Re: [FFmpeg-devel] [PATCH 3/3] lavu/hwcontext: clarify behavior on av_hwframe_map() failure

[James Almer <jamrial@gmail.com>]

On 1/19/2022 10:40 AM, Anton Khirnov wrote:
> Clear anything that av_hwframe_map() might have done to the destination
> frame, but leave caller-provided fields unchanged.
> ---
>   libavutil/hwcontext.c | 23 +++++++++++++++++++++--
>   libavutil/hwcontext.h |  4 ++++
>   2 files changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/libavutil/hwcontext.c b/libavutil/hwcontext.c
> index 31c7840dba..ae33da1262 100644
> --- a/libavutil/hwcontext.c
> +++ b/libavutil/hwcontext.c
> @@ -18,6 +18,7 @@
>   
>   #include "config.h"
>   
> +#include "avassert.h"
>   #include "buffer.h"
>   #include "common.h"
>   #include "hwcontext.h"
> @@ -788,6 +789,8 @@ fail:
>   
>   int av_hwframe_map(AVFrame *dst, const AVFrame *src, int flags)
>   {
> +    AVBufferRef    *orig_dst_frames = dst->hw_frames_ctx;
> +    enum AVPixelFormat orig_dst_fmt = dst->format;
>       AVHWFramesContext *src_frames, *dst_frames;
>       HWMapDescriptor *hwmap;
>       int ret;
> @@ -825,7 +828,7 @@ int av_hwframe_map(AVFrame *dst, const AVFrame *src, int flags)
>               ret = src_frames->internal->hw_type->map_from(src_frames,
>                                                             dst, src, flags);
>               if (ret != AVERROR(ENOSYS))
> -                return ret;
> +                goto fail;

What if ret is 0? Is the stuff in the fail label meant to run on success?

>           }
>       }
>   
> @@ -837,11 +840,27 @@ int av_hwframe_map(AVFrame *dst, const AVFrame *src, int flags)
>               ret = dst_frames->internal->hw_type->map_to(dst_frames,
>                                                           dst, src, flags);
>               if (ret != AVERROR(ENOSYS))
> -                return ret;
> +                goto fail;
>           }
>       }
>   
>       return AVERROR(ENOSYS);
> +
> +fail:
> +    // if the caller provided dst frames context, it should be preserved
> +    // by this function
> +    av_assert0(orig_dst_frames == NULL ||
> +               orig_dst_frames == dst->hw_frames_ctx);
> +
> +    // preserve user-provided dst frame fields, but clean
> +    // anything we might have set
> +    dst->hw_frames_ctx = NULL;
> +    av_frame_unref(dst);
> +
> +    dst->hw_frames_ctx = orig_dst_frames;
> +    dst->format        = orig_dst_fmt;
> +
> +    return ret;
>   }
>   
>   int av_hwframe_ctx_create_derived(AVBufferRef **derived_frame_ctx,
> diff --git a/libavutil/hwcontext.h b/libavutil/hwcontext.h
> index 04d19d89c2..c18b7e1e8b 100644
> --- a/libavutil/hwcontext.h
> +++ b/libavutil/hwcontext.h
> @@ -571,6 +571,10 @@ enum {
>    * possible with the given arguments and hwframe setup, while other return
>    * values indicate that it failed somehow.
>    *
> + * On failure, the destination frame will be left blank, except for the
> + * hw_frames_ctx/format fields thay may have been set by the caller - those will
> + * be preserved as they were.
> + *
>    * @param dst Destination frame, to contain the mapping.
>    * @param src Source frame, to be mapped.
>    * @param flags Some combination of AV_HWFRAME_MAP_* flags.
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