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Re: Can git choose perl at runtime?

On Wed, Dec 19 2018, John Passaro wrote:

> I recently submitted my first patch using OSX and found the experience
> frustrating, for reasons that have come up on the list before,
> concerning git-send-email and perl dependencies that you need to be
> root to update.
> Last seen here:
> https://public-inbox.org/git/878t55qga6.fsf@xxxxxxxxxxxxxxxxxxx/
> The struggle is that Mac's package manager Homebrew has opted,
> apparently with some finality, to no longer support linking to a user
> perl at build time. PERL_PATH is hard-coded to link to the system
> perl, which means the user needs sudo to install the SSL libraries
> required for send-email. So for send-email to work, you need to either
> sudo cpan or build git yourself. The obvious solution here would be to
> do /usr/bin/env perl, but in the above message Aevar pointed out
> pitfalls with that.
> It seems that choosing perl at compile time necessarily comes with
> tradeoffs. So I wonder if there is a way we can support choosing a
> perl at runtime without breaking the existing mechanism of linking to
> perl at compile time.
> I'm picturing adding an executable "git-perl" to libexec that checks
> config core.perlPath and envvar GIT_PERL_PATH, in some order. Having
> chosen one of these or the build-time PERL_PATH as a last resort, it
> exec's the correct perl executable.
> Then relevant scripts (e.g. git-add--interactive, git-send-email)
> invoke git-perl instead of /usr/bin/perl, and the makefile no longer
> replaces that with PERL_PATH -- instead that will be used at runtime
> via git-perl when we can be sure the user does not explicitly prefer
> something different.
> That does mean we have a new command to support and document: "git
> perl". If it is preferred to keep this hidden as an implementation
> detail, we could call the executable something like "util-git-perl"
> instead so that it doesn't show up when scanning libexec for git
> commands.
> Does this seem like a good idea? I'd be happy to work on a patch.

I see no problem with this. As I noted in my message you linked to doing
this unconditionally is a bad idea, but we can just do it with a config,
e.g. this works:

    diff --git a/perl/header_templates/fixed_prefix.template.pl b/perl/header_templates/fixed_prefix.template.pl
    index 857b4391a4..f96e2ecd11 100644
    --- a/perl/header_templates/fixed_prefix.template.pl
    +++ b/perl/header_templates/fixed_prefix.template.pl
    @@ -1 +1,7 @@
    +BEGIN {
    +    chomp(my $perlPath = `git config --get core.perlPath`);;
    +    if ($perlPath and $^X ne $perlPath) {
    +       exec($perlPath, $0, @ARGV);
    +    }
     use lib (split(/@@PATHSEP@@/, $ENV{GITPERLLIB} || '@@INSTLIBDIR@@'));

Here you just optionally set core.perlPath in your config and if set
it'll chainload to the new interpreter you point at.

I leave wondering if you also want a setting for @INC there, dealing
with perl/header_templates/runtime_prefix.template.pl and docs/tests as
an exercise for the reader :)