Merge branch 'master' into fix-fragment-loop

doc/design.md

@@ -73,6 +73,7 @@ design idea is pretty simple :
     **Feature: Media Zigzagging**
 
     If there is a backup stream, Media Zigzagging will go through all available levels in `primary` and `backup` streams. Behavior has a dual constraint, where fragment retry limits and level limits are accounted in the same time.
+    When the lowest level has been reached, zigzagging will be adjusted to start from the highest level until retry limits are not reached.
 
     ![Media Zigzagging Explanation](./media-zigzagging.png) 
 

src/controller/level-controller.js

@@ -290,7 +290,7 @@ export default class LevelController extends EventHandler {
     let {config} = this.hls;
     let {details: errorDetails} = errorEvent;
     let level = this._levels[levelIndex];
-    let redundantLevels, delay;
+    let redundantLevels, delay, nextLevel;
 
     level.loadError++;
     level.fragmentError = fragmentError;
@@ -326,10 +326,12 @@ export default class LevelController extends EventHandler {
         level.urlId = (level.urlId + 1) % redundantLevels;
         level.details = undefined;
       } else {
-        // Switch-down if more renditions are available
-        if (this.manualLevelIndex === -1 && levelIndex !== 0) {
-          logger.warn(`level controller, ${errorDetails}: switch-down to ${levelIndex - 1}`);
-          this.hls.nextAutoLevel = this.currentLevelIndex = levelIndex - 1;
+        // Search for available level
+        if (this.manualLevelIndex === -1) {
+          // When lowest level has been reached, let's start hunt from the top
+          nextLevel = (levelIndex === 0) ? this._levels.length - 1 : levelIndex - 1;
+          logger.warn(`level controller, ${errorDetails}: switch to ${nextLevel}`);
+          this.hls.nextAutoLevel = this.currentLevelIndex = nextLevel;
         } else if (fragmentError === true) {
           // Allow fragment retry as long as configuration allows.
           // reset this._level so that another call to set level() will trigger again a frag load