fix: Fix race that allows multiple text streams to be loaded (#5129)

When enabling text visibility and selecting a new text language
simultaneously, a race condition can occur that allows multiple text
streams to be loaded in the same text media state.
Tracking a sequence id, updates `loadNewTextStream_` to only create a
new text media state with the text stream from the last call to
`loadNewTextStream_`.

Resolves: #4821

---------

Co-authored-by: Dan Sparacio <[email protected]>
Co-authored-by: Casey Occhialini <[email protected]>

CONTRIBUTORS

@@ -117,6 +117,8 @@ Vasanth Polipelli <[email protected]>
 Vignesh Venkatasubramanian <[email protected]>
 Vincent Valot <[email protected]>
 Wayne Morgan <[email protected]>
+Wen Ren <[email protected]>
+Will Harris <[email protected]>
 Yohann Connell <[email protected]>
 Raymond Cheng <[email protected]>
 Janroel Koppen <[email protected]>

lib/media/streaming_engine.js

@@ -77,6 +77,9 @@ shaka.media.StreamingEngine = class {
     /** @private {?shaka.extern.Stream} */
     this.currentTextStream_ = null;
 
+    /** @private {number} */
+    this.textStreamSequenceId_ = 0;
+
     /**
      * Maps a content type, e.g., 'audio', 'video', or 'text', to a MediaState.
      *
@@ -220,6 +223,8 @@ shaka.media.StreamingEngine = class {
     const ContentType = shaka.util.ManifestParserUtils.ContentType;
     goog.asserts.assert(!this.mediaStates_.has(ContentType.TEXT),
         'Should not call loadNewTextStream_ while streaming text!');
+    this.textStreamSequenceId_++;
+    const currentSequenceId = this.textStreamSequenceId_;
 
     try {
       // Clear MediaSource's buffered text, so that the new text stream will
@@ -241,7 +246,7 @@ shaka.media.StreamingEngine = class {
     const streamText =
         textDisplayer.isTextVisible() || this.config_.alwaysStreamText;
 
-    if (streamText) {
+    if (streamText && (this.textStreamSequenceId_ == currentSequenceId)) {
       const state = this.createMediaState_(stream);
       this.mediaStates_.set(ContentType.TEXT, state);
       this.scheduleUpdate_(state, 0);

test/player_integration.js

@@ -336,6 +336,45 @@ describe('Player', () => {
       player.setTextTrackVisibility(true);
       expect(getTracksActive()).toEqual([false, true]);
     });
+
+    // https://github.com/shaka-project/shaka-player/issues/4821
+    it('loads a single text stream', async () => {
+      player.configure({preferredTextLanguage: 'en'});
+      await player.load('test:sintel_no_text_compiled');
+
+      // Add preferred language text track.
+      const locationUri = new goog.Uri(location.href);
+      const partialUri = new goog.Uri('/base/test/test/assets/text-clip.vtt');
+      const absoluteUri = locationUri.resolve(partialUri);
+      await player.addTextTrackAsync(
+          absoluteUri.toString(), 'en', 'subtitles', 'text/vtt');
+
+      // Add alternate language text track.
+      // Two text tracks with same timings but different text
+      // are necessary for test.
+      const partialUri2 =
+      new goog.Uri('/base/test/test/assets/text-clip-alt.vtt');
+      const absoluteUri2 = locationUri.resolve(partialUri2);
+      await player.addTextTrackAsync(
+          absoluteUri2.toString(), 'fr', 'subtitles', 'text/vtt');
+
+      const textTracks = player.getTextTracks();
+      expect(textTracks.length).toBe(2);
+      expect(textTracks[0].language).toBe('en');
+      expect(textTracks[1].language).toBe('fr');
+
+      // Enable text visibilty and immediately change language.
+      // Only one set of cues should be active.
+      // Cues should be of the selected language track.
+      player.setTextTrackVisibility(true);
+      player.selectTextLanguage('fr');
+      video.currentTime = 5;
+      video.play();
+      await waiter.waitForMovementOrFailOnTimeout(video, 10);
+
+      expect(video.textTracks[0].activeCues.length).toBe(1);
+      expect(player.getTextTracks()[1].active).toBe(true);
+    });
   });  // describe('setTextTrackVisibility')
 
   describe('plays', () => {

test/test/assets/text-clip-alt.vtt

@@ -0,0 +1,63 @@
+WEBVTT
+
+00:04.000 --> 00:07.300
+Journal de bord du capitaine.
+Date stellaire 41636.9.
+
+00:07.500 --> 00:11.800
+Comme nous le craignions, l'épave
+du cargo Odin de la Fédération,
+
+00:12.100 --> 00:16.400
+percuté par un astéroïde,
+ne révèle aucun signe de vie.
+
+00:16.700 --> 00:19.000
+Mais trois nacelles
+de sauvetage manquaient,
+
+00:19.300 --> 00:21.900
+laissant supposer
+qu'il y aurait des survivants.
+
+00:22.700 --> 00:27.700
+- Mise sur orbite Angel One, paré.
+- Quel genre d'endroit, Data ?
+
+00:28.200 --> 00:31.600
+Planète de Classe-M.
+Faune et flore à base de carbone.
+
+00:31.900 --> 00:34.300
+Population éparse
+et forme de vie intelligente.
+
+00:34.600 --> 00:38.400
+Développement technique équivalent
+au milieu du 20ème siècle terrien.
+
+00:38.800 --> 00:41.000
+C'est presque un retour aux sources.
+
+00:41.300 --> 00:43.600
+Si jamais des survivants
+ont pu aller si loin,
+
+00:43.800 --> 00:49.000
+c'était la planète la plus proche.
+La distance qui nous a pris 2 jours
+
+00:49.500 --> 00:52.400
+aurait pris 5 mois aux nacelles
+de sauvetage du Odin.
+
+00:52.600 --> 00:54.700
+Cinq mois, six jours, onze heures,
+deux min...
+
+00:54.900 --> 00:58.400
+- Merci, Data.
+- ..et 57 secondes.
+
+00:58.600 --> 01:01.300
+Signal audio provenant d'Angel One.