Re: [FFmpeg-devel] [PATCH] avformat/mov: discard data streams with all zero sample_delta

["\"zhilizhao(赵志立)\"" <quinkblack@foxmail.com>]

> On Jul 6, 2022, at 12:09 PM, Gyan Doshi <ffmpeg@gyani.pro> wrote:
> 
> 
> 
> On 2022-07-06 08:23 am, "zhilizhao(赵志立)" wrote:
>> 
>>> On Jul 5, 2022, at 10:33 PM, Gyan Doshi <ffmpeg@gyani.pro> wrote:
>>> 
>>> 
>>> 
>>> On 2022-07-05 07:05 pm, "zhilizhao(赵志立)" wrote:
>>>>> On Jul 5, 2022, at 8:07 PM, Gyan Doshi <ffmpeg@gyani.pro> wrote:
>>>>> 
>>>>> 
>>>>> 
>>>>> On 2022-07-05 01:20 pm, Zhao Zhili wrote:
>>>>>> From: Zhao Zhili <zhilizhao@tencent.com>
>>>>>> 
>>>>>> Streams with all zero sample_delta in 'stts' have all zero dts.
>>>>>> They have higher chance be chose by mov_find_next_sample(), which
>>>>>> leads to seek again and again.
>>>>>> 
>>>>>> For example, GoPro created a 'GoPro SOS' stream:
>>>>>>   Stream #0:4[0x5](eng): Data: none (fdsc / 0x63736466), 13 kb/s (default)
>>>>>>     Metadata:
>>>>>>       creation_time   : 2022-06-21T08:49:19.000000Z
>>>>>>       handler_name    : GoPro SOS
>>>>>> 
>>>>>> With 'ffprobe -show_frames http://example.com/gopro.mp4', ffprobe
>>>>>> blocks until all samples in 'GoPro SOS' stream are consumed first.
>>>>>> 
>>>>>> Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
>>>>>> ---
>>>>>>  libavformat/mov.c | 14 ++++++++++++++
>>>>>>  1 file changed, 14 insertions(+)
>>>>>> 
>>>>>> diff --git a/libavformat/mov.c b/libavformat/mov.c
>>>>>> index 88669faa70..2a4eb79f27 100644
>>>>>> --- a/libavformat/mov.c
>>>>>> +++ b/libavformat/mov.c
>>>>>> @@ -3062,6 +3062,20 @@ static int mov_read_stts(MOVContext *c, AVIOContext *pb, MOVAtom atom)
>>>>>>      st->nb_frames= total_sample_count;
>>>>>>      if (duration)
>>>>>>          st->duration= FFMIN(st->duration, duration);
>>>>>> +
>>>>>> +    // All samples have zero duration. They have higher chance be chose by
>>>>>> +    // mov_find_next_sample, which leads to seek again and again.
>>>>>> +    //
>>>>>> +    // It's AVERROR_INVALIDDATA actually, but such files exist in the wild.
>>>>>> +    // So only mark data stream as discarded for safety.
>>>>>> +    if (!duration && sc->stts_count &&
>>>>>> +            st->codecpar->codec_type == AVMEDIA_TYPE_DATA) {
>>>>>> +        av_log(c->fc, AV_LOG_WARNING,
>>>>>> +               "All samples in data stream index:id [%d:%d] have zero duration, "
>>>>>> +               "discard the stream\n",
>>>>>> +               st->index, st->id);
>>>>>> +        st->discard = AVDISCARD_ALL;
>>>>>> +    }
>>>>>>      sc->track_end = duration;
>>>>>>      return 0;
>>>>>>  }
>>>>> So this will allow audio and video streams to be demuxed, but not data?  That distinction seems arbitrary.
>>>> Disable audio/video streams may create regression. It’s unlikely for random
>>>> and broken data stream.
>>>> 
>>>>> Print a warning and assign a duration to each sample. Either 1 or if not zero/Inf, st->duration/st->nb_frames.
>>>> Set sample_duration to 1 doesn’t work. Dts still far behind other streams.
>>>> 
>>>> Set sample_duration st->duration/st->nb_frames works for me, but I prefer
>>>> current strategy for the following reasons:
>>>> 
>>>> 1. AVDISCARD_ALL is more close to AVERROR_INVALIDDATA by giving up instead
>>>> of trying correction and hope it works, which may not, e.g., st->duration
>>>> is broken, or bad interleave even though we fixed sample_duration.
>>> It's not about hoping that it works.  It's about not preventing the user from acquiring the stream payload.
>>> 
>>> Can you test if setting -discard:d none -i INPUT allows reading the stream with your patch?
>> Yes it does allow reading the stream. ’stts’ box is parsed during
>> avformat_find_stream_info(), AVStream->discard flag can be modified
>> after that. The patch has no effect if user changed AVStream->discard
>> flag.
> 
> What's the duration of the demuxed stream?

The demuxed data track has correct duration since there is a check
```
    if (duration)
        st->duration= FFMIN(st->duration, duration);
```
st->duration comes from ‘mdhd’ and not overwrite in this case.

Every packet has zero as timestamp, as expected:

./ffmpeg -debug_ts  -discard:d none -i ~/tmp/gopro.mp4 -map 0:4 -c copy -copy_unknown -f data /tmp/test

demuxer -> ist_index:4 type:data next_dts:0 next_dts_time:0 next_pts:0 next_pts_time:0 pkt_pts:0 pkt_pts_time:0 pkt_dts:0 pkt_dts_time:0 duration:0 duration_time:0 off:0 off_time:0
demuxer+ffmpeg -> ist_index:4 type:data pkt_pts:0 pkt_pts_time:0 pkt_dts:0 pkt_dts_time:0 duration:0 duration_time:0 off:0 off_time:0
muxer <- type:data pkt_pts:0 pkt_pts_time:0 pkt_dts:0 pkt_dts_time:0 duration:0 duration_time:0 size:16

> 
> Regards,
> Gyan
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