[arch-dev-public] libalpm hook and wrapper for detecting broken perl modules

[Florian Pritz]

Hi,

TLDR: Skip to the ##TLDR marker below.

We've just had another perl update and as always people with locally
built perl modules ran into issues with random perl tools (including
cpan) not working.

I've looked into this back in 2014 already [1] and back then I chose to
implement versioned dependencies to catch such issues. However I have
not pushed for those dependencies to get adopted by AUR packages since
doing so will force users to remove those packages prior to being able
to update and the main goal was to detect incorrectly rebuilt packages
in our repository (there were 3 from multiple maintainers IIRC).

Naively I thought posting instructions to arch-dev-public on how to find
broken modules would be sufficient. Turn out that not all users of the
AUR read this list and some have unknowingly installed modules with cpan
directly so even if they read the post, they wouldn't expect to have to
run the check.

##TLDR

Talking to some folks in #perl pointed me towards a hopefully satisfying
solution. I plan to version the architecture specific directories and
introduce a libalpm hook that checks for old directories and warns the
user by printing a list of affected AUR/cpanplus-dist-arch packages,
respectively a list of files for modules installed directly with cpan.

The new directory layout would be
"/usr/lib/perl5/$baseversion/{core,vendor,site}_perl" with $baseversion
being "5.26" right now. Currently the layout is
"/usr/lib/perl5/{core,vendor,site}_perl". The check for old versions
would then be rather simple since it just has to look for directories
beside the current version.

During the old discussion, Justin raised the issue that if the
directories are versioned, perl modules that are not rebuilt will just
be missing. This could lead to software not detecting them any more and
disabling a feature or using a slower module to do the job.

One possibility to counter this would be to replace the perl executable
with a wrapper that performs the check and prints errors to stderr.
Optionally it could also exit with an error code rather than continuing
to run the script. I'm not sure if I want this or not, but I'm leaning
towards a yes since exiting will make the error more difficult to miss.

If there are no objections I'll start working on this in the next couple
of weeks.

[1]
https://lists.archlinux.org/pipermail/arch-dev-public/2014-June/026380.html


Florian

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