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

[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.

> +            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.