[FFmpeg-devel] [PATCH] avformat/mov: Fix decoding fragmented MP4 with multiple sample entries and empty stsc

[Dimitry Andric]

On 3 Apr 2025, at 22:02, Dimitry Andric <dimitry at unified-streaming.com> wrote:
> 
> When decoding fragmented MP4 files that have an empty stsc box, and
> instead contain sample description indexes in their tfhd boxes, the mov
> demuxer does not notify the decoder whenever the current sample
> description index changes. If the SPS or PPS changed sufficiently, this
> can lead to unexpected decoding errors.
> 
> To fix this, in mov_finalize_packet(), when stsc_data is not available,
> use get_frag_stream_info_from_pkt() to get at the current fragment
> stream info, and retrieve the current sample description index from
> there. Then use that index in a similar manner as the stsc case.
> 
> Signed-off-by: Dimitry Andric <dimitry at unified-streaming.com>
> ---
> libavformat/mov.c | 50 ++++++++++++++++++++++++++++-------------------
> 1 file changed, 30 insertions(+), 20 deletions(-)
> 
> diff --git a/libavformat/mov.c b/libavformat/mov.c
> index 452690090c..ead89192f4 100644
> --- a/libavformat/mov.c
> +++ b/libavformat/mov.c
> @@ -10756,25 +10756,29 @@ static int mov_switch_root(AVFormatContext *s, int64_t target, int index)
>     return 1;
> }
> 
> -static int mov_change_extradata(AVStream *st, AVPacket *pkt)
> +static int mov_change_extradata(AVStream *st, AVPacket *pkt, int stsd_id)
> {
>     MOVStreamContext *sc = st->priv_data;
>     uint8_t *side, *extradata;
>     int extradata_size;
> 
> -    /* Save the current index. */
> -    sc->last_stsd_index = sc->stsc_data[sc->stsc_index].id - 1;
> +    if (stsd_id > 0 &&
> +        stsd_id - 1 < sc->stsd_count &&
> +        stsd_id - 1 != sc->last_stsd_index) {
> +        /* Save the current index. */
> +        sc->last_stsd_index = stsd_id - 1;
> 
> -    /* Notify the decoder that extradata changed. */
> -    extradata_size = sc->extradata_size[sc->last_stsd_index];
> -    extradata = sc->extradata[sc->last_stsd_index];
> -    if (st->discard != AVDISCARD_ALL && extradata_size > 0 && extradata) {
> -        side = av_packet_new_side_data(pkt,
> -                                       AV_PKT_DATA_NEW_EXTRADATA,
> -                                       extradata_size);
> -        if (!side)
> -            return AVERROR(ENOMEM);
> -        memcpy(side, extradata, extradata_size);
> +        /* Notify the decoder that extradata changed. */
> +        extradata_size = sc->extradata_size[sc->last_stsd_index];
> +        extradata = sc->extradata[sc->last_stsd_index];
> +        if (st->discard != AVDISCARD_ALL && extradata_size > 0 && extradata) {
> +            side = av_packet_new_side_data(pkt,
> +                                           AV_PKT_DATA_NEW_EXTRADATA,
> +                                           extradata_size);
> +            if (!side)
> +                return AVERROR(ENOMEM);
> +            memcpy(side, extradata, extradata_size);
> +        }
>     }
> 
>     return 0;
> @@ -10893,13 +10897,10 @@ static int mov_finalize_packet(AVFormatContext *s, AVStream *st, AVIndexEntry *s
> 
>     /* Multiple stsd handling. */
>     if (sc->stsc_data) {
> -        if (sc->stsc_data[sc->stsc_index].id > 0 &&
> -            sc->stsc_data[sc->stsc_index].id - 1 < sc->stsd_count &&
> -            sc->stsc_data[sc->stsc_index].id - 1 != sc->last_stsd_index) {
> -            int ret = mov_change_extradata(st, pkt);
> -            if (ret < 0)
> -                return ret;
> -        }
> +        int stsd_id = sc->stsc_data[sc->stsc_index].id;
> +        int ret = mov_change_extradata(st, pkt, stsd_id);
> +        if (ret < 0)
> +            return ret;
> 
>         /* Update the stsc index for the next sample */
>         sc->stsc_sample++;
> @@ -10908,6 +10909,15 @@ static int mov_finalize_packet(AVFormatContext *s, AVStream *st, AVIndexEntry *s
>             sc->stsc_index++;
>             sc->stsc_sample = 0;
>         }
> +    } else {
> +        MOVContext *mov = s->priv_data;
> +        MOVFragmentStreamInfo *frag_stream_info = get_frag_stream_info_from_pkt(&mov->frag_index, pkt, sc->id);
> +        if (frag_stream_info) {
> +            int stsd_id = frag_stream_info->stsd_id;
> +            int ret = mov_change_extradata(st, pkt, stsd_id);
> +            if (ret < 0)
> +                return ret;
> +        }
>     }
> 
>     return 0;
> -- 
> 2.43.0
> 

Any comments on this patch?

-Dimitry