Audio player: Throttle concurrent loading of audio metadata.

ui/file_manager/audio_player/js/audio_player.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * Overrided metadata worker's path.
  * @type {string}
  */
 ContentMetadataProvider.WORKER_SCRIPT = '/js/metadata_worker.js';
 
@@ skipping 29 matching lines @@
    * @type {?boolean}
    * @private
    */
   // Initial value is null. It'll be set in load().
   this.isTrackInfoExpanded_ = null;
 
   this.player_ =
     /** @type {AudioPlayerElement} */ (document.querySelector('audio-player'));
   this.player_.tracks = [];
 
+  /**
+   * Queue to throttle concurrent reading of audio file metadata.
+   * Here we loads up to 25 songs concurrently to cover the number of songs in
+   * an album in most cases. This number should not be too large so that the
+   * number of open file descriptors will not hit the system limit.
+   * @private {AsyncUtil.ConcurrentQueue}
+   */
+  this.loadMetadataQueue_ = new AsyncUtil.ConcurrentQueue(25);
+
   // Restore the saved state from local storage, and update the local storage
   // if the states are changed.
   var STORAGE_PREFIX = 'audioplayer-';
   var KEYS_TO_SAVE_STATES =
       ['shuffle',
        'repeat-mode',
        'volume',
        'playlist-expanded',
        'track-info-expanded'];
   var storageKeys = KEYS_TO_SAVE_STATES.map(a => STORAGE_PREFIX + a);
@@ skipping 149 matching lines @@
     }.bind(this));
   }.bind(this));
 };
 
 /**
  * Loads metadata for a track.
  * @param {number} track Track number.
  * @private
  */
 AudioPlayer.prototype.loadMetadata_ = function(track) {
-  this.fetchMetadata_(
-      this.entries_[track], this.displayMetadata_.bind(this, track));
+  this.loadMetadataQueue_.run(function(callback) {
+    this.fetchMetadata_(this.entries_[track], function(metadata) {
+      this.displayMetadata_(track, metadata);
+      callback();
+    }.bind(this));
+  }.bind(this));
 };
 
 /**
  * Displays track's metadata.
  * @param {number} track Track number.
  * @param {Object} metadata Metadata object.
  * @param {string=} opt_error Error message.
  * @private
  */
 AudioPlayer.prototype.displayMetadata_ = function(track, metadata, opt_error) {
@@ skipping 377 matching lines @@
   // TODO(yoshiki): Handle error in better way.
   this.title = metadata.mediaTitle || this.getDefaultTitle();
   this.artist = error || metadata.mediaArtist || this.getDefaultArtist();
   this.artworkUrl = metadata.contentThumbnailUrl || "";
 };
 
 // Starts loading the audio player.
 window.addEventListener('DOMContentLoaded', function(e) {
   AudioPlayer.load();
 });