[FFmpeg-devel] [PATCH 2/3] flvdec: Mark the demuxer as allowing discontinuous timestamps

[Derek Buitenhuis]

On 03/01/2019 01:33, Michael Niedermayer wrote:
> The file looks like 2 files concatenated, even with FLV main header between 
> them. 

Yes, they all seem to be. I was under the impression FLV didn't have a header to
check against like this. Seems I was wrong.

> the patch below should make this work with sequential demuxing assuming the
> 2 files dont change streams somehow with the normal demuxer.
> 
> not sure this is the best way to do it ...

[...]

> also trying a random other flv related tool, FLVTool2 1.0.6 does not seem to
> demux the 2nd file of the 2 correctly. Which makes me suspect more that the
> file is invalid. Not that this would fundamentally change anything

Some do, some don't. Classic multimedia.

I would also be OK with a patch that simply stops demuxing like many programs
seem to do, instead of outputting the packets. Up to you which you prefer; I have
no strong opinion between the two.

> diff --git a/libavformat/flvdec.c b/libavformat/flvdec.c
> index 4b9f46902b..c9423da31c 100644
> --- a/libavformat/flvdec.c
> +++ b/libavformat/flvdec.c

[...]

>  static int probe(AVProbeData *p, int live)
> @@ -917,6 +919,17 @@ static int resync(AVFormatContext *s)
>          flv->resync_buffer[j ] =
>          flv->resync_buffer[j1] = avio_r8(s->pb);
>  
> +        if (i >= 8) {
> +            uint8_t *d = flv->resync_buffer + j1 - 8;
> +            if (d[0] == 'F' &&
> +                d[1] == 'L' &&
> +                d[2] == 'V' &&
> +                d[3] < 5 && d[5] == 0) {
> +                av_log(s, AV_LOG_WARNING, "Concatenated FLV detected, might fail to demux, decode and seek %Ld\n", flv->last_ts);
> +                flv->time_offset = flv->last_ts + 1;
> +            }
> +        }

How does this affect seeking? That is, is it safe? If it beaks seeking,
it may be better to not output the concatenated packets at all.

Cheers,
- Derek