Carp can inhibit object destruction

This is a bug report for perl from perl-bug@ton.iguana.be,
generated with the help of perlbug 1.42 running under perl 5.30.3.


-----------------------------------------------------------------
[Please describe your issue here]

I reported it as a Carp bug, but there is some suspecious behaviour in the core
around @DB::args going on (see later in the report).

The version of Carp on my system is 1.50

Consider the following program:
========================
#!/usr/bin/perl -l
package Foo;
use Carp;

sub bar {
    splice(@_, 0, 1);
    # die "Boom!";
    croak "Boom!";
}

package main;
sub DESTROY { print "DESTROY" }

{
    my $obj  = bless [];
    eval { Foo::bar(undef, $obj)};
    print "ERR=$@";
    # @DB::args = ();
    # eval { Carp::croak "zz" };
}
print "END";
========================

It creates an obkect $obj inside a lexical scope.
This object should stop existing at the end of the scope, so it should print
"DESTROY". After that the program should print the last line and say "END"

So we expect:
DESTROY
END

We however get:
END
DESTROY

The object survived until global destruction.

Uncommenting any of the commented out lines will make the program work as
expected. Also commenting the "splice" statement will make things work.

What is happening here is that the "croak" will call its internal "caller_info"
sub which will use caller() inside package DB to populate @DB::args.
(even though "croak" and "carp" mostly shouldn't need that).

So the object ends up with an extra reference in  @DB::args which makes it
survive to the end of the program. That is why clearing @DB::args or doing
another "croak" (which fill @DB::args with other data) fixes it. It also 
explains why "die" instead of "croak" fixes this.

The case for the "splice" is less clear. perlfunc talks about "caller" inside
DB and warns that it is possible to get hold of values that don't exist on the
stack anymore (making you able to access freed memory). However, this doesn't 
really seem to be the cause here. Tracing the code through Carp for the failing
and succeeding case is see EXACTLY the same code executed
(confirmed with Devel::Cover)

Also consider the following program:

========================
#!/usr/bin/perl -l
package Foo;
use Carp;

sub bar {
    croak "Boom!";
}

package main;
use Data::Dumper;
sub DESTROY { print "DESTROY" }

{
    my $obj  = bless [];
    eval { Foo::bar(undef, $obj)};
    print "ERR=$@";
    # Comment out the following line to see a difference
    print Dumper(\@DB::args);
}
print "END";
========================

This doesn't have the splice. So I wouldn't expect @DB::args to be contentious
here, but it still displays the bug (END printed first).

However, if you comment out the 'print Dumper(\@DB::args);' line, it suddenly
starts working as expected. This is why I suspect this might be a perl bug
instead of a bug in Carp. This is also the point where I stopped tracing
things since I don't want to dive into the perl guts at this point.

It is however quite easy to at least hide the problem in Carp by not visibly
modifying @DB::args. This seems to be a minimal change to Carp.pm to paper over
the problem:

========================
--- /usr/lib/x86_64-linux-gnu/perl-base/Carp.pm 2020-06-07 09:56:32.000000000 +0200
+++ ./Carp.pm	2020-11-15 13:04:08.024086999 +0100
@@ -301,6 +301,7 @@
 }
 
 sub caller_info {
+    local @DB::args;
     my $i = shift(@_) + 1;
     my %call_info;
     my $cgc = _cgc();
========================

With that all versions of the programs work as expected.

[Please do not change anything below this line]
-----------------------------------------------------------------
---
Flags:
    category=library
    severity=low
    module=Carp
---
Site configuration information for perl 5.30.3:

Configured by Debian at Sun Jun  7 07:56:32 UTC 2020.

Summary of my perl5 (revision 5 version 30 subversion 3) configuration:
   
