fix: null check return value of selectPlaylist (#781)

This is #779 but against master.

.gitignore

@@ -38,7 +38,7 @@ test/dist/
 
 # this file is generated and shouldn't be checked in
 src/decrypter-worker.worker.js
+src/mse/transmuxer-worker.worker.js
 src/transmuxer-worker.worker.js
 
 .eslintcache
-.yo-rc.json

src/master-playlist-controller.js

@@ -43,6 +43,47 @@ const sumLoaderStat = function(stat) {
   return this.audioSegmentLoader_[stat] +
          this.mainSegmentLoader_[stat];
 };
+const shouldSwitchToMedia = function({
+  currentPlaylist,
+  nextPlaylist,
+  forwardBuffer,
+  bufferLowWaterLine,
+  duration,
+  log
+}) {
+  // we have no other playlist to switch to
+  if (!nextPlaylist) {
+    videojs.log.warn('We received no playlist to switch to. Please check your stream.');
+    return false;
+  }
+
+  // If the playlist is live, then we want to not take low water line into account.
+  // This is because in LIVE, the player plays 3 segments from the end of the
+  // playlist, and if `BUFFER_LOW_WATER_LINE` is greater than the duration availble
+  // in those segments, a viewer will never experience a rendition upswitch.
+  if (!currentPlaylist.endList) {
+    return true;
+  }
+
+  // For the same reason as LIVE, we ignore the low water line when the VOD
+  // duration is below the max potential low water line
+  if (duration < Config.MAX_BUFFER_LOW_WATER_LINE) {
+    return true;
+  }
+
+  // we want to switch down to lower resolutions quickly to continue playback, but
+  if (nextPlaylist.attributes.BANDWIDTH < currentPlaylist.attributes.BANDWIDTH) {
+    return true;
+  }
+
+  // ensure we have some buffer before we switch up to prevent us running out of
+  // buffer while loading a higher rendition.
+  if (forwardBuffer >= bufferLowWaterLine) {
+    return true;
+  }
+
+  return false;
+};
 
 /**
  * the master playlist controller controller all interactons
@@ -432,19 +473,14 @@ export class MasterPlaylistController extends videojs.EventTarget {
 
       const bufferLowWaterLine = this.bufferLowWaterLine();
 
-      // If the playlist is live, then we want to not take low water line into account.
-      // This is because in LIVE, the player plays 3 segments from the end of the
-      // playlist, and if `BUFFER_LOW_WATER_LINE` is greater than the duration availble
-      // in those segments, a viewer will never experience a rendition upswitch.
-      if (!currentPlaylist.endList ||
-          // For the same reason as LIVE, we ignore the low water line when the VOD
-          // duration is below the max potential low water line
-          this.duration() < Config.MAX_BUFFER_LOW_WATER_LINE ||
-          // we want to switch down to lower resolutions quickly to continue playback, but
-          nextPlaylist.attributes.BANDWIDTH < currentPlaylist.attributes.BANDWIDTH ||
-          // ensure we have some buffer before we switch up to prevent us running out of
-          // buffer while loading a higher rendition.
-          forwardBuffer >= bufferLowWaterLine) {
+      if (shouldSwitchToMedia({
+        currentPlaylist,
+        nextPlaylist,
+        forwardBuffer,
+        bufferLowWaterLine,
+        duration: this.duration(),
+        log: this.logger_
+      })) {
         this.masterPlaylistLoader_.media(nextPlaylist);
       }
 

test/master-playlist-controller.test.js

@@ -1458,6 +1458,37 @@ QUnit.test('selects a playlist after main/combined segment downloads', function(
   assert.equal(this.player.tech_.hls.stats.bandwidth, 4194304, 'default bandwidth');
 });
 
+QUnit.test('does not select a playlist after segment downloads if only one playlist', function(assert) {
+  const origWarn = videojs.log.warn;
+  let calls = 0;
+  const warnings = [];
+
+  videojs.log.warn = (text) => warnings.push(text);
+  this.masterPlaylistController.selectPlaylist = () => {
+    calls++;
+    return null;
+  };
+  this.masterPlaylistController.mediaSource.trigger('sourceopen');
+
+  // master
+  this.standardXHRResponse(this.requests.shift());
+  // media
+  this.standardXHRResponse(this.requests.shift());
+
+  // "downloaded" a segment
+  this.masterPlaylistController.mainSegmentLoader_.trigger('bandwidthupdate');
+  assert.strictEqual(calls, 2, 'selects after the initial segment');
+
+  assert.equal(warnings.length, 1, 'one warning logged');
+  assert.equal(
+    warnings[0],
+    'We received no playlist to switch to. Please check your stream.',
+    'we logged the correct warning'
+  );
+
+  videojs.log.warn = origWarn;
+});
+
 QUnit.test('re-triggers bandwidthupdate events on the tech', function(assert) {
   this.masterPlaylistController.mediaSource.trigger('sourceopen');
   // master