Re: [gentoo-dev] [RFC] Forced/automatic USE flag constraints (codename: ENFORCED_USE)

[Michał Górny]

On wto, 2017-05-30 at 20:46 +0200, Alexis Ballier wrote:
> On Tue, 30 May 2017 20:11:38 +0200
> Michał Górny <mgorny@g.o> wrote:
> [...]
> > > > Of course, we could just validate all the possible cases via
> > > > repoman, and reject the ebuild if there's at least one conflict
> > > > between them. Not sure how to express that properly in the spec
> > > > though. Not sure how it would work practically.
> > >
> > > Adding a 2^n check to repoman isn't gonna work well.
> > >
> >
> > I'm not saying it's the most optimal algorithm of verifying
> > the correctness of the constraints. It's just the one that's quite
> > obvious -- relatively simple and reliable. If someone can come up with
> > something better that covers at least the most common cases, I'm all
> > for it.
> >
> > That said, this wouldn't be that much of a problem if we can keep the
> > n low. For a start, we can rule out all flags that don't appear
> > in REQUIRED_USE at all. Furthermore, I think we could also ignore
> > the constraints for flags that don't appear there at least 'twice',
> > and so on.
>
> :)
>
> You're applying classical techniques to lower the size of a SAT
> instance so that your exponential algorithm does not explode, but it's
> still hard.
>
> I'm not sure what you want: If you want to detect that there is an
> impossible constraint, well, ebuild writer will notice soon enough when
> testing it. If you want to detect that there is a way to have a
> conflict between useflags, then there will be valid cases where this
> will happen.
>
> That said, assuming we have REQUIRED_USE in CNF form, its negation is
> in DNF form. Solving SAT on DNF formulas is easy (linear I think), and
> this would give you an assignment of useflags triggering an impossible
> constraint. e.g. 'foo? ( bar )' with USE='foo -bar' in make.conf.
> This could be used to trigger a repoman warning but basically every
> single ebuild would trigger those.

Not sure if we understand each other.

I'd like the constraints to be plain straightforward, to the point of
having only one acceptable solution. No special Portage-style algorithms
that attempt to provide a reasonable solution to unreasonable input,
resulting in horrible solutions that need 20 more hacks every few
months.

For example:

  foo? ( bar )

would mean 'if you have USE=foo, then USE=bar is enabled as well'. Not
'find some random solution which satisfies this'. In other words, here
changing USE=foo into USE=-foo is not an acceptable solution.

Now, this also means that every constraint that can't be solved in this
easy fashion is invalid. We want to detect that, and warn the developer.
Some of those could be tricky. Simple example:

  foo? ( baz ) bar? ( !baz )

This one is invalid because USE='foo bar' requires both 'baz' and '!baz'
as a solution. Remember that we don't want to do any changes besides
what's explicitly written there, no guessing. However, the following
should be valid:

  foo? ( baz ) bar? ( !foo !baz )

Because now we clearly indicate that USE=bar disables USE=foo,
and therefore makes the first constraint inapplicable. It clearly
indicates course of action for all combinations:

  foo bar baz -> foo bar baz
   F   F   F  -> [valid]
   F   F   T  -> [valid]
   F   T   F  -> [valid]
   F   T   T  ->  F   T   F
   T   F   F  ->  T   F   T
   T   F   T  -> [valid]
   T   T   F  ->  F   T   F
   T   T   T  ->  F   T   F

-- 
Best regards,
Michał Górny