preaction

I ran into a frustrating problem the other day:

$ git add -i
/usr/bin/perl: symbol lookup error: ~/perl5/lib/perl5/x86_64-linux-thread-multi/auto/List/Util/Util.so:
undefined symbol: Perl_xs_apiversion_bootcheck
fatal: 'add--interactive' appears to be a git command, but we were not
able to execute it. Maybe git-add--interactive is broken?

I've seen this error message from Perl a lot. It basically means that I'm trying to load an XS module compiled for a different version of Perl. Since git is directly trying to run /usr/bin/perl (5.10.1) as opposed to the perlbrew Perl I have installed (5.16.3), the error comes as no surprise: PERL5LIB is checked before Perl's built-in libraries. So if you have a local::lib (which adds its directories to PERL5LIB) and try to use those modules in a different Perl, things may not work as you expected.

What is more surprising is that Git explicitly uses /usr/bin/perl in the first place. Some Google-fu brought up a thread on the Git mailing list about changing to #!/usr/bin/env perl, but it appears this was rejected. According to another post in that thread, it is possible to set PERL_PATH when running make on Git, but that did not seem to work for me.

But the Git Perl scripts all seem to have one thing in common: They all add the paths in the GITPERLLIB environment variable to the front of @INC as the first thing they do. GITPERLLIB is treated as a :-delimited list of directories, like PERL5LIB. So if we fill in GITPERLLIB with the right directories, we can ensure that the right List::Util version is found first.

The right directories are part of Perl's Config. This configuration is available to us in Perl scripts through the Config module which provides a %Config hash. There are three "layers" of Perl library paths, "core", "vendor", and "site", configured in the following Config keys:

• core => 'archlib', 'privlib'
• vendor => 'vendorarch', 'vendorlib'
• site => 'sitearch', 'sitelib'

The "core" libraries are just that, the core Perl 5 libraries. The "vendor" libraries are additional libraries that your vendor may have provided in their Perl distribution. The "site" libraries are the CPAN libraries you've downloaded and installed via the cpan client (unless you're using local::lib, which overrides the install directories).

Armed with these Config keys, we can make a GITPERLLIB that overrides our local::lib directories. So now, in my .zshrc, I have:

# Fix git perl scripts in case of local::lib
# If we install modules for a different arch in local::lib, we'll get some problems
if [[ -x /usr/bin/perl ]]; then
    export GITPERLLIB=`/usr/bin/perl -MConfig -e'print join ":", grep { $_ } map { $Config{$_} } qw( sitearch sitelib vendorarch vendorlib archlib privlib )'`
fi

Now I can do my proper git add --interactive again!