feat(HLS): Take into account the parsing time for manifest schedule u…

…pdate (#5678)

lib/hls/hls_parser.js

@@ -9,6 +9,7 @@ goog.provide('shaka.hls.HlsParser');
 
 goog.require('goog.Uri');
 goog.require('goog.asserts');
+goog.require('shaka.abr.Ewma');
 goog.require('shaka.hls.ManifestTextParser');
 goog.require('shaka.hls.Playlist');
 goog.require('shaka.hls.PlaylistType');
@@ -215,6 +216,13 @@ shaka.hls.HlsParser = class {
 
     /** @private {boolean} */
     this.lowLatencyByterangeOptimization_ = false;
+
+    /**
+     * An ewma that tracks how long updates take.
+     * This is to mitigate issues caused by slow parsing on embedded devices.
+     * @private {!shaka.abr.Ewma}
+     */
+    this.averageUpdateDuration_ = new shaka.abr.Ewma(5);
   }
 
 
@@ -3679,12 +3687,20 @@ shaka.hls.HlsParser = class {
     }
 
     try {
+      const startTime = Date.now();
       await this.update();
 
+      // Keep track of how long the longest manifest update took.
+      const endTime = Date.now();
+
       // This may have converted to VOD, in which case we stop updating.
       if (this.isLive_()) {
+        const updateDuration = (endTime - startTime) / 1000.0;
+        this.averageUpdateDuration_.sample(1, updateDuration);
         const delay = this.getUpdatePlaylistDelay_();
-        this.updatePlaylistTimer_.tickAfter(/* seconds= */ delay);
+        const finalDelay = Math.max(0,
+            delay - this.averageUpdateDuration_.getEstimate());
+        this.updatePlaylistTimer_.tickAfter(/* seconds= */ finalDelay);
       }
     } catch (error) {
       // Detect a call to stop() during this.update()