feat: Set segmentPrefetchLimit to 2 by default for low latency stream…

…ing (shaka-project#5275)

docs/tutorials/config.md

@@ -122,18 +122,20 @@ buffering settings) while some will not have any effect until the next call to
 #### Low latency streaming
 
 With `.streaming.lowLatencyMode` set to true,
-`.streaming.inaccurateManifestTolerance` is set to 0 by default, and
-`.streaming.rebufferingGoal` is set to 0.01 by default.
+`.streaming.inaccurateManifestTolerance` is set to 0 by default,
+`.streaming.rebufferingGoal` is set to 0.01 by default, and
+`.streaming.segmentPrefetchLimit` is set to 2 by default.
 
-To customize the values of inaccurateManifestTolerance and rebufferingGoal
-with low latency mode, you can set the fields in the same or subsequent
-call to configure().
+To customize the values of inaccurateManifestTolerance, rebufferingGoal and
+segmentPrefetchLimit with low latency mode, you can set the fields in the same
+or subsequent call to configure().
 ```js
 player.configure({
   streaming: {
     lowLatencyMode: true,
     inaccurateManifestTolerance: 0,
     rebufferingGoal: 0.01,
+    segmentPrefetchLimit: 2,
   }
 });
 

lib/player.js

@@ -3205,15 +3205,19 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     }
 
     // If lowLatencyMode is enabled, and inaccurateManifestTolerance and
-    // rebufferingGoal are not specified, set inaccurateManifestTolerance to 0
-    // and rebufferingGoal to 0.01 by default for low latency streaming.
+    // rebufferingGoal and segmentPrefetchLimit are not specified, set
+    // inaccurateManifestTolerance to 0 and rebufferingGoal to 0.01 and
+    // segmentPrefetchLimit to 2 by default for low latency streaming.
     if (config['streaming'] && config['streaming']['lowLatencyMode']) {
       if (config['streaming']['inaccurateManifestTolerance'] == undefined) {
         config['streaming']['inaccurateManifestTolerance'] = 0;
       }
       if (config['streaming']['rebufferingGoal'] == undefined) {
         config['streaming']['rebufferingGoal'] = 0.01;
       }
+      if (config['streaming']['segmentPrefetchLimit'] == undefined) {
+        config['streaming']['segmentPrefetchLimit'] = 2;
+      }
     }
     const ret = shaka.util.PlayerConfiguration.mergeConfigObjects(
         this.config_, config, this.defaultConfig_());

lib/util/player_configuration.js

@@ -206,6 +206,8 @@ shaka.util.PlayerConfiguration = class {
       observeQualityChanges: false,
       maxDisabledTime: 30,
       parsePrftBox: false,
+      // When low latency streaming is enabled, segmentPrefetchLimit will
+      // default to 2 if not specified.
       segmentPrefetchLimit: 0,
     };
 

test/player_unit.js

@@ -1291,19 +1291,23 @@ describe('Player', () => {
           lowLatencyMode: true,
           rebufferingGoal: 1,
           inaccurateManifestTolerance: 1,
+          segmentPrefetchLimit: 1,
         },
       });
       expect(player.getConfiguration().streaming.rebufferingGoal).toBe(1);
       expect(player.getConfiguration().streaming.inaccurateManifestTolerance)
           .toBe(1);
+      expect(player.getConfiguration().streaming.segmentPrefetchLimit).toBe(1);
 
       // When low latency streaming gets enabled, rebufferingGoal will default
-      // to 0.01 if unless specified, and inaccurateManifestTolerance will
-      // default to 0 unless specified.
+      // to 0.01 if unless specified, inaccurateManifestTolerance will
+      // default to 0 unless specified, and segmentPrefetchLimit will
+      // default to 2 unless specified.
       player.configure('streaming.lowLatencyMode', true);
       expect(player.getConfiguration().streaming.rebufferingGoal).toBe(0.01);
       expect(player.getConfiguration().streaming.inaccurateManifestTolerance)
           .toBe(0);
+      expect(player.getConfiguration().streaming.segmentPrefetchLimit).toBe(2);
     });
   });