On Thu, 1 Apr 2010 00:31:09 -0700
Brian Harring <ferringb@gmail.com> wrote:
> > Cycle breaking requires explicit instructions from the ebuilds in
> > question (many of which are system things, which further
> > complicates it) along with support from Portage, so it's a distant
> > future, lot of work thing.
> 
> Nonsense.  Note I said 'use cycle', not the generic 'cycle
> breaking'. USE induced cycles don't require explicit instructions
> from the ebuild at all- the PM itself can search the solution space
> (toggling flags as needed) to search out a way around the cycle.
> 
> Consider user configuration w/ USE=X, pkg_a w/ DEPEND "X? ( pkg_b )", 
> pkg_b w/ DEPEND "pkg_a".  To be clear, you're claiming that the 
> ebuild itself (and only the ebuild) is the the one able to state-
> 
> emerge pkg_a[-X]
> emerge pkg_a[X]
> 
> As demonstrated, that cycle is easily broken.  A lot of the cycles 
> users run into originate that way also.

Congratulations. You just turned on 'build' and 'bootstrap', and turned
off 'acl'.

> And as I've already laid out in the bug, pkg_pretend has it's own set 
> of issues when compared to pkg_setup due to it being non temporal, 
> thus having high false positive potentials.

These are exactly the same issues that pkg_setup has. You can't block a
useful feature simply because developers could theoretically screw
things up by using it.

> The main council push for pkg_pretend was to move use constraint 
> checking to pre build.  VALID_USE does that cleaner and enabling use 
> cycle breaking to be built; as such I'm pushing it up to them unless 
> someone can find significant *real* flaws.

No, VALID_USE addresses a *subset* of the issues. It's not a
replacement for pkg_pretend.

> > When in the distant future Portage
> > becomes able to deal with cycle breaking, ebuilds can be converted
> > to use something like VALID_USE when they're also updated to export
> > information on which of their flags can safely be toggled.
> 
> You're being short sighted.  VALID_USE is useful now for representing 
> use states that are allowed; that data itself is useful for use cycle 
> breaking.  Added bonus of enabling better functionality via a
> superior solutions, basically.

Simply adding VALID_USE won't let you do cycle breaking. You also need
extensive lists of which flags for which packages can safely be toggled
and when without breaking the system, and the only way you'll get those
lists is if developers care enough to update their ebuilds to provide
them.

-- 
Ciaran McCreesh