[FFmpeg-devel] [PATCH 12/18] lavf/dv: make returning the video packet optional

[Anton Khirnov]

Quoting Andreas Rheinhardt (2022-08-24 18:03:05)
> Anton Khirnov:
> > The mov demuxer only returns DV audio, video packets are discarded.
> > 
> > It first reads the data to be parsed into a packet. Then both this
> > packet and the pointer to its data are passed together to
> > avpriv_dv_produce_packet(), which parses the data and partially
> > overwrites the packet. This is confusing and potentially dangerous, so
> > just pass NULL and avoid pointless packet modification.
> > ---
> >  libavformat/dv.c  | 19 +++++++++++--------
> >  libavformat/mov.c |  2 +-
> >  2 files changed, 12 insertions(+), 9 deletions(-)
> > 
> > diff --git a/libavformat/dv.c b/libavformat/dv.c
> > index 303cecf9bb..f88fe62349 100644
> > --- a/libavformat/dv.c
> > +++ b/libavformat/dv.c
> > @@ -430,14 +430,17 @@ int avpriv_dv_produce_packet(DVDemuxContext *c, AVPacket *pkt,
> >          }
> >      }
> >  
> > -    /* Now it's time to return video packet */
> > -    size = dv_extract_video_info(c, buf);
> > -    pkt->data         = buf;
> > -    pkt->pos          = pos;
> > -    pkt->size         = size;
> > -    pkt->flags       |= AV_PKT_FLAG_KEY;
> > -    pkt->stream_index = c->vst->index;
> > -    pkt->pts          = c->frames;
> > +    /* return the video packet, if the caller wants it */
> > +    if (pkt) {
> > +        size = dv_extract_video_info(c, buf);
> > +
> > +        pkt->data         = buf;
> > +        pkt->pos          = pos;
> > +        pkt->size         = size;
> > +        pkt->flags       |= AV_PKT_FLAG_KEY;
> > +        pkt->stream_index = c->vst->index;
> > +        pkt->pts          = c->frames;
> > +    }
> >  
> >      c->frames++;
> >  
> > diff --git a/libavformat/mov.c b/libavformat/mov.c
> > index 1d8c5b2904..a2b429e52f 100644
> > --- a/libavformat/mov.c
> > +++ b/libavformat/mov.c
> > @@ -8768,7 +8768,7 @@ static int mov_read_packet(AVFormatContext *s, AVPacket *pkt)
> >          }
> >  #if CONFIG_DV_DEMUXER
> >          if (mov->dv_demux && sc->dv_audio_container) {
> > -            ret = avpriv_dv_produce_packet(mov->dv_demux, pkt, pkt->data, pkt->size, pkt->pos);
> > +            ret = avpriv_dv_produce_packet(mov->dv_demux, NULL, pkt->data, pkt->size, pkt->pos);
> >              av_packet_unref(pkt);
> >              if (ret < 0)
> >                  return ret;
> 
> 1. LGTM.
> 2. The way mov handles dv audio is very broken:
> a) I don't see anything that guarantees that the
> samplerate of the inner demuxer and the sample rate reported to the user
> (presumably taken from mov structures) coincide.
> b) dv_fctx and dv_demux are part of MOVContext, not MOVStreamContext. If
> there were multiple MOVStreams with dv-audio, the older dv_fctx and
> dv_demux would leak. Do you see anything that guarantees that they will
>  only be at most one MOVStream for dv-audio?

Not that I can see. I'd say the best thing to do here is fail when a
dvaudio stream already exists - trying to support it without having any
valid samples seems pointless. I'll send a patch.

> c) The former would be easily fixable by moving the fields to
> MOVStreamContext. But there is actually another bug: The underlying dv
> stream can contain multiple audio streams in this one MOVStream and only
> the first of these is forwarded.
> https://samples.ffmpeg.org/ffmpeg-bugs/trac/ticket4671/dir1.tar.bz2
> contains a sample where this actually occurs: The DV stream has two
> stereo tracks, only the first of which is forwarded (use -enable_drefs 1
> to be able to play it; the non-forwarded seems to be silent in this
> case). Are these actually independent audios or are these stereo pairs
> part of a single streams with more than two channels?

Cannot answer this. I would ignore this until and unless someone
actually comes complaining about this.

-- 
Anton Khirnov