[perl #120959] @DB::args inconsitently reflects changes to @_

# New Ticket Created by  Jim Avera 
# Please include the string:  [perl #120959]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=120959 >


This is a bug report for perl from james_avera@yahoo.com,
generated with the help of perlbug 1.39 running under perl 5.14.2.

-----------------------------------------------------------------
When caller() is called from package DB, it sets @DB::args
"to be the arguments with which the subroutine was invoked...
@DB::args will contain modifications the subroutine makes to @_ or
its contents, not the original values at call time" [perlfunc]

However this is not always the case.

If the calling sub (not in package DB) modifies @_ before calling into
package DB, then @DB::args may or may not reflect the changes.
Specifically, "shift @_" is effectively un-done and the removed arg
re-appears in @DB::args, BUT only if no other changes have been made
to @_, such as 'unshift' or 'push'.

Is this a bug, a side-effect of the optimizer, or intentional behavior?

On the surface, it seems like @DB::args should either always get
the current state of @_ (my preference), or always get the args
before any modifications.

[This matters to me because @_ isn't displayed reliably in a
a debug tool -- http://abhweb.org/jima/perl/lib/Vis.pm ]

#!/usr/bin/perl
