[FFmpeg-devel] [PATCH 1/2] avcodec, avformat: deprecate anything related to side data merging

wm4 nfxjfg at googlemail.com
Thu Mar 16 22:32:20 EET 2017

On Thu, 16 Mar 2017 21:01:04 +0100
Michael Niedermayer <michael at niedermayer.cc> wrote:

> On Thu, Mar 16, 2017 at 05:20:51AM +0100, wm4 wrote:
> > This patch deprecates anything that has to do with merging/splitting
> > side data. Automatic side data merging (and splitting), as well as all
> > API symbols involved in it, are removed completely.
> > 
> > Two FF_API_ defines are dedicated to deprecating API symbols related to
> > this: FF_API_MERGE_SD_API removes av_packet_split/merge_side_data in
> > libavcodec, and FF_API_LAVF_KEEPSIDE_FLAG deprecates
> > AVFMT_FLAG_KEEP_SIDE_DATA in libavformat.
> > 
> > Since it was claimed that changing the default from merging side data to
> > not doing it is an ABI change, there are two additional FF_API_ defines,
> > which stop using the side data merging/splitting by default (and remove
> > any code in avformat/avcodec doing this): FF_API_MERGE_SD in libavcodec,
> > and FF_API_LAVF_MERGE_SD in libavformat.
> > 
> > It is very much intended that FF_API_MERGE_SD and FF_API_LAVF_MERGE_SD
> > are quickly defined to 0 in the next ABI bump, while the API symbols are
> > retained for a longer time for the sake of compatibility.
> > AVFMT_FLAG_KEEP_SIDE_DATA will (very much intentionally) do nothing for
> > most of the time it will still be defined. Keep in mind that no code
> > exists that actually tries to unset this flag for any reason, nor does
> > such code need to exist. Code setting this flag explicitly will work as
> > before. Thus it's ok for AVFMT_FLAG_KEEP_SIDE_DATA to do nothing once
> > side data merging has been removed from libavformat.
> > 
> > In order to avoid that anyone in the future does this incorrectly, here
> > is a small guide how to update the internal code on bumps:
> > 
> > - next ABI bump (probably soon):
> >   - define FF_API_LAVF_MERGE_SD to 0, and remove all code covered by it
> >   - define FF_API_MERGE_SD to 0, and remove all code covered by it
> > - next API bump (typically two years in the future or so):
> >   - define FF_API_LAVF_MERGE_SD to 0, and remove all code covered by it
> >   - define FF_API_MERGE_SD_API to 0, and remove all code covered by it
> > 
> > This forces anyone who actually wants packet side data to temporarily
> > use deprecated API to get it all. If you ask me, this is batshit fucked
> > up crazy, but it's how we roll. Making AVFMT_FLAG_KEEP_SIDE_DATA to be
> > set by default was rejected as an ABI change, so I'm going all the way
> > to get rid of this once and for all.
> > ---  
> What is the advantage to deprecate the API ?
> Applications which depend on the current default would need the code
> provided by the API when the default changes

The API deprecation is intentionally decoupled from the ABI bump, so
API users which really want to use those functions get enough time to
fix their code (I think the "standard" is 2 years).

I'll push this on Saturday or Monday then, with the changes suggested
by the others.

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