[Rpm-ecosystem] Rich deps syntax finalization

[James Antill]

On Wed, 2015-08-26 at 10:29 +0200, Florian Festi wrote:
> On 08/25/2015 06:12 PM, Pavel Odvody wrote:
> > On Tue, 2015-08-25 at 18:01 +0200, Florian Festi wrote:
> >> Well, as Requires are (logically) all connected with an AND this is
> >> equivalent to:
> >>
> >> Requires: PkgA
> >> Requires: (PkgB IF NOT PkgC)
> >>
> >> And as IF is equivalent to OR NOT this is equivalent to
> >>
> >> Requires: PkgA
> >> Requires: (PkgB OR PkgC)
> >>
> > This is  a great remark - Arguments are short-circuit evaluated.
> 
> Short-circuit evaluation is a concept that has no meaning here.
[...]
> Short-circuit evaluation means some sub expressions are not looked at at
> all. But this is only relevant if evaluating some expressions can have
> side effects.

 But there is a side effect, something might be installed/removed. And
there will be expectations based on what is seen (so I suspect that your
first statement is wrong ... as the expectations will be different).
 Eg.

pkgA: Requires: pkgB IF NOT pkgC

...or:

pkgA: Requires: pkgC OR pkgB

...now what happens:

pkgA: To be installed
pkgB: NOT installed
pkgC:     installed
if: pkgA will now not allow pkgC to be removed, without installing pkgB.
or: pkgA will now not allow pkgC to be removed, without installing pkgB.

pkgA: To be installed
pkgB:     installed
pkgC:     installed
if: pkgB and pkgC can't be removed, but either one can.
or: pkgB and pkgC can't be removed, but either one can.

...these look the same, right? But:

pkgA: To be installed
pkgB:     installed
pkgC: NOT installed
if: pkgA will now not allow pkgB to be removed, without removing pkgA.  
    In theory pkgC can be installed ... but I doubt many would predict
    that outcome.
or: pkgA will now not allow pkgB to be removed, without installing pkgC.

pkgA: To be installed
pkgB: NOT available
pkgC: NOT installed
if: pkgC is installed, and user is confused.
or: pkgC is installed.

pkgA: To be installed
pkgB: NOT installed
pkgC: NOT installed
if: pkgB should be installed. Installing pkgC is technically legal but
    would be a _big_ surprise, I think. Installing both would be lol.
with short circuit:
or: pkgB should be installed. Installing pkgC is technically legal but
    would be a surprise. Installing both would be weird.
without short circuit:
or: pkgB or pkgC should be installed. Having both be installed would be
    weird.

...of course "IF" has similar problems with:

pkgA: Requires: pkgB IF pkgC

...and what happens when you remove pkgB later (can remove pkgA or pkgC,
and I would guess the closest to the "correct" answer is: remove pkgA
unless pkgC was installed after pkgA).
 A decent amount of this is mitigated due to users not removing things
much, although the DNF autoremove leaf defaults will make that worse.

>  As the matching does not have side effects you cannot tell
> from outside whether the implementation uses short circuit evaluation or
> not. So it is an implementation detail that's not your business.

 You can tell what happens when you install and/or remove things.
 I know the DNF maintainers currently hope that they can get away with
an undocumented compare_providers, and this would be a huge extension of
that, but I don't think they can.