Re: [gentoo-portage-dev] Dynamic USE dependencies

[Rich Freeman]

On Thu, Apr 2, 2015 at 12:56 PM, Kent Fredric <kentfredric@×××××.com> wrote:
>
> On 3 April 2015 at 05:32, Rich Freeman <rich0@g.o> wrote:
>>
>> Out of curiosity, what is keeping us from having USE flag dependencies
>> handled dynamically, in the same way that package dependencies are?
>> If portage can figure out that I need libxml2 installed even if I
>> don't put it in /var/lib/portage/world, why can't it figure out that I
>> need it built with USE=icu even if I don't put that in
>> /etc/portage/package.use?
>
> I'd say its more a concept issue than an application issue.
>
> USE flags often signify a need for code to be recompiled to grant the
> feature.
>
> How do you disambiguate between USE flags that *do* need a recompile to
> enable their power, and those that *dont* need a recompile to enable their
> power.

Why is this necessary?  If a USE flag changes, just rebuild the application.

>
> Or even clarify to portage that, "The older version of X that didn't have
> IUSE=foo, actually had feature foo, but just didn't have the use flag" vs
> "The older version of X that didn't have IUSE=foo, didnt have feature foo or
> the IUSE".

Do what --newuse does.  If a flag is added/removed, then rebuild the
application.

>
> "Is this new useflag defaulted on because it already existed, or is it
> defaulted on because its a new feature and its awesome"
>
> "Is this new useflag defaulted off because it didnt already exist, or did it
> exist and were disabling the feature because its bad"
>

I'd suggest that the algorithm is:

1.  When installing a new or updated package:
1a. set all the flags in accordance with configuration (explicit
set/unset in profiles, make.conf, package.use)
1b. if these explicitly conflict with package dependencies then fail
with an immediate error (that is, config says USE=foo, and package dep
says atom[-foo] and so on
1c. additionally set flags as needed to satisfy package dependencies
when they don't conflict with explicit configuration

2.  When evaluating the dep tree to see if a package needs to be
rebuilt, just keep the existing USE configuration unless it conflicts
with explicit configuration or a package dependency.

So, the goal would be to avoid rebuilding stuff just because we can.
However, we would rebuild things if a user wants a flag to be set
differently, or if there is now a dependency problem.


-- 
Rich