perl debugger reports undefined behaviour when using the X and V commands

[sir-ragna]

Module: Perl debugger (core_perl)

Description

When attempting to do data examination with the X command or V main reports undefined behavior.

Use of each() on hash after insertion without resetting hash iterator results in undefined behavior, Perl interpreter: 0x55a84e5f5010 at /usr/share/perl5/core_perl/dumpvar.pl line 488.
at /usr/share/perl5/core_perl/dumpvar.pl line 488.
        main::dumpvar("main", -1) called at /usr/share/perl5/core_perl/perl5db.pl line 3801
        eval {...} called at /usr/share/perl5/core_perl/perl5db.pl line 3796
        DB::Obj::_handle_V_command_and_X_command(DB::Obj=HASH(0x55a84e624750)) called at /usr/share/perl5/core_perl/perl5db.pl line 3175
        DB::DB called at main.pl line 4

Steps to Reproduce

1. Create a minimal script.

# file: example.pl
my $foo = "bar";

2. Start the debugger with perl -d example.pl.
3. Enter the X command (or V main).
4. Peruse the output for a line that reports undefined behavoir

Use of each() on hash after insertion without resetting hash iterator results in undefined behavior, Perl interpreter: 0x55eff1ca2010 at /usr/share/perl5/core_perl/dumpvar.pl line 488.
at /usr/share/perl5/core_perl/dumpvar.pl line 488.
        main::dumpvar("main", -1) called at /usr/share/perl5/core_perl/perl5db.pl line 3801
        eval {...} called at /usr/share/perl5/core_perl/perl5db.pl line 3796
        DB::Obj::_handle_V_command_and_X_command(DB::Obj=HASH(0x55eff1cd16f0)) called at /usr/share/perl5/core_perl/perl5db.pl line 3175
        DB::DB called at example.pl line 2

This can be reproduced with other files as well.

# file: example2.pl
use strict;

sub foo {
    my $p1 = shift;
    print $p1, "\n";
}

foo('hello');

The above sample with the same steps results in a very similar error message:

Use of each() on hash after insertion without resetting hash iterator results in undefined behavior, Perl interpreter: 0x5563db0de010 at /usr/share/perl5/core_perl/dumpvar.pl line 488.
at /usr/share/perl5/core_perl/dumpvar.pl line 488.
        main::dumpvar("main", -1) called at /usr/share/perl5/core_perl/perl5db.pl line 3801
        eval {...} called at /usr/share/perl5/core_perl/perl5db.pl line 3796
        DB::Obj::_handle_V_command_and_X_command(DB::Obj=HASH(0x5563db10d790)) called at /usr/share/perl5/core_perl/perl5db.pl line 3175
        DB::DB called at example2.pl line 9

Expected behavior

I would like the data examination functions in the debugger not to trigger any undefined behavior.

Perl configuration

This is perl 5, version 42, subversion 0 (v5.42.0) built for x86_64-linux-thread-multi

Copyright 1987-2025, Larry Wall

Perl may be copied only under the terms of either the Artistic License or the
GNU General Public License, which may be found in the Perl 5 source kit.

Complete documentation for Perl, including FAQ lists, should be found on
this system using "man perl" or "perldoc perl".  If you have access to the
Internet, point your browser at https://www.perl.org/, the Perl Home Page.

[jkeenan]

@sir-ragna, thanks for spotting that.

AFAICT, the warning was added to the Perl core distribution by @demerphq in this 2013 commit:

commit a7b39f85d7caac0822fdcd78400e131a95f11148 (HEAD)
Author:     Yves Orton <demerphq@gmail.com>
AuthorDate: Mon Mar 18 00:28:03 2013 +0100
Commit:     Yves Orton <demerphq@gmail.com>
CommitDate: Tue Mar 19 00:23:12 2013 +0100

    detect each() after insert and produce warnings when we do

That would have been in the 5.17 development cycle. But I don't think that commit had any particular focus on the debugger.

Using a variety of older perl releases (installed via perlbrew) and running your first example through the debugger in those releases suggests that 5.18 was the first production release to display this warning.

Can someone familiar with the debugger take a closer look at this?

[jkeenan]

Fixed by 84b1a74 in #23669. Closing.