[FFmpeg-devel] [PATCH] avcodec: Require avoptions for the user to set max_pixels.

[Michael Niedermayer]

On Sun, Dec 11, 2016 at 01:54:28PM +0100, wm4 wrote:
> On Sat, 10 Dec 2016 23:01:04 +0100
> Michael Niedermayer <michael at niedermayer.cc> wrote:
> 
> > When we will backport this, it will be inevitably in a different location
> > in AVCodecContext in each release and master. 3.0, 3.1, 3.2 and master
> > use the same soname though and must have a binary compatible interface.
> > It thus can only saftely be accessed through AVOptions
> > 
> > It may be enough to require this only in the releases but that could be
> > rather confusing.
> > 
> > Signed-off-by: Michael Niedermayer <michael at niedermayer.cc>
> > ---
> >  libavcodec/avcodec.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
> > index 02234aee67..8123092ac0 100644
> > --- a/libavcodec/avcodec.h
> > +++ b/libavcodec/avcodec.h
> > @@ -3573,8 +3573,8 @@ typedef struct AVCodecContext {
> >      /**
> >       * The number of pixels per image to maximally accept.
> >       *
> > -     * - decoding: set by user
> > -     * - encoding: set by user
> > +     * - decoding: set by user through AVOptions (NO direct access)
> > +     * - encoding: set by user through AVOptions (NO direct access)
> >       */
> >      int64_t max_pixels;
> >  
> 
> Why? We gave up the Libav ABI compat. abomination.

Its explained in the patch comment above

max_pixels should to be backported as it allows users to control memory
use from large images better, avoid some OOMs and fixes issues which
some people consider security bugs
if its backported it will not be in the same location relative to the
start of AVCodecContext in master, 3.2, 3.1, 3.0
master, 3.2, 3.1, 3.0 all have the same soname
libs using the same soname need to be binary compatible
direct access to one location will not work and thus be binary
incompatible if the field is at a different location

Thus releases cannot use direct access to the max_pixels field.
One could say it differently in that if 3.0.X would access max_pixels
directly at byte offset 123 then an application built against that
and linked to 3.2 will access another field before max_pixels in 3.2

AVOptions solves that corner case of backporting added fields.
This restriction can simply be droped on the next ABI major bump

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Its not that you shouldnt use gotos but rather that you should write
readable code and code with gotos often but not always is less readable
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