== null &&
+              position <= appendedPTS
+            ) {
+              // 9 is a magic number. remove parts from lookup after a match but keep some short seeks back.
+              if (i > 9) {
+                this.activeParts = activeParts.slice(i - 9);
+              }
+              return activePart;
             }
-            return activePart;
           }
+        } else if (
+          mainFragEntity.body.start <= position &&
+          mainFragEntity.appendedPTS !== null &&
+          position <= mainFragEntity.appendedPTS
+        ) {
+          return mainFragEntity.body;
         }
-      } else if (
-        activeFragment.start <= position &&
-        activeFragment.appendedPTS !== undefined &&
-        position <= activeFragment.appendedPTS
-      ) {
-        return activeFragment;
       }
     }
     return this.getBufferedFrag(position, levelType);
@@ -362,6 +361,7 @@ export class FragmentTracker implements ComponentAPI {
     const fragKey = getFragmentKey(frag);
     this.fragments[fragKey] = {
       body: frag,
+      appendedPTS: null,
       loaded: data,
       buffered: false,
       range: Object.create(null),
@@ -373,10 +373,23 @@ export class FragmentTracker implements ComponentAPI {
     data: BufferAppendedData
   ) {
     const { frag, part, timeRanges } = data;
+    let mainFragEntity = this.mainFragEntity;
     if (frag.type === PlaylistLevelType.MAIN) {
-      if (this.activeFragment !== frag) {
-        this.activeFragment = frag;
-        frag.appendedPTS = undefined;
+      const lastMainFrag = mainFragEntity ? mainFragEntity.body : null;
+      if (lastMainFrag !== frag) {
+        if (mainFragEntity && lastMainFrag && lastMainFrag.sn !== frag.sn) {
+          // archive frag for getBufferedFrag()
+          mainFragEntity.buffered = true;
+          this.fragments[getFragmentKey(lastMainFrag)] = mainFragEntity;
+        }
+        const fragKey = getFragmentKey(frag);
+        mainFragEntity = this.mainFragEntity = this.fragments[fragKey] || {
+          body: frag,
+          appendedPTS: null,
+          loaded: null,
+          buffered: false,
+          range: Object.create(null),
+        };
       }
       if (part) {
         let activeParts = this.activeParts;
@@ -393,17 +406,20 @@ export class FragmentTracker implements ComponentAPI {
     Object.keys(timeRanges).forEach((elementaryStream: SourceBufferName) => {
       const timeRange = timeRanges[elementaryStream] as TimeRanges;
       this.detectEvictedFragments(elementaryStream, timeRange);
-      if (!part && frag.type === PlaylistLevelType.MAIN) {
+      if (!part && mainFragEntity) {
         const streamInfo = frag.elementaryStreams[elementaryStream];
         if (!streamInfo) {
           return;
         }
         for (let i = 0; i < timeRange.length; i++) {
           const rangeEnd = timeRange.end(i);
           if (rangeEnd <= streamInfo.endPTS && rangeEnd > streamInfo.startPTS) {
-            frag.appendedPTS = Math.max(rangeEnd, frag.appendedPTS || 0);
+            mainFragEntity.appendedPTS = Math.max(
+              rangeEnd,
+              mainFragEntity.appendedPTS || 0
+            );
           } else {
-            frag.appendedPTS = streamInfo.endPTS;
+            mainFragEntity.appendedPTS = streamInfo.endPTS;
           }
         }
       }
@@ -446,7 +462,9 @@ export class FragmentTracker implements ComponentAPI {
     const fragKey = getFragmentKey(fragment);
     fragment.stats.loaded = 0;
     fragment.clearElementaryStreamInfo();
-    fragment.appendedPTS = undefined;
+    if (this.mainFragEntity === this.fragments[fragKey]) {
+      this.mainFragEntity = null;
+    }
     delete this.fragments[fragKey];
     if (fragment.endList) {
       delete this.endListFragments[fragment.type];
@@ -456,7 +474,7 @@ export class FragmentTracker implements ComponentAPI {
   public removeAllFragments() {
     this.fragments = Object.create(null);
     this.endListFragments = Object.create(null);
-    this.activeFragment = null;
+    this.mainFragEntity = null;
     this.activeParts = null;
   }
 }

src/controller/level-helper.ts

@@ -199,7 +199,6 @@ export function mergeDetails(
       ) {
         newFrag.start = newFrag.startPTS = oldFrag.startPTS as number;
         newFrag.startDTS = oldFrag.startDTS;
-        newFrag.appendedPTS = oldFrag.appendedPTS;
         newFrag.maxStartPTS = oldFrag.maxStartPTS;
 
         newFrag.endPTS = oldFrag.endPTS;

src/loader/fragment.ts

@@ -120,8 +120,6 @@ export class Fragment extends BaseSegment {
   public startPTS?: number;
   // The ending Presentation Time Stamp (PTS) of the fragment. Set after transmux complete.
   public endPTS?: number;
-  // The latest Presentation Time Stamp (PTS) appended to the buffer.
-  public appendedPTS?: number;
   // The starting Decode Time Stamp (DTS) of the fragment. Set after transmux complete.
   public startDTS!: number;
   // The ending Decode Time Stamp (DTS) of the fragment. Set after transmux complete.

src/types/fragment-tracker.ts

@@ -4,6 +4,9 @@ import type { FragLoadedData } from './events';
 
 export interface FragmentEntity {
   body: Fragment;
+  // appendedPTS is the latest buffered presentation time within the fragment's time range.
+  // It is used to determine: which fragment is appended at any given position, and hls.currentLevel.
+  appendedPTS: number | null;
   loaded: FragLoadedData | null;
   buffered: boolean;
   range: { [key in SourceBufferName]: FragmentBufferedRange };