Re: [gentoo-portage-dev] [RFC] Depending on "active" version

[Alec Warner]

>
>
> Hmm; one could get the same benefit by introducing a new interface
> (e.g. pkg_env_check()) which is defined to return true if the
> environment is ok, false otherwise (with some text to stdout, perhaps).
> The package manager would then run this function, after building the
> depgraph and finding the candidate packages to merge, for each
> candidate package - if any package fails is env_check, none of the
> packages get emerged.  Note this is then completely independent of
> depgraph creation.
>
> In the 'tr1' example, I'd imagine something like this:
>
> use.local.desc: boost: Use boost library for tr1 rather than gcc's.
>
> ebuild:
>
> ...
> inherit ... toolchain-funcs versionator ...
> ...
> DEPEND=... boost? ( dev-libs/boost ) ...
> ...
> pkg_env_check() {
> 	use boost && return 0
> 	version_is_at_least "4.1" $(gcc-version) && return 0
> 	echo "Either USE boost, or switch to gcc later than 4.1"
> }
>
>
> (with a default definition, "pkg_env_check() { return 0; }" )

In an ideal system you'd want this stuff in the metadata cache so that
the resolver can deal with it up front.  All resolution is a brute force
metadata search; and assuming we had all the necessary data up front, we
can optimize the search there (see pkgcore and restriction subsystem)
versus IMHO doing a 'dumb' search and then running through a list of
criteria for inclusion.

This is the same reason why built_with_use in pkg_setup is really just
use_deps; these metadata should be included during resolution, not after.

With that said, I'm not sure how easy it would be to rewrite that code;
and this is simpler in that it's just a few bash functions as opposed to
 more resolver foo.
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