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#!/usr/bin/perl
package evil;

use 5.010001;
use strict;
use warnings;

use Carp;

my $INTERMEDIATE = __PACKAGE__.'/intermediate';
my $LAX          = __PACKAGE__.'/lax';

our $VERSION = 0.003002;

our %tainted;
our %wants_strict;

sub import {
	croak "Cannot load evil module when \"no evil ':strict'\" is in effect" if %wants_strict;

	my $hinthash = (caller 0)[10] || {};
	croak "Current module requested no evilness" if $hinthash->{$LAX};

	$hinthash = (caller 3)[10] || {};
	croak "Cannot load evil module when parent requested \"no evil ':lax'\"" if $hinthash->{$LAX};

	my $level = 4;
	my @caller;
	while (@caller = caller $level) {
		$hinthash = $caller[10] || {};
		croak "Cannot load evil module when ancestor requested \"no evil ':intermediate'\""
		  if $hinthash->{$INTERMEDIATE};
		$level++;
	}

	$tainted{caller()} = 1;
}

sub unimport {
	my $strict_arg = grep /^:strict$/i, @_;
	my $intermediate_arg = grep /^:intermediate$/i, @_;
	my $lax_arg = grep /^:lax$/i, @_;
	my $disable_arg = grep /^:disable$/i, @_;

	if (!$disable_arg && $tainted{caller()}) { # caller is evil
		croak 'Current module is evil'
	}

	if ($strict_arg) {
		$wants_strict{caller()} = 1;
		croak "Evil module already loaded. Cannot enforce \"no evil ':strict'\"" if %tainted
	} elsif ($lax_arg) {
		$^H{$LAX} = 1
	} elsif ($disable_arg) {
		delete $wants_strict{caller()};
		delete $^H{$LAX};
		delete $^H{$INTERMEDIATE};
	} else { # $intermediate_arg or no arg
		$^H{$INTERMEDIATE} = $^H{$LAX} = 1
	}
}

1;
__END__

=encoding utf-8

=head1 NAME

evil - RFC 3514 (evil bit) implementation for Perl modules

=head1 SYNOPSIS

  # in A.pm
  package A;
  use evil;

  # in B.pm
  package B;
  no evil ':strict';
  use A; # <dies>

  # in C.pm
  package C;
  use A;

  # in D.pm
  package D;
  no evil;
  use C; # <dies>

  # in E.pm
  package E;
  no evil ':lax';
  use C; # does not die, as C is not evil

  # in F.pm
  package F;
  use C;
  no evil;
  # does not die, as modules loaded before the pragma are ignored

=head1 DESCRIPTION

L<RFC3514|https://www.ietf.org/rfc/rfc3514.txt> introduces a new flag
called the "evil bit" in all IP packets. The intention is to simplify
the work of firewalls. Software that sends IP packets with malicious
intent must set the evil bit to true, and firewalls can simply drop
such packets.

The evil pragma is a Perl implementation of the same concept. With