[FFmpeg-devel] [PATCH] avcodec/snow: ensure current_picture is writable before modifying its data

[James Almer]

On 5/29/2020 5:37 PM, Michael Niedermayer wrote:
> On Fri, May 29, 2020 at 02:20:37PM -0300, James Almer wrote:
>> current_picture was not writable here because a reference existed in
>> at least avctx->coded_frame, and potentially elsewhere if the caller
>> created new ones from it.
> 
> Please elaborate when and how the encoder internal buffer becomes
> read only
> i simply took your code and replaced
> 
> -        ret = av_frame_make_writable(s->current_picture);
> -        if (ret < 0)
> -            return ret;
> +        ret = av_frame_is_writable(s->current_picture);
> +        if (ret <= 0)
> +            return -1;
> 
> and fate is fully happy which tests both the encoder and the filters
> using it
> also if this is for coded_frame then shouldnt it be under FF_API_CODED_FRAME?
> iam clearly missing something here

You need to also move the av_frame_unref(avctx->coded_frame) line back 
to where it was before my patch. I moved it before this check to ensure 
at least the reference stored there is freed before current_picture is 
written to, but there of course could still exist other references 
created by the user, and that's what this make_writable() call is for.

And since this specific chunk is not strictly coded_frame related, it 
doesn't need to be under that guard.

> 
> thx
> 
> [...]
> 
> 
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