Re: [gentoo-dev] RFC: USE=libav as replacement for broken || ( libav:= ffmpeg:= )

[Alexis Ballier]

On Mon, 19 Jan 2015 20:31:45 +0100
Michał Górny <mgorny@g.o> wrote:

> Hello,
>
> As we've discussed multiple times, the following kind of dependencies
> is completely broken and can't work:
>
>   || ( media-libs/libav:= media-libs/ffmpeg:= )

See end of the email.

> For this reason, I would like to employ the solution used by Exherbo.
> More specifically, use:
>
>   libav? ( media-libs/libav:= )
>   !libav? ( media-libs/ffmpeg:= )
>
> This has two advantages:
>
> 1. gives users more explicit control over whether they want to use
> libav or ffmpeg. Since the two have mutual blockers, right now random
> packages could have tried to force you to switch from one to the
> other. However, most often Portage would just give you terribly
> unreadable blockers.


This sounds good to me; there are a few things to consider though:

- Whatever default useflag is enabled, people will have "terribly
  unreadable blockers" if they use the other implementation. It is even
  worse if you want to follow upstreams in their preferred
  implementation: some want libav, others ffmpeg...
- This is hidden by the proposal but it enforces that a package has
  equal visibility of both implementations (while the || requires only
  one); libav tends to have a faster code cleaning rate and thus a
  slower unmasking rate in gentoo. I agree with that requirement but
  this might end-up being unpractical since this means packages
  keywording and stabilization will have the slowest pace: mpv has been
  masked for 4 months and counting for this reason.
- What to do with packages like mplayer that haven't been building
  against libav for a year or so ?
- ffmpeg dep is media-video/ffmpeg:0= (not media-libs but more
  importantly the :0=)
- It is a good opportunity to kill virtual/ffmpeg and at the same time
  workaround a subslot and := deps limitation with virtuals.


> 2. Subslots work correctly. Rebuilds are forced when the chosen
> library is upgraded. Moreover, USE flag change causes a rebuild when
> user decides to change the ffmpeg provider.


No offense, but this argument is complete crap. You should rather fix
portage bugs than propose to introduce tree-wide changes to hide them...
More precisely: || ( a:= b c:= d ) is perfectly defined (in the "what
it means" sense, not in PMS sense). When the package is built, if 'a' is
satisfied then a (and its subslot) is added to the subslot list of the
package; ditto for c. You end up with a list of subslot deps, that you
can store in vdb or whatever, and use that to decide when to rebuild
the package.


Alexis.