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From: Martin Vaeth <martin@mvath.de>
Subject: [gentoo-portage-dev] Re: Dynamic USE dependencies
Date: Wed, 8 Apr 2015 07:45:41 +0000 (UTC)
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Rich Freeman <rich0@gentoo.org> wrote:
>
> Keep in mind that keeping track of past decisions made by portage does
> not require user-editable config files in /etc.

Yes, but you might not always agree with portage's decisions,
and the resolution might be non-unique.
Although the user might always influence it with /etc/portage/package.use
it might be better to keep this transparent to the user - therefore,
I suggested a directory in /etc and not in /var
(Moreover, a cache would not make sense, since the
previous decision could always be trivially calculated from /var/db/pkg,
and the user-selected state of the USE-flag for the corresponding
package. You could not check the validity of such a cache, since
you cannot check whether USE or /etc/portage/package.use has changed)

> That said, portage still has to spend time basically re-validating the
> consistency of the entire system

*Validating* consistency is not time-consuming. Looking for possibilities
to unset use-flags is a much more time-consuming story.
If everything is consistent or can easily be resolved, you hardly need
any backtracking...