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On Thu, 01 Jun 2017 23:31:25 +0200 |
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Michał Górny <mgorny@g.o> wrote: |
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[...] |
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> > There are probably dozens of ways to make that non deterministic. |
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> > Here's one: USE='-*'. Apply '|| ( cli cgi fpm apache2 embed phpdbg |
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> > )' last; this enables 'cli'. Since it's the last one, REQUIRED_USE |
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> > is not satisfied because of 'cli? ( ^^ ( readline libedit ) )'. |
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> > If you process it first, then you enable cli and readline and are |
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> > good. |
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> |
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> You just take a second iteration, and things fall into place. That's |
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> not a problem. |
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> |
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> > > > Also, what happens if we applied all the constraints and |
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> > > > obtained some useflags setting that still fails REQUIRED_USE |
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> > > > check ? |
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> > > |
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> > > It can't happen. If you can apply all the constraints, then |
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> > > implicitly REQUIRED_USE is satisfied. If you can't apply all the |
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> > > constraints, then it just fails. Of course, we want to ultimately |
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> > > avoid that case. |
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> > |
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> > See the php example above. How do you ensure this does not happen? |
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> > |
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> > Note that your assertion 'If you can apply all the constraints, then |
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> > implicitly REQUIRED_USE is satisfied.' is false: You're changing the |
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> > values of useflags, thus might violate some previously checked |
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> > constraint. |
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> |
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> So you reiterate. Either you reach a solution (preferably there's only |
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> a single valid solution you can reach) or you hit a previous state |
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> which indicates a loop and you fail. |
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So, PM, for every ebuild, will need to store the (at most) 2^n states it |
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has already seen while trying to solve REQUIRED_USE and iterate until |
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it cannot proceed anymore or falls into a previous state (so that's 2^n |
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time too). That way we go from a linear time & linear space algorithm |
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to one in exponential time & space. That's probably not a good idea. |
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I think it's better to limit the number of iterations to some constant. |
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I'd say 1, then fail if REQUIRED_USE is still not satisfied. Maybe 2 or |
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3 can be useful but I think it'd be much harder to provide automated |
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checks for that and much harder for ebuild writers to understand what |
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will happen with the REQUIRED_USE they're about to write. |
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[...] |
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|
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Alexis. |
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