[FFmpeg-devel] [PATCH v9 2/6] avcodec/webp: separate VP8 decoding

[Andreas Rheinhardt]

Thilo Borgmann via ffmpeg-devel:
> Am 28.01.24 um 11:29 schrieb Anton Khirnov:
>> Quoting Thilo Borgmann via ffmpeg-devel (2024-01-25 16:39:19)
>>> Am 25.01.24 um 11:04 schrieb Anton Khirnov:
>>>> Quoting Thilo Borgmann via ffmpeg-devel (2023-12-31 13:30:14)
>>>>> ---
>>>>>    libavcodec/webp.c | 50
>>>>> +++++++++++++++++++++++++++++++++++++++++------
>>>>>    1 file changed, 44 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/libavcodec/webp.c b/libavcodec/webp.c
>>>>> index 4fd107aa0c..58a20b73da 100644
>>>>> --- a/libavcodec/webp.c
>>>>> +++ b/libavcodec/webp.c
>>>>> @@ -194,6 +194,7 @@ typedef struct WebPContext {
>>>>>        AVFrame *alpha_frame;               /* AVFrame for alpha
>>>>> data decompressed from VP8L */
>>>>>        AVPacket *pkt;                      /* AVPacket to be passed
>>>>> to the underlying VP8 decoder */
>>>>>        AVCodecContext *avctx;              /* parent AVCodecContext */
>>>>> +    AVCodecContext *avctx_vp8;          /* wrapper context for VP8
>>>>> decoder */
>>>>
>>>> Nested codec contexts are in general highly undesirable and should be
>>>> avoided whenever possible.
>>>
>>> AFAICT we do it that way in the other codecs as well (cri, ftr, imm5,
>>> tdsc, tiff). So what do you suggest to do to avoid having it nested?
>>
>> Integrating the two decoders directly, as is done now.
>>
>> With nesting it is very tricky to handle all the corner cases properly,
>> especially passing through all the options to the innner decoder, like
>> direct rendering, other user callbacks, etc. It should only be done as a
>> last resort and there should be a strong argument to do it this way.
> 
> IIUC that was what the patch still did some some versions ago.
> It brought us the data races in animated files, decoupling the decoder
> solving the issue.
> 

If one keeps the codecs integrated, then one needs at the very least
change the check for whether to call ff_thread_finish_setup() as I did:
https://ffmpeg.org/pipermail/ffmpeg-devel/2024-January/320490.html
This will not be enough: E.g. changing the dimensions in VP8 code and
then reverting that change in WebP (as has been done in the earlier
version of your patch which made me propose that these decoders should
be separated) will have to be avoided.

- Andreas