applehttp: Use half the target duration as interval if the playlist didn't update

This is mandated in draft-pantos-http-live-streaming-07, section
6.3.4.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/applehttp.c

@@ -376,7 +376,6 @@ static int read_data(void *opaque, uint8_t *buf, int buf_size)
 
 restart:
     if (!v->input) {
-reload:
         /* If this is a live stream and the reload interval has elapsed since
          * the last playlist reload, reload the variant playlists now. */
         int64_t reload_interval = v->n_segments > 0 ?
@@ -384,10 +383,16 @@ reload:
                                   v->target_duration;
         reload_interval *= 1000000;
 
+reload:
         if (!v->finished &&
-            av_gettime() - v->last_load_time >= reload_interval &&
-            (ret = parse_playlist(c, v->url, v, NULL)) < 0)
+            av_gettime() - v->last_load_time >= reload_interval) {
+            if ((ret = parse_playlist(c, v->url, v, NULL)) < 0)
                 return ret;
+            /* If we need to reload the playlist again below (if
+             * there's still no more segments), switch to a reload
+             * interval of half the target duration. */
+            reload_interval = v->target_duration * 500000;
+        }
         if (v->cur_seq_no < v->start_seq_no) {
             av_log(NULL, AV_LOG_WARNING,
                    "skipping %d segments ahead, expired from playlists\n",