[FFmpeg-devel] [PATCH 2/2 v2] avformat/demux: extract extradata from packets when context update is requested

[James Almer]

On 4/21/2024 12:32 PM, Andreas Rheinhardt wrote:
> James Almer:
>> If the demuxer doesn't set extradata in the stream's codecpar, a
>> need_context_update request will delete the previously extracted extradata in
>> the stream's internal AVCodecContext.
>> As we can't ensure the old extradata is valid for the stream in its post
>> context update request state, try to get extradata from the new packet instead
>> of attempting to preserve the old in some form.
>>
>> Signed-off-by: James Almer <jamrial at gmail.com>
>> ---
>>   libavformat/demux.c | 12 +++++++++++-
>>   1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/libavformat/demux.c b/libavformat/demux.c
>> index abfd5fee7d..39aa1cd4e3 100644
>> --- a/libavformat/demux.c
>> +++ b/libavformat/demux.c
>> @@ -1319,6 +1319,8 @@ fail:
>>       return ret;
>>   }
>>   
>> +static int extract_extradata(FFFormatContext *si, AVStream *st, const AVPacket *pkt);
>> +
>>   static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
>>   {
>>       FFFormatContext *const si = ffformatcontext(s);
>> @@ -1373,6 +1375,11 @@ static int read_frame_internal(AVFormatContext *s, AVPacket *pkt)
>>                   return ret;
>>               }
>>   
>> +            if (!sti->avctx->extradata &&
>> +                (ret = extract_extradata(si, st, pkt)) < 0) {
>> +                av_packet_unref(pkt);
>> +                return ret;
>> +            }
>>               sti->codec_desc = avcodec_descriptor_get(sti->avctx->codec_id);
>>   
>>               sti->need_context_update = 0;
>> @@ -2470,6 +2477,8 @@ static int extract_extradata(FFFormatContext *si, AVStream *st, const AVPacket *
>>       if (ret < 0)
>>           return ret;
>>   
>> +    av_bsf_flush(sti->extract_extradata.bsf);
>> +
>>       ret = av_bsf_send_packet(sti->extract_extradata.bsf, pkt_ref);
>>       if (ret < 0) {
>>           av_packet_unref(pkt_ref);
>> @@ -3090,7 +3099,8 @@ find_stream_info_err:
>>           err = codec_close(sti);
>>           if (err < 0 && ret >= 0)
>>               ret = err;
>> -        av_bsf_free(&sti->extract_extradata.bsf);
>> +        if (sti->extract_extradata.bsf)
>> +            av_bsf_flush(sti->extract_extradata.bsf);
>>       }
>>       if (ic->pb) {
>>           FFIOContext *const ctx = ffiocontext(ic->pb);
> 
> This will keep the BSF around for longer than necessary, although only a
> tiny minority of demuxers ever set need_context_update at all.

It's not exactly a massive context, so i don't think it's a problem. But 
if you prefer, i could add an FF_INFMT_FLAG_ for this.