Bug#839026: perl: maintainer test to check for uninstallable dual life module packages

[Niko Tyni]

Package: perl
Version: 5.24.1~rc3-3
Severity: wishlist

In the Perl 5.24 transition we made four dual life module packages in
sid uninstallable because the bundled version was newer. 

In itself this is normal and fine: there's no need for a separate package
with an older version of a bundled module, so those can be removed (from
either testing or also from unstable) without losing any functionality.

However, any versioned dependencies on these packages must first
be changed, at least as long as we don't use versioned Provides (see
#758100). It seems that we forgot to check these versioned dependencies
before the upload, as libio-compress-lzma-perl became uninstallable and
unbuildable due to such a dependency. See #838933.

Speaking for myself, I know that I completely forgot about checking this,
and I suspect we were just lucky that the collateral damage was limited
to only one package, which was easily (and quickly) fixed.

The attached patch adds a maintainer test to debian/t/control.t that
fails for each package in sid (or, more precisely, whatever is in the
local apt cache) that becomes uninstallable because of the current Breaks
relations in debian/control.

The idea is that this should make us less prone to forget this step,
but I'm not sure if the uninstallability of those packages in sid is
going to be long-standing enough that the test failures become sort of
false positives and we become blind to them.

Possibly we should go further and iterate over all packages hunting for
versioned dependencies on these uninstallable packages. I'm not sure how
cleanly this could be implemented, and I'm still not sure where to draw
the line for failures.

Filing this to at least track the issue. Thoughts?
-- 
Niko Tyni   ntyni at debian.org
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