stream-controller: avoid fragLoopLoading error on streams with fragme…

…nts not starting with K-Frames

when there is a significant startPTS offset between audio and video and frag look up always detect that same fragment,
force loading previous fragment, to try to get last keyframe ... this usually helps fill the buffer hole

related to https://github.com/dailymotion/hls.js/issues/517

src/controller/stream-controller.js

@@ -285,8 +285,21 @@ class StreamController extends EventHandler {
             //logger.log('find SN matching with pos:' +  bufferEnd + ':' + frag.sn);
             if (fragPrevious && frag.level === fragPrevious.level && frag.sn === fragPrevious.sn) {
               if (frag.sn < levelDetails.endSN) {
-                frag = fragments[frag.sn + 1 - levelDetails.startSN];
-                logger.log(`SN just loaded, load next one: ${frag.sn}`);
+                let deltaPTS = fragPrevious.deltaPTS,
+                    curSNIdx = frag.sn - levelDetails.startSN;
+                // if there is a significant delta between audio and video, larger than max allowed hole,
+                // it might be because video fragment does not start with a keyframe.
+                // let's try to load previous fragment again to get last keyframe
+                // then we will reload again current fragment (that way we should be able to fill the buffer hole ...)
+                if (deltaPTS && deltaPTS > config.maxBufferHole) {
+                  frag = fragments[curSNIdx-1];
+                  logger.warn(`SN just loaded, with large PTS gap between audio and video, maybe frag is not starting with a keyframe ? load previous one to try to overcome this`);
+                  // decrement previous frag load counter to avoid frag loop loading error when next fragment will get reloaded
+                  fragPrevious.loadCounter--;
+                } else {
+                  frag = fragments[curSNIdx+1];
+                  logger.log(`SN just loaded, load next one: ${frag.sn}`);
+                }
               } else {
                 // have we reached end of VOD playlist ?
                 if (!levelDetails.live) {

src/helper/level-helper.js

@@ -71,6 +71,13 @@ class LevelHelper {
     fragments = details.fragments;
     frag = fragments[fragIdx];
     if(!isNaN(frag.startPTS)) {
+      // delta PTS between audio and video
+      let deltaPTS = Math.abs(frag.startPTS-startPTS);
+      if (isNaN(frag.deltaPTS)) {
+        frag.deltaPTS = deltaPTS;
+      } else {
+        frag.deltaPTS = Math.max(deltaPTS,frag.deltaPTS);
+      }
       startPTS = Math.min(startPTS,frag.startPTS);
       endPTS = Math.max(endPTS, frag.endPTS);
       startDTS = Math.min(startDTS,frag.startDTS);