[FFmpeg-devel] [PATCH] lavc/pngdec: always create a copy for APNG_DISPOSE_OP_BACKGROUND

[Andreas Rheinhardt]

Anton Khirnov:
> Calling av_frame_make_writable() from decoders is tricky, especially
> when frame threading is used. It is much simpler and safer to just make
> a private copy of the frame.
> This is not expected to have a major performance impact, since
> APNG_DISPOSE_OP_BACKGROUND is not used often and
> av_frame_make_writable() would typically make a copy anyway.
> 
> Found-by: James Almer <jamrial at gmail.com>
> ---
>  libavcodec/pngdec.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/libavcodec/pngdec.c b/libavcodec/pngdec.c
> index 562c5ffea4..3e509e8ae0 100644
> --- a/libavcodec/pngdec.c
> +++ b/libavcodec/pngdec.c
> @@ -89,6 +89,8 @@ typedef struct PNGDecContext {
>      int has_trns;
>      uint8_t transparent_color_be[6];
>  
> +    uint8_t *background_buf;
> +    unsigned background_buf_allocated;
>      uint32_t palette[256];
>      uint8_t *crow_buf;
>      uint8_t *last_row;
> @@ -1079,19 +1081,20 @@ static int handle_p_frame_apng(AVCodecContext *avctx, PNGDecContext *s,
>      ff_thread_await_progress(&s->last_picture, INT_MAX, 0);
>  
>      // need to reset a rectangle to background:
> -    // create a new writable copy
>      if (s->last_dispose_op == APNG_DISPOSE_OP_BACKGROUND) {
> -        int ret = av_frame_make_writable(s->last_picture.f);
> -        if (ret < 0)
> -            return ret;
> +        av_fast_malloc(&s->background_buf, &s->background_buf_allocated,
> +                       src_stride * p->height);

No check for whether the frame is already writable?

> +        if (!s->background_buf)
> +            return AVERROR(ENOMEM);
>  
> -        src        = s->last_picture.f->data[0];
> -        src_stride = s->last_picture.f->linesize[0];
> +        memcpy(s->background_buf, src, src_stride * p->height);
>  
>          for (y = s->last_y_offset; y < s->last_y_offset + s->last_h; y++) {
> -            memset(s->last_picture.f->data[0] + src_stride * y +
> +            memset(s->background_buf + src_stride * y +
>                     s->bpp * s->last_x_offset, 0, s->bpp * s->last_w);
>          }
> +
> +        src = s->background_buf;
>      }
>  
>      // copy unchanged rectangles from the last frame
> @@ -1716,6 +1719,7 @@ static av_cold int png_dec_end(AVCodecContext *avctx)
>      s->last_row_size = 0;
>      av_freep(&s->tmp_row);
>      s->tmp_row_size = 0;
> +    av_freep(&s->background_buf);
>  
>      av_freep(&s->iccp_data);
>      av_dict_free(&s->frame_metadata);
>