From apm@one.com

Hi,

I've been having troubles with Perl calling the DESTROY method on my
objects twice (*in the same process*) during global destruction when the
Perl code forks. This happens often when using a module like
Net​::Server​::PreFork.

The problem has been reproduced on​:
Perl 5.10/Debian on Debian squeeze
Perl 5.14.2/Ubuntu on Ubuntu 12.04
Perl 5.18/Debian on Debian jessie
Perl 5.20-RC1 from upstream Perl

A colleague of mine was so kind to boil the steps to reproduce further
down to this snippet not using any modules​:

#!/usr/bin/perl

package MyModule;

use Devel​::Peek;

sub new {
print "IN NEW\n";
return bless {};
}

sub DESTROY {
print "======= BEGIN DESTROY​:".$$."========\n";
print Dump(@​_);
print "============ END DESTROY​:".$$."========\n";
}

1;

package main;

open my $fh, ">", "/tmp/some.pid";
print $fh $$;
close $fh;

my $hardtodestroy = new MyModule;
my $pid = fork;

if ($pid) {

print "Parent​: $$, child​: $pid\n";

$SIG{'TERM'} = sub { kill 15, $pid; exit 0; };
select undef,undef,undef, 300;

print "stopped\n";
kill 15, $pid;
exit 0;
}

$SIG{'TERM'} = sub { exit 0; };
select undef,undef,undef, 300;
exit 0;

Description​:

In one terminal run​:

$ while /bin/true ; perl destroybug.pl ; done

In another terminal run this​:

$ kill 15 -$(cat /tmp/some.pid)

Many times you will see the child calling destroy twice.

XS modules trying to use DESTROY to free() pointers kept in PVMG/IV
scalars will have free() called twice, causing glibc to complain.

regards,
Peter Mogensen