Re: [gentoo-pms] Runtime-switchable USE flags

[David Leverton]

Ulrich Mueller wrote:
>>>>>> On Thu, 26 Jun 2014, David Leverton wrote:
>
>> If, for example, we make subversion an IUSE_RUNTIME flag of git, and
>> some other package depends on dev-vcs/git[-subversion], and the user
>> has the subversion flag disabled for git but happens to have
>> subversion installed anyway, should the dependency be considered
>> satisfied (in which case it's fairly pointless)? Or does it
>> magically get treated like a blocker on dev-vcs/subversion?
>
> It should be treated like a regular USE flag for dependency
> resolution. So in your example, the dependency would be satisfied.

That makes that sort of dependency fairly useless then - it forces the 
user to mess with their configuration, but it doesn't actually influence 
what gets installed.  If the package is somehow incompatible with 
git-svn being available and working (OK, it's a pretty contrived 
example...) then the dependency doesn't do anything to help the 
situation.  If we're OK with that then fine, but it's a potential point 
of confusion for ebuild authors.

> More interesting example: Some other package could depend on
> dev-vcs/git[-perl]. "perl" is a regular USE flag in git, but is has
> "subversion? ( perl )" in REQUIRED_USE. Therefore dev-vcs/git[-perl]
> is effectively equivalent to dev-vcs/git[-perl,-subversion]. However,
> the restrictions in current GLEP 62 don't cover this case.

I don't think that's really an issue.  If the package is incompatible 
with the perl support in git then it should say so, and if git-svn 
happens to be a casualty then the PM can enforce that just fine.

> Your arguments are valid, but they apply to regular USE flags, too.
> So I fail to see why we would need special treatment for runtime
> switchable flags here.

In theory at least, if a regular USE flag is disabled it's supposed to 
actively remove support for that feature when the package is 
built/installed.  Currently if you depend on dev-vcs/git[-subversion] 
then you know git-svn isn't installed at all, regardless of whether 
subversion itself is.  With IUSE_RUNTIME, the only difference between a 
flag being enabled and disabled is that if it's enabled then you know 
the deps for that feature are installed, but if it's disabled then they 
might be installed anyway, so depending on that state doesn't guarantee 
anything.

> If necessary, we can always impose additional restrictions by tree
> policy (as we do for "noblah" flags, for example), but I'd rather not
> restrict what the package manager can do. Especially, if we can simply
> go with normal USE flag logic in one case, but need special casing for
> runtime flags in the other case.

If we're OK with leaving a potential trap then that's fine, just making 
sure people are aware.  (Perhaps we could have a repoman check instead?)

>
> Ulrich
>