  Platform:
    osname=linux
    osvers=4.19.0
    archname=x86_64-linux-gnu-thread-multi
    uname='linux localhost 4.19.0 #1 smp debian 4.19.0 x86_64 gnulinux '
    config_args='-Dmksymlinks -Dusethreads -Duselargefiles -Dcc=x86_64-linux-gnu-gcc -Dcpp=x86_64-linux-gnu-cpp -Dld=x86_64-linux-gnu-gcc -Dccflags=-DDEBIAN -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -fdebug-prefix-map=/build/perl-MTdOjL/perl-5.30.3=. -fstack-protector-strong -Wformat -Werror=format-security -Dldflags= -Wl,-z,relro -Dlddlflags=-shared -Wl,-z,relro -Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr -Dprivlib=/usr/share/perl/5.30 -Darchlib=/usr/lib/x86_64-linux-gnu/perl/5.30 -Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 -Dvendorarch=/usr/lib/x86_64-linux-gnu/perl5/5.30 -Dsiteprefix=/usr/local -Dsitelib=/usr/local/share/perl/5.30.3 -Dsitearch=/usr/local/lib/x86_64-linux-gnu/perl/5.30.3 -Dman1dir=/usr/share/man/man1 -Dman3dir=/usr/share/man/man3 -Dsiteman1dir=/usr/local/man/man1 -Dsiteman3dir=/usr/local/man/man3 -Duse64bitint -Dman1ext=1 -Dman3ext=3perl
-Dpager=/usr/bin/sensible-pager -Uafs -Ud_csh -Ud_ualarm -Uusesfio -Uusenm -Ui_libutil -Ui_xlocale -Uversiononly -DDEBUGGING=-g -Doptimize=-O2 -dEs -Duseshrplib -Dlibperl=libperl.so.5.30.3'
    hint=recommended
    useposix=true
    d_sigaction=define
    useithreads=define
    usemultiplicity=define
    use64bitint=define
    use64bitall=define
    uselongdouble=undef
    usemymalloc=n
    default_inc_excludes_dot=define
    bincompat5005=undef
  Compiler:
    cc='x86_64-linux-gnu-gcc'
    ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64'
    optimize='-O2 -g'
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN -fwrapv -fno-strict-aliasing -pipe -I/usr/local/include'
    ccversion=''
    gccversion='9.3.0'
    gccosandvers=''
    intsize=4
    longsize=8
    ptrsize=8
    doublesize=8
    byteorder=12345678
    doublekind=3
    d_longlong=define
    longlongsize=8
    d_longdbl=define
    longdblsize=16
    longdblkind=3
    ivtype='long'
    ivsize=8
    nvtype='double'
    nvsize=8
    Off_t='off_t'
    lseeksize=8
    alignbytes=8
    prototype=define
  Linker and Libraries:
    ld='x86_64-linux-gnu-gcc'
    ldflags =' -fstack-protector-strong -L/usr/local/lib'
    libpth=/usr/local/lib /usr/include/x86_64-linux-gnu /usr/lib /lib/x86_64-linux-gnu /lib/../lib /usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib
    libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
    perllibs=-ldl -lm -lpthread -lc -lcrypt
    libc=libc-2.30.so
    so=so
    useshrplib=true
    libperl=libperl.so.5.30
    gnulibc_version='2.30'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs
    dlext=so
    d_dlsymun=undef
    ccdlflags='-Wl,-E'
    cccdlflags='-fPIC'
    lddlflags='-shared -L/usr/local/lib -fstack-protector-strong'

