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 <jamr...@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);
>
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