[FFmpeg-devel] [PATCH 14/21] avformat/matroskadec: Use proper levels after discontínuity

[Andreas Rheinhardt]

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 at googlemail.com>
---
 libavformat/matroskadec.c | 59 +++++++++++++++++++++++----------------
 1 file changed, 35 insertions(+), 24 deletions(-)

diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index 0179e5426e..42f1c21042 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -737,13 +737,24 @@ static const char *const matroska_doctypes[] = { "matroska", "webm" };
 
 static int matroska_read_close(AVFormatContext *s);
 
+static int matroska_reset_status(MatroskaDemuxContext *matroska,
+                                 uint32_t id, int64_t position)
+{
+    matroska->current_id = id;
+    matroska->num_levels = 1;
+    matroska->current_cluster.pos = 0;
+
+    if (position >= 0)
+        return avio_seek(matroska->ctx->pb, position, SEEK_SET);
+
+    return 0;
+}
+
 static int matroska_resync(MatroskaDemuxContext *matroska, int64_t last_pos)
 {
     AVIOContext *pb = matroska->ctx->pb;
     int64_t ret;
     uint32_t id;
-    matroska->current_id = 0;
-    matroska->num_levels = 0;
 
     /* seek to next position to resync from */
     if ((ret = avio_seek(pb, last_pos + 1, SEEK_SET)) < 0) {
@@ -759,7 +770,14 @@ 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 further 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);
@@ -1610,18 +1628,12 @@ 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;
 }
@@ -3535,9 +3547,7 @@ 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)
@@ -3557,8 +3567,8 @@ 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;
@@ -3568,18 +3578,16 @@ 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;
 }
 
@@ -3662,8 +3670,8 @@ 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) {
@@ -3677,7 +3685,10 @@ 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, MATROSKA_ID_CLUSTER, before_pos);
+
     return rv;
 }
 
-- 
2.19.2