[FFmpeg-devel] [PATCH] decode.c: Handle 0-size packets in compat_decode

[wm4]

On Mon,  3 Jul 2017 20:57:21 +0200
Reimar Döffinger <Reimar.Doeffinger at gmx.de> wrote:

> The old API did that just fine, and if we provide
> a compatibility layer it should at least be compatible.
> For the test-case (feeding AVParser output directly to
> decoder, failing to discard 0-size packets) just discarding
> 0-size packets at the start works, but not sure if in some
> cases we might want to pass them on to e.g. allow retrieving
> additional pending frames.
> ---
>  libavcodec/decode.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/libavcodec/decode.c b/libavcodec/decode.c
> index 052f93d82f..c63090f137 100644
> --- a/libavcodec/decode.c
> +++ b/libavcodec/decode.c
> @@ -842,6 +842,7 @@ static int compat_decode(AVCodecContext *avctx, AVFrame *frame,
>      avci->compat_decode = 1;
>  
>      if (avci->compat_decode_partial_size > 0 &&
> +        pkt->size &&
>          avci->compat_decode_partial_size != pkt->size) {
>          av_log(avctx, AV_LOG_ERROR,
>                 "Got unexpected packet size after a partial decode\n");
> @@ -902,7 +903,10 @@ finish:
>              ret = FFMIN(avci->compat_decode_consumed, pkt->size);
>      }
>      avci->compat_decode_consumed = 0;
> -    avci->compat_decode_partial_size = (ret >= 0) ? pkt->size - ret : 0;
> +    // for compatibility with old API behaviour handle
> +    // 0-size specially
> +    if (pkt->size)
> +        avci->compat_decode_partial_size = (ret >= 0) ? pkt->size - ret : 0;
>  
>      return ret;
>  }

I thought it was decided that 0-sized packets are always disallowed for
encoding? Also, 0-sized packets are drain packets even with the old
API, and thus could never properly work. You just relied in implicit
restart behavior (i.e. leaving the drained state by feeding a new
non-0-sized packet), which also didn't work with all decoders.