[FFmpeg-devel] [PATCH] avcodec/avcodec: don't free AVOption settable fields in avcodec_close()

[James Almer]

On 3/20/2022 8:34 PM, Andreas Rheinhardt wrote:
> James Almer:
>>
>>
>> On 3/20/2022 8:26 PM, Andreas Rheinhardt wrote:
>>> James Almer:
>>>> It can uninitialize fields that may still be used after the context
>>>> was closed,
>>>> so do it instead in avcodec_free_context().
>>>>
>>>> Signed-off-by: James Almer <jamrial at gmail.com>
>>>> ---
>>>>    libavcodec/avcodec.c | 1 -
>>>>    libavcodec/options.c | 2 +-
>>>>    2 files changed, 1 insertion(+), 2 deletions(-)
>>>>
>>>> diff --git a/libavcodec/avcodec.c b/libavcodec/avcodec.c
>>>> index 38bdaad4fa..122d09b63a 100644
>>>> --- a/libavcodec/avcodec.c
>>>> +++ b/libavcodec/avcodec.c
>>>> @@ -524,7 +524,6 @@ av_cold int avcodec_close(AVCodecContext *avctx)
>>>>          if (avctx->priv_data && avctx->codec &&
>>>> avctx->codec->priv_class)
>>>>            av_opt_free(avctx->priv_data);
>>>> -    av_opt_free(avctx);
>>>>        av_freep(&avctx->priv_data);
>>>>        if (av_codec_is_encoder(avctx->codec)) {
>>>>            av_freep(&avctx->extradata);
>>>> diff --git a/libavcodec/options.c b/libavcodec/options.c
>>>> index 33f11480a7..91335415c1 100644
>>>> --- a/libavcodec/options.c
>>>> +++ b/libavcodec/options.c
>>>> @@ -172,7 +172,7 @@ void avcodec_free_context(AVCodecContext **pavctx)
>>>>        av_freep(&avctx->intra_matrix);
>>>>        av_freep(&avctx->inter_matrix);
>>>>        av_freep(&avctx->rc_override);
>>>> -    av_channel_layout_uninit(&avctx->ch_layout);
>>>> +    av_opt_free(avctx);
>>>>          av_freep(pavctx);
>>>>    }
>>>
>>> This will lead to memleaks for users that use avcodec_close(avctx) +
>>> av_free(avctx) to free an AVCodecContext (e.g. our frame-threaded
>>> encoders do this). Notice that avcodec_free_context() violates the
>>> documentation of AVCodecContext.extradata (documented to not be freed
>>> for decoders) and AVCodecContext.subtitle_header and
>>> AVCodecContext.rc_override (documented to not be freed by lavc for
>>> encoders), so there is a reason for using it instead of
>>> avcodec_free_context() (even when not reusing the context).
>>
>> That's an absolute mess of a situation. av_free(avctx) should not be an
>> allowed or supported scenario when avcodec_free_context() exists. And
>> why is the latter violating its own documentation?
>>
> 
> It is not violating its own documentation, but the documentation of the
> relevant AVCodecContext fields. IIRC Anton wanted a function that just
> frees the whole context, even if this meant that fields which are
> documented as being owned by the user are freed. Even documenting the
> current state of affairs in avcodec.h doesn't change the fact that there
> is a valid reason to use avcodec_close()+av_free(), so we can't pretend
> it doesn't happen.
> 
> - Andreas

Ok, do i add a codecpar copy like i suggested in 
http://ffmpeg.org/pipermail/ffmpeg-devel/2022-March/294312.html, then? 
It works, but it feels really weird doing that in what's the cleanup 
portion of the function.
Alternatively, add the dance from 
https://patchwork.ffmpeg.org/project/ffmpeg/patch/20220319030407.45503-1-jamrial@gmail.com/ 
which should have the same effect and never fail, unlike param copy.