[FFmpeg-devel] [PATCH v2] hevcdec: skip slices with missing PPS instead of skipping the entire packet

[Hendrik Leppkes]

On Thu, Dec 30, 2021 at 4:13 PM James Almer <jamrial at gmail.com> wrote:
>
>
>
> On 12/30/2021 12:08 PM, Hendrik Leppkes wrote:
> > Aborting decoding of the entire packet on a missing PPS can result in
> > missing the actual PPS on streams with badly ordered NALs, where the
> > SPS/PPS/VPS are stitched to the back of the previous frame, instead of
> > the beginning of the next frame.
> >
> > Instead, skip the undecodable slice, and let the decoder process further
> > NALs in the same packet.
> >
> > If this happens on the first slice, the entire frame will be discarded
> > later, otherwise on other slices the decode error flag will be set to
> > indicate a missing/corrupt slice.
> > ---
> >   libavcodec/hevcdec.c | 4 +++-
> >   1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/libavcodec/hevcdec.c b/libavcodec/hevcdec.c
> > index 3aa70e2245..89381db240 100644
> > --- a/libavcodec/hevcdec.c
> > +++ b/libavcodec/hevcdec.c
> > @@ -575,7 +575,9 @@ static int hls_slice_header(HEVCContext *s)
> >       sh->pps_id = get_ue_golomb_long(gb);
> >       if (sh->pps_id >= HEVC_MAX_PPS_COUNT || !s->ps.pps_list[sh->pps_id]) {
> >           av_log(s->avctx, AV_LOG_ERROR, "PPS id out of range: %d\n", sh->pps_id);
> > -        return AVERROR_INVALIDDATA;
> > +        if (s->ref)
> > +            s->ref->frame->decode_error_flags |= FF_DECODE_ERROR_DECODE_SLICES;
>
> Did you check frame threading decoding with tsan after this? Touching
> the output frame at this point could result in thread sync races.
>
> Maybe it's better to implement signaling slice decoding errors in a
> separate patch that properly stores such events internally in the
> per-thread context, then sets the flag in the output frame before
> returning, like it's done in h264.
> Like i said it's not a blocker for this fix, so it can be done later.
>

As far as I can tell, there is no threading issues here. There is no
slice-threading here (only through wave-front parallel processing,
which comes later), and no other thread would ever read or write the
error field. The HEVCContext "s" is per-thread, and s->ref is accessed
quite freely by the surrounding code.
But I can take it out if there is any remaining concern. In reality,
its quite likely that the entire frame is undecodable in this scenario
anyway, and the change is only to possibly recover decoding of future
frames - not partially decode this one, although that might be a bonus
in some rare cases.

- Hendrik