avformat/matroskadec: Use proper levels after discontínuity

The earlier code set the level to zero upon seeking and after a
discontinuity although in both cases parsing (re)starts at a level 1
element.

Also set the segment's length to unkown if an error occured in order not
to drop any valid data that happens to be beyond the designated end of
the segment.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/matroskadec.c

@@ -762,8 +762,6 @@ static int matroska_resync(MatroskaDemuxContext *matroska, int64_t last_pos)
 {
     AVIOContext *pb = matroska->ctx->pb;
     uint32_t id;
-    matroska->current_id = 0;
-    matroska->num_levels = 0;
 
     /* Try to seek to the last position to resync from. If this doesn't work,
      * we resync from the earliest position available: The start of the buffer. */
@@ -783,7 +781,13 @@ static int matroska_resync(MatroskaDemuxContext *matroska, int64_t last_pos)
             id == MATROSKA_ID_CUES     || id == MATROSKA_ID_TAGS        ||
             id == MATROSKA_ID_SEEKHEAD || id == MATROSKA_ID_ATTACHMENTS ||
             id == MATROSKA_ID_CLUSTER  || id == MATROSKA_ID_CHAPTERS) {
-            matroska->current_id = id;
+            /* Prepare the context for parsing of a level 1 element. */
+            matroska_reset_status(matroska, id, -1);
+            /* Given that we are here means that an error has occured,
+             * so treat the segment as unknown length in order not to
+             * discard valid data that happens to be beyond the designated
+             * end of the segment. */
+            matroska->levels[0].length = EBML_UNKNOWN_LENGTH;
             return 0;
         }
         id = (id << 8) | avio_r8(pb);
@@ -1679,18 +1683,11 @@ static int matroska_parse_seekhead_entry(MatroskaDemuxContext *matroska,
             matroska->current_id                   = 0;
 
             ret = ebml_parse(matroska, matroska_segment, matroska);
-
-            /* remove dummy level */
-            while (matroska->num_levels) {
-                uint64_t length = matroska->levels[--matroska->num_levels].length;
-                if (length == EBML_UNKNOWN_LENGTH)
-                    break;
-            }
         }
     }
-    /* seek back */
-    avio_seek(matroska->ctx->pb, before_pos, SEEK_SET);
-    matroska->current_id = saved_id;
+    /* Seek back - notice that in all instances where this is used it is safe
+     * to set the level to 1 and unset the position of the current cluster. */
+    matroska_reset_status(matroska, saved_id, before_pos);
 
     return ret;
 }
@@ -3604,9 +3601,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
     timestamp = FFMAX(timestamp, st->index_entries[0].timestamp);
 
     if ((index = av_index_search_timestamp(st, timestamp, flags)) < 0 || index == st->nb_index_entries - 1) {
-        avio_seek(s->pb, st->index_entries[st->nb_index_entries - 1].pos,
-                  SEEK_SET);
-        matroska->current_id = 0;
+        matroska_reset_status(matroska, 0, st->index_entries[st->nb_index_entries - 1].pos);
         while ((index = av_index_search_timestamp(st, timestamp, flags)) < 0 || index == st->nb_index_entries - 1) {
             matroska_clear_queue(matroska);
             if (matroska_parse_cluster(matroska) < 0)
@@ -3626,8 +3621,8 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
         tracks[i].end_timecode         = 0;
     }
 
-    avio_seek(s->pb, st->index_entries[index].pos, SEEK_SET);
-    matroska->current_id       = 0;
+    /* We seek to a level 1 element, so set the appropriate status. */
+    matroska_reset_status(matroska, 0, st->index_entries[index].pos);
     if (flags & AVSEEK_FLAG_ANY) {
         st->skip_to_keyframe = 0;
         matroska->skip_to_timecode = timestamp;
@@ -3637,18 +3632,16 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
     }
     matroska->skip_to_keyframe = 1;
     matroska->done             = 0;
-    matroska->num_levels       = 0;
     ff_update_cur_dts(s, st, st->index_entries[index].timestamp);
     return 0;
 err:
     // slightly hackish but allows proper fallback to
     // the generic seeking code.
+    matroska_reset_status(matroska, 0, -1);
     matroska_clear_queue(matroska);
-    matroska->current_id = 0;
     st->skip_to_keyframe =
     matroska->skip_to_keyframe = 0;
     matroska->done = 0;
-    matroska->num_levels = 0;
     return -1;
 }
 
@@ -3711,6 +3704,7 @@ static CueDesc get_cue_desc(AVFormatContext *s, int64_t ts, int64_t cues_start)
 static int webm_clusters_start_with_keyframe(AVFormatContext *s)
 {
     MatroskaDemuxContext *matroska = s->priv_data;
+    uint32_t id = matroska->current_id;
     int64_t cluster_pos, before_pos;
     int index, rv = 1;
     if (s->streams[0]->nb_index_entries <= 0) return 0;
@@ -3731,8 +3725,8 @@ static int webm_clusters_start_with_keyframe(AVFormatContext *s)
         read = ebml_read_length(matroska, matroska->ctx->pb, &cluster_length);
         if (read < 0)
             break;
-        avio_seek(s->pb, cluster_pos, SEEK_SET);
-        matroska->current_id = 0;
+
+        matroska_reset_status(matroska, 0, cluster_pos);
         matroska_clear_queue(matroska);
         if (matroska_parse_cluster(matroska) < 0 ||
             !matroska->queue) {
@@ -3746,7 +3740,10 @@ static int webm_clusters_start_with_keyframe(AVFormatContext *s)
             break;
         }
     }
-    avio_seek(s->pb, before_pos, SEEK_SET);
+
+    /* Restore the status after matroska_read_header: */
+    matroska_reset_status(matroska, id, before_pos);
+
     return rv;
 }