matroskadec: prevent seeking before the first keyframe of the reference stream

Originally committed as revision 15301 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -139,6 +139,7 @@ typedef struct {
     EbmlList encodings;
 
     AVStream *stream;
+    int64_t first_timecode;
     int64_t end_timecode;
 } MatroskaTrack;
 
@@ -1181,6 +1182,7 @@ static int matroska_read_header(AVFormatContext *s, AVFormatParameters *ap)
         st = track->stream = av_new_stream(s, 0);
         if (st == NULL)
             return AVERROR(ENOMEM);
+        track->first_timecode = AV_NOPTS_VALUE;
 
         if (!strcmp(track->codec_id, "V_MS/VFW/FOURCC")
             && track->codec_priv.size >= 40
@@ -1466,8 +1468,11 @@ static int matroska_parse_block(MatroskaDemuxContext *matroska, uint8_t *data,
         if (track->type == MATROSKA_TRACK_TYPE_SUBTITLE
             && timecode < track->end_timecode)
             is_keyframe = 0;  /* overlapping subtitles are not key frame */
-        if (is_keyframe)
+        if (is_keyframe) {
             av_add_index_entry(st, cluster_pos, timecode, 0,0,AVINDEX_KEYFRAME);
+            if (track->first_timecode == AV_NOPTS_VALUE)
+                track->first_timecode = timecode;
+        }
         track->end_timecode = FFMAX(track->end_timecode, timecode+duration);
     }
 
@@ -1692,9 +1697,15 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
     MatroskaTrack *tracks = matroska->tracks.elem;
     AVStream *st = s->streams[stream_index];
     int i, index, index_sub, index_min;
+    int64_t first_timecode = 0;
+
+    for (i=0; i < matroska->tracks.nb_elem; i++)
+        if (tracks[i].stream->index == stream_index &&
+            tracks[i].first_timecode != AV_NOPTS_VALUE)
+            first_timecode = tracks[i].first_timecode;
 
-    if (timestamp < 0)
-        timestamp = 0;
+    if (timestamp < first_timecode)
+        timestamp = first_timecode;
 
     if ((index = av_index_search_timestamp(st, timestamp, flags)) < 0) {
         if (st->nb_index_entries)