[gentoo-dev] use.mask and no* flags

[Ciaran McCreesh]

Looks like some people aren't sure about how use.mask works, or why arch
developers tend to hate noblah USE flags... Hopefully this'll clear
things up.

Consider a hypothetical package named 'vplayer' which plays videos [1].
This package has optional support, via USE flags, for various sound and
video output modes, video codecs and so on.

One of vplayer's optional features is support for the 'fakemedia' codec,
which is unfortunately only available as a dodgy x86 binary. So how do
we handle this? We *could* do something like the following: >

    RDEPEND="x86? ( fakemedia? ( >=media-libs/fakemedia-1.1 ) )"

< However, that's not really very nice -- what happens when an amd64
binary is made as well? Also, users on other archs will see the
fakemedia flag listed in emerge -pv output, even though it's not
actually available.

Or, from the other side... vplayer supports output via ALSA as one
option. Unfortunately, ALSA isn't available on sparc [2] or alpha. So,
you could do: >

    DEPEND="!sparc? ( !alpha? ( media-libs/alsa-lib ) )"

< Again, this is messy, and alsa still shows up as an option in emerge
-pv.  Also, when we get ALSA working on sparc, we'd have to go through
every single ebuild that does this and remove the !sparc? stuff.

The solution is use.mask. Each profile can have a use.mask file which
can be used to force certain USE flags to be disabled on a given arch
(or, for that matter, on, say, selinux systems or a particular subarch).
Say the'fakemedia' USE flag was masked on non-x86, the following would
be totally legal and wouldn't break anything: >

    RDEPEND="fakemedia? ( >=media-libs/fakemedia-1-1 )"

< Anyone on a non-x86 box would see the following when doing emerge -pv
vplayer: >

    [ebuild   R   ] media-video/vplayer-1.2 alsa -arts (-fakemedia)

< So, if you're thinking about doing an arch? DEPEND, please think
again. It's easy to get things added to use.mask, just file a bug or ask
the arch people on IRC. We'd much rather add a line into a couple of
textfiles than have to screw around with arch? flags. Really, the only
time you should be using arch? flags for *DEPEND [3] is when a
particular feature usually works on a given arch but doesn't for one
particular package (for example, maybe vplayer's opengl support is
broken on 64bit systems).

Now we get onto those pesky no* flags. Currently there is no way for us
to force a USE flag on on a given profile [4]. So, if you do something
like this: >

    RDEPEND="!noalsa? ( media-libs/alsa )"

< you're screwing over those archs that don't have alsa support, and
there's nothing we can do about it short of those hideous arch? flags.

[1]: This isn't a dig at the mplayer guys, just that sound/video apps
are an easy example here :)

[2]: Well... Not entirely true... ALSA itself will mostly work on some
sparc boxes with a 2.6.x kernel, but the mixer hates our userland.

[3]: In other places it does of course have perfectly valid uses.

[4]: This may or may not be a portage deficiency. Point is, it's an
issue, and whilst complaining that portage needs to be fixed is a viable
long term solution, right now it's no use. Although if someone files a
bug requesting a use.force, please Cc: me.

-- 
Ciaran McCreesh : Gentoo Developer (Sparc, MIPS, Vim, Fluxbox)
Mail            : ciaranm at gentoo.org
Web             : http://dev.gentoo.org/~ciaranm