[FFmpeg-devel] [PATCH] avformat/mux: fix overflow in case one of dts in not set

[Andreas Rheinhardt]

Michael Niedermayer:
> On Thu, Sep 30, 2021 at 12:36:17PM +0200, Paul B Mahol wrote:
>> Signed-off-by: Paul B Mahol <onemda at gmail.com>
>> ---
>>  libavformat/mux.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/libavformat/mux.c b/libavformat/mux.c
>> index 2053a5636e..583328b123 100644
>> --- a/libavformat/mux.c
>> +++ b/libavformat/mux.c
>> @@ -949,7 +949,7 @@ int ff_interleave_packet_per_dts(AVFormatContext *s, AVPacket *out,
>>              last_dts = av_rescale_q(last->pkt.dts,
>>                                      st->time_base,
>>                                      AV_TIME_BASE_Q);
>> -            delta_dts = FFMAX(delta_dts, last_dts - top_dts);
>> +            delta_dts = FFMAX(delta_dts, (top_dts == AV_NOPTS_VALUE || last_dts == AV_NOPTS_VALUE) ? 0 : last_dts - top_dts);
>>          }
> 
> In which case are packets without DTS interleaved by DTS ?
> 

E.g. in case of attached pictures. Other possible cases are e.g. the
null muxer or more generally muxers with AVFMT_NOTIMESTAMPS.
An alternative solution for this that I have been thinking about for a
while but which I have not implemented yet goes as follows:
1. Use a dedicated interleave function for the null muxer that does not
interleave at all, but just returns the packets as they are (it doesn't
matter for the null muxer).
2. For attached pictures (not timed thumbnails) the concept of
timestamps does not really make sense; ergo don't interleave them, but
let the generic muxing code put them into AVStream->attached_pic or
AVStream->attachment (to be added as a replacement for attached_pic)
before the interleavement functions get called. Also check generically
that there is only one packet for such a stream and remove the checks in
e.g. flacenc and several other muxers for this purpose. These muxers
allow only a single non-attached-pic stream, so there is no problem with
interleavement at all.
(Attached pics are currently somewhat broken: Such streams are counted
among the interleaved streams, so that after the attached pics have been
received and consumed the ordinary stream is only written if there are
so many packets that the max_interleave_delta criterion kicks in,
leading to completely unnecessary delay.)

- Andreas