[FFmpeg-devel] [PATCH 05/37] avformat/matroskadec: Get rid of cluster size field assumption

[Andreas Rheinhardt]

James Almer:
> On 5/16/2019 7:29 PM, Andreas Rheinhardt wrote:
>> The earlier code relied on the length of clusters always being coded on
>> eight bytes as was the behaviour of libavformat's Matroska muxer until
>> recently. But given that our own Matroska muxer now (and mkvmerge from
>> time immemorial) creates files that don't conform to this assumption,
>> it is high time to get rid of this assumption.
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at gmail.com>
>> ---
>> I originally planed for this patch to get merged before my Matroska muxer
>> patches.
>>  libavformat/matroskadec.c | 15 +++++++++------
>>  1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
>> index 4dd933ef74..6fd5537f5a 100644
>> --- a/libavformat/matroskadec.c
>> +++ b/libavformat/matroskadec.c
>> @@ -3711,15 +3711,17 @@ static int webm_clusters_start_with_keyframe(AVFormatContext *s)
>>      cluster_pos = s->streams[0]->index_entries[index].pos;
>>      before_pos = avio_tell(s->pb);
>>      while (1) {
>> -        int64_t cluster_id = 0, cluster_length = 0;
>> +        uint64_t cluster_id, cluster_length;
>> +        int read;
>>          AVPacket *pkt;
>>          avio_seek(s->pb, cluster_pos, SEEK_SET);
>>          // read cluster id and length
>> -        ebml_read_num(matroska, matroska->ctx->pb, 4, &cluster_id);
>> -        ebml_read_length(matroska, matroska->ctx->pb, &cluster_length);
>> -        if (cluster_id != 0xF43B675) { // done with all clusters
>> +        read = ebml_read_num(matroska, matroska->ctx->pb, 4, &cluster_id);
>> +        if (read < 0 || cluster_id != 0xF43B675) // done with all clusters
> 
> This could be changed to use the matroska.h define instead.
> 
The defines in matroska.h include the whole EBML-numbers, including
the length descriptor bit (or VINT_MARKER in the parlance of the
current EBML specifications), whereas ebml_read_length returns the
number encoded by the EBML number (i.e. without VINT_MARKER). So one
can't simply replace the number by MATROSKA_ID_CLUSTER and so I didn't
do it.
>> +            break;
>> +        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_clear_queue(matroska);
>> @@ -3728,7 +3730,8 @@ static int webm_clusters_start_with_keyframe(AVFormatContext *s)
>>              break;
>>          }
>>          pkt = &matroska->queue->pkt;
>> -        cluster_pos += cluster_length + 12; // 12 is the offset of the cluster id and length.
>> +        // 4 + read is the length of the cluster id and the cluster length field.
>> +        cluster_pos += 4 + read + cluster_length;
>>          if (!(pkt->flags & AV_PKT_FLAG_KEY)) {
>>              rv = 0;
>>              break;
> 
> Applied, thanks.
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