[FFmpeg-devel] [RFC PATCH] avformat/utils: always seek back after avformat_find_stream_info()

[Hendrik Leppkes]

On Wed, Apr 10, 2019 at 9:55 AM Aman Gupta <ffmpeg at tmm1.net> wrote:
>
> On Tue, Apr 9, 2019 at 9:49 PM Hendrik Leppkes <h.leppkes at gmail.com> wrote:
>
> > On Wed, Apr 10, 2019 at 2:21 AM Aman Gupta <ffmpeg at tmm1.net> wrote:
> > >
> > > From: Aman Gupta <aman at tmm1.net>
> > >
> > > Previously, the initial seek position was recorded into
> > > old_offset at the beginning of avformat_find_stream_info(),
> > > and passed into estimate_timings(). In the case of mpegts
> > > with a known filesize, it was further passed into
> > > estimate_timings_from_pts() which called avio_seek(SEEK_SET)
> > > after doing its timing related seeks. (Interestingly, this
> > > seeked all the way back to the initial position before
> > > the probe, rather than only back before the timing
> > > related seeks to the end of the file).
> > >
> > > With this commit, we pull the avio_seek() out of the mpegts
> > > specific code-path and unconditionally seek all streams back
> > > to their original position after probing is complete.
> > >
> > > This effectively prevents data that is consumed during
> > > avformat_find_stream_info() from being discarded, so packets
> > > contained at the beginning of a file are still passed back
> > > to the user for playback.
> > >
> >
> > I don't think I ever had a case where data was apparently lost from
> > the beginning of the stream. Can you give examples of what this fixes?
>
>
> Perhaps I'm missing something.
>
> If I see the debug log message "After avformat_find_stream_info() pos:
> 1000000 bytes  seeks:0", are the packets from the first megabyte of the
> file still returned from API? Maybe they are being cached somewhere and I
> did not notice.
>

As far as I know, the packets read during stream info probing are
being cached in a buffer and returned to the caller on av_read_frame.

- Hendrik