[FFmpeg-devel] [PATCH 1/3] avcodec/libwebpenc_animencoder: Don't return pkt without data/side-data

[James Almer]

On 4/10/2021 9:39 PM, Andreas Rheinhardt wrote:
> They are essentially forbidden in our encoding API as they are
> considered empty. So just set the data, but leave the size at zero.

This doesn't seem like a good solution. You're propagating dummy packets 
when the encoder didn't produce any. It's an ugly hack to workaround a 
misbehaving muxer.
As this is a AV_CODEC_CAP_DELAY encoder, you're supposed to set 
got_packet to 0 in this scenario.

The muxer should then not expect this encoder to return more than one 
packet, which is all it ever truly produces, to assume it effectively 
came from it.

> 
> (The old encoding API allowed such packets: It used buffer_pkt_valid
> to record whether the packet is empty or not. This has been changed
> in 827d6fe73d2f5472c1c2128eb14fab6a4db29032 which broke said encoder.
> Said regression has been reported in #9179 which this commit partially
> fixes.)
> 
> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
> ---
>   libavcodec/libwebpenc_animencoder.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/libavcodec/libwebpenc_animencoder.c b/libavcodec/libwebpenc_animencoder.c
> index 7f35a0b939..d871e85d43 100644
> --- a/libavcodec/libwebpenc_animencoder.c
> +++ b/libavcodec/libwebpenc_animencoder.c
> @@ -34,6 +34,7 @@ typedef struct LibWebPAnimContext {
>       WebPAnimEncoder *enc;     // the main AnimEncoder object
>       int64_t prev_frame_pts;   // pts of the previously encoded frame.
>       int done;                 // If true, we have assembled the bitstream already
> +    uint8_t padding_buf[AV_INPUT_BUFFER_PADDING_SIZE];
>   } LibWebPAnimContext;
>   
>   static av_cold int libwebp_anim_encode_init(AVCodecContext *avctx)
> @@ -103,6 +104,8 @@ static int libwebp_anim_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
>           }
>   
>           pkt->pts = pkt->dts = frame->pts;
> +        // Packets without data and side-data are not supported by the API
> +        pkt->data = s->padding_buf;
>           s->prev_frame_pts = frame->pts;  // Save for next frame.
>           ret = 0;
>           *got_packet = 1;
>