Locally applied patches:
    DEBPKG:debian/cpan_definstalldirs - Provide a sensible INSTALLDIRS default for modules installed from CPAN.
    DEBPKG:debian/db_file_ver - https://bugs.debian.org/340047 Remove overly restrictive DB_File version check.
    DEBPKG:debian/doc_info - Replace generic man(1) instructions with Debian-specific information.
    DEBPKG:debian/enc2xs_inc - https://bugs.debian.org/290336 Tweak enc2xs to follow symlinks and ignore missing @INC directories.
    DEBPKG:debian/errno_ver - https://bugs.debian.org/343351 Remove Errno version check due to upgrade problems with long-running processes.
    DEBPKG:debian/libperl_embed_doc - https://bugs.debian.org/186778 Note that libperl-dev package is required for embedded linking
    DEBPKG:fixes/respect_umask - Respect umask during installation
    DEBPKG:debian/writable_site_dirs - Set umask approproately for site install directories
    DEBPKG:debian/extutils_set_libperl_path - EU:MM: set location of libperl.a under /usr/lib
    DEBPKG:debian/no_packlist_perllocal - Don't install .packlist or perllocal.pod for perl or vendor
    DEBPKG:debian/fakeroot - Postpone LD_LIBRARY_PATH evaluation to the binary targets.
    DEBPKG:debian/instmodsh_doc - Debian policy doesn't install .packlist files for core or vendor.
    DEBPKG:debian/ld_run_path - Remove standard libs from LD_RUN_PATH as per Debian policy.
    DEBPKG:debian/libnet_config_path - Set location of libnet.cfg to /etc/perl/Net as /usr may not be writable.
    DEBPKG:debian/perlivp - https://bugs.debian.org/510895 Make perlivp skip include directories in /usr/local
    DEBPKG:debian/squelch-locale-warnings - https://bugs.debian.org/508764 Squelch locale warnings in Debian package maintainer scripts
    DEBPKG:debian/patchlevel - https://bugs.debian.org/567489 List packaged patches for 5.30.3-4 in patchlevel.h
    DEBPKG:fixes/document_makemaker_ccflags - https://bugs.debian.org/628522 [rt.cpan.org #68613] Document that CCFLAGS should include $Config{ccflags}
    DEBPKG:debian/find_html2text - https://bugs.debian.org/640479 Configure CPAN::Distribution with correct name of html2text
    DEBPKG:debian/perl5db-x-terminal-emulator.patch - https://bugs.debian.org/668490 Invoke x-terminal-emulator rather than xterm in perl5db.pl
    DEBPKG:debian/cpan-missing-site-dirs - https://bugs.debian.org/688842 Fix CPAN::FirstTime defaults with nonexisting site dirs if a parent is writable
    DEBPKG:fixes/memoize_storable_nstore - [rt.cpan.org #77790] https://bugs.debian.org/587650 Memoize::Storable: respect 'nstore' option not respected
    DEBPKG:debian/makemaker-pasthru - https://bugs.debian.org/758471 Pass LD settings through to subdirectories
    DEBPKG:debian/makemaker-manext - https://bugs.debian.org/247370 Make EU::MakeMaker honour MANnEXT settings in generated manpage headers
    DEBPKG:debian/kfreebsd-softupdates - https://bugs.debian.org/796798 Work around Debian Bug#796798
    DEBPKG:fixes/autodie-scope - https://bugs.debian.org/798096 Fix a scoping issue with "no autodie" and the "system" sub
    DEBPKG:fixes/memoize-pod - [rt.cpan.org #89441] Fix POD errors in Memoize
    DEBPKG:debian/hurd-softupdates - https://bugs.debian.org/822735 Fix t/op/stat.t failures on hurd
    DEBPKG:fixes/math_complex_doc_great_circle - https://bugs.debian.org/697567 [rt.cpan.org #114104] Math::Trig: clarify definition of great_circle_midpoint
    DEBPKG:fixes/math_complex_doc_see_also - https://bugs.debian.org/697568 [rt.cpan.org #114105] Math::Trig: add missing SEE ALSO
    DEBPKG:fixes/math_complex_doc_angle_units - https://bugs.debian.org/731505 [rt.cpan.org #114106] Math::Trig: document angle units
    DEBPKG:fixes/cpan_web_link - https://bugs.debian.org/367291 CPAN: Add link to main CPAN web site
    DEBPKG:debian/hppa_op_optimize_workaround - https://bugs.debian.org/838613 Temporarily lower the optimization of op.c on hppa due to gcc-6 problems
    DEBPKG:debian/installman-utf8 - https://bugs.debian.org/840211 Generate man pages with UTF-8 characters
    DEBPKG:fixes/getopt-long-4 - https://bugs.debian.org/864544 [rt.cpan.org #122068] Fix issue #122068.
    DEBPKG:debian/hppa_opmini_optimize_workaround - https://bugs.debian.org/869122 Lower the optimization level of opmini.c on hppa
    DEBPKG:debian/sh4_op_optimize_workaround - https://bugs.debian.org/869373 Also lower the optimization level of op.c and opmini.c on sh4
    DEBPKG:debian/perldoc-pager - https://bugs.debian.org/870340 [rt.cpan.org #120229] Fix perldoc terminal escapes when sensible-pager is less
    DEBPKG:debian/prune_libs - https://bugs.debian.org/128355 Prune the list of libraries wanted to what we actually need.
    DEBPKG:debian/mod_paths - Tweak @INC ordering for Debian
    DEBPKG:debian/configure-regen - https://bugs.debian.org/762638 Regenerate Configure et al. after probe unit changes
    DEBPKG:debian/deprecate-with-apt - https://bugs.debian.org/747628 Point users to Debian packages of deprecated core modules
    DEBPKG:debian/disable-stack-check - https://bugs.debian.org/902779 [GH #16607] Disable debugperl stack extension checks for binary compatibility with perl
    DEBPKG:fixes/eumm-usrmerge - https://bugs.debian.org/913637 Avoid mangling /bin non-perl shebangs on merged-/usr systems
    DEBPKG:debian/perlbug-editor - https://bugs.debian.org/922609 Use "editor" as the default perlbug editor, as per Debian policy
    DEBPKG:fixes/configure-fixup - Sync Configure with metaconfig
    DEBPKG:fixes/posix-mb-sbuild-fix - [8f8f6a1] Fix edge case test failure in ext/POSIX/t/mb.t
    DEBPKG:fixes/perlbug-github - [31fa749] perlbug - Change default behavior to save to file
    DEBPKG:fixes/io-socket-ip-nov4 - https://bugs.debian.org/962019 Fix test failures in IO::Socket::IP with an IPv6-only host
    DEBPKG:debian/eu-mm-perl-base - https://bugs.debian.org/962138 Suppress an ExtUtils::MakeMaker warning about our non-default @INC

---
@INC for perl 5.30.3:
    /etc/perl
    /usr/local/lib/x86_64-linux-gnu/perl/5.30.3
    /usr/local/share/perl/5.30.3
    /usr/lib/x86_64-linux-gnu/perl5/5.30
    /usr/share/perl5
    /usr/lib/x86_64-linux-gnu/perl-base
    /usr/lib/x86_64-linux-gnu/perl/5.30
    /usr/share/perl/5.30
    /usr/local/lib/site_perl
    /usr/local/lib/x86_64-linux-gnu/perl/5.30.0
    /usr/local/share/perl/5.30.0

---
Environment for perl 5.30.3:
    HOME=/home/ton
    LANG=en_US.utf8
    LANGUAGE=
    LC_TIME=C
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/home/ton/bin:/home/ton/bin.SampleSetup:/usr/local/bin:/usr/local/sbin:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/bin:/usr/lib/jvm/java-10-openjdk-amd64/bin:/usr/lib/fvwm/2.6.8:/usr/bin:/usr/sbin:/bin:/sbin:/opt/containerd/bin:/snap/bin:.:/home/ton/chromiumos/depot_tools:/home/ton/chromiumos/depot_tools
    PERL_BADLANG (unset)
    SHELL=/bin/bash
0
perl
11/15/2020 1:09:44 PM
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