TracksStore keeps the last reference in case of several being communi…

…cated

As #1623 reported, there seem to be an issue seen on lower-end
devices where the `TracksStore` gets multiple track references for the
same buffer type (`audio`, `video`...) and Period combination.

This should __NEVER__ happen as per the `TracksStore` API, there should
only be one reference linked to the `TracksStore` for any such
combination - if the external logic wanted to update the track
reference, it had to call `removeTrackReference` with the previous one
(or `resetPeriodObjects` to just reset everything) before adding the new
one.

Turns out it does happen in some unknown and difficult to reproduce
scenario. I guess a race condition allows for some cleaning-up phase to
be lost somewhere (either a reset is being skipped - which mostly happens
when `RELOADING`, or either some `PeriodStream` cleaning is being missed,
which happens when Period switching).

So what I propose here isn't a real fix for that problem, but to add
some resilience into the TracksStore by considering the last track
reference for a buffer type + Period combination - which IMO
makes more sense API-wise than keeping the first one and should "fix"
cases of infinite rebuffering (as I suppose some code awaited
indefinitely that a track choice be performed through that new track
reference).

The absence of clearing of the previous reference still indicates a
leak (though the leak should not matter anymore for the `TracksStore`
once the new one is added) and still indicate that something went
wrong somewhere, so I kept the `log.error` call which will help us
debug other issues that may have their source in that same weird
race condition.

I'm still looking how such race condition is even possible in the
meantime.

src/main_thread/tracks_store/tracks_store.ts

@@ -363,6 +363,7 @@ export default class TracksStore extends EventEmitter<ITracksStoreEvents> {
     period: IPeriodMetadata,
     adaptationRef: SharedReference<IAdaptationChoice | null | undefined>,
   ): void {
+    log.debug("TS: Adding Track Reference", bufferType, period.id);
     let periodObj = getPeriodItem(this._storedPeriodInfo, period.id);
     if (periodObj === undefined) {
       // The Period has not yet been added.
@@ -383,7 +384,7 @@ export default class TracksStore extends EventEmitter<ITracksStoreEvents> {
       log.error(
         `TS: Subject already added for ${bufferType} ` + `and Period ${period.start}`,
       );
-      return;
+      periodObj[bufferType].dispatcher.dispose();
     }
 
     const dispatcher = new TrackDispatcher(adaptationRef);
@@ -502,6 +503,7 @@ export default class TracksStore extends EventEmitter<ITracksStoreEvents> {
     bufferType: "audio" | "text" | "video",
     periodId: string,
   ): void {
+    log.debug("TS: Removing Track Reference", bufferType, periodId);
     let periodIndex;
     for (let i = 0; i < this._storedPeriodInfo.length; i++) {
       const periodI = this._storedPeriodInfo[i];
@@ -600,6 +602,7 @@ export default class TracksStore extends EventEmitter<ITracksStoreEvents> {
    * You might want to call this API when restarting playback.
    */
   public resetPeriodObjects(): void {
+    log.debug("TS: Resetting Period Objects");
     for (let i = this._storedPeriodInfo.length - 1; i >= 0; i--) {
       const storedObj = this._storedPeriodInfo[i];
       storedObj.audio.dispatcher?.dispose();