package Git::LoadCPAN;
use 5.008;
use strict;
use warnings;

=head1 NAME

Git::LoadCPAN - Wrapper for loading modules from the CPAN (OS) or Git's own copy

=head1 DESCRIPTION

The Perl code in Git depends on some modules from the CPAN, but we
don't want to make those a hard requirement for anyone building from
source.

Therefore the L<Git::LoadCPAN> namespace shipped with Git contains
wrapper modules like C<Git::LoadCPAN::Module::Name> that will first
attempt to load C<Module::Name> from the OS, and if that doesn't work
will fall back on C<FromCPAN::Module::Name> shipped with Git itself.

Usually distributors will not ship with Git's Git::FromCPAN tree at
all via the C<NO_PERL_CPAN_FALLBACKS> option, preferring to use their
own packaging of CPAN modules instead.

This module is only intended to be used for code shipping in the
C<git.git> repository. Use it for anything else at your peril!

=cut

# NO_PERL_CPAN_FALLBACKS_STR evades the sed search-replace from the
# Makefile, and allows for detecting whether the module is loaded from
# perl/Git as opposed to perl/build/Git, which is useful for one-off
# testing without having Error.pm et al installed.
use constant NO_PERL_CPAN_FALLBACKS_STR => '@@' . 'NO_PERL_CPAN_FALLBACKS' . '@@';
use constant NO_PERL_CPAN_FALLBACKS => (
	q[@@NO_PERL_CPAN_FALLBACKS@@] ne ''
	and
	q[@@NO_PERL_CPAN_FALLBACKS@@] ne NO_PERL_CPAN_FALLBACKS_STR
);

sub import {
	shift;
	my $caller = caller;
	my %args = @_;
	my $module = exists $args{module} ? delete $args{module} : die "BUG: Expected 'module' parameter!";
	my $import = exists $args{import} ? delete $args{import} : die "BUG: Expected 'import' parameter!";
	die "BUG: Too many arguments!" if keys %args;

	# Foo::Bar to Foo/Bar.pm
	my $package_pm = $module;
	$package_pm =~ s[::][/]g;
	$package_pm .= '.pm';

	eval {
		require $package_pm;
		1;
	} or do {
		my $error = $@ || "Zombie Error";

		if (NO_PERL_CPAN_FALLBACKS) {
			chomp(my $error = sprintf <<'THEY_PROMISED', $module);
BUG: The '%s' module is not here, but NO_PERL_CPAN_FALLBACKS was set!

Git needs this Perl module from the CPAN, and will by default ship
with a copy of it. This Git was built with NO_PERL_CPAN_FALLBACKS,
meaning that whoever built it promised to provide this module.

You're seeing this error because they broke that promise, and we can't
load our fallback version, since we were asked not to install it.

If you're seeing this error and didn't package Git yourself the
package you're using is broken, or your system is broken. This error
won't appear if Git is built without NO_PERL_CPAN_FALLBACKS (instead
we'll use our fallback version of the module).
THEY_PROMISED
			die $error;
		}

		my $Git_LoadCPAN_pm_path = $INC{"Git/LoadCPAN.pm"} || die "BUG: Should have our own path from %INC!";

		require File::Basename;
		my $Git_LoadCPAN_pm_root = File::Basename::dirname($Git_LoadCPAN_pm_path) || die "BUG: Can't figure out lib/Git dirname from '$Git_LoadCPAN_pm_path'!";

		require File::Spec;
		my $Git_pm_FromCPAN_root = File::Spec->catdir($Git_LoadCPAN_pm_root, '..', 'FromCPAN');
		die "BUG: '$Git_pm_FromCPAN_root' should be a directory!" unless -d $Git_pm_FromCPAN_root;

		local @INC = ($Git_pm_FromCPAN_root, @INC);
		require $package_pm;
	};

	if ($import) {
		no strict 'refs';
		*{"${caller}::import"} = sub {
			shift;
			use strict 'refs';
			unshift @_, $module;
			goto &{"${module}::import"};
		};
		use strict 'refs';
	}
}

1;