[DRE-maint] Bug#591419: librmagick-ruby should not be installable with imagemagick

[Antonio Terceiro]

On Mon, Dec 05, 2016 at 09:18:07AM +0100, Nish Aravamudan wrote:
> > However, I fail to see how this would fix the issue, since it won't help
> > with the very problem that a new version of ImageMagick that has no
> > SONAME bump breaks ruby-rmagick.
> 
> So you're right that it does not "fix" the issue of ImageMagick itself
> -- given that is something (as you've responded in the other bug) that
> src:imagemagick needs to fix.
> 
> > There is a new version of ruby-rmagick which claims support for
> > ImageMagick 6.9, and apparently shifts the dependency from libmagickcore
> > to libmagickwand ... this might be the correct fix.

This wouldn't fix the issue either, I was wrong.

> However, what the above patch does is, at least for libMagickCore itself
> (and that could be adjusted for libMagickWand as well) is ignore the ABI
> values from the symbols file -- that's not entirely 'correct', but what
> it means is that if the ABI is not correctly describing minimum version,
> we will insert a dependency on the version of imagemagick actually used
> to build the package at build-time. That does mean the dependency may be
> strictly higher than it needs to be, but it also helps accurately
> reflect that ruby-rmagick should be rebuilt (so it forces an imagemagick
> transition of sorts on every version bump?). Although perhaps the shell
> snippet should make the dependency strictly =  (rather than >=).

A >= dependency would be useless, because a new package with a ABI
break would satisfy that dependency just fine.

A strict dependency (= v) would make ruby-rmagick immediately
uninstallable when *any* later version comes along, not only one that
has an ABI break. So even security fixes, or regular new upstream
versions that do not break the ABI, would invalidate the dependency
chain of ruby-rmagick.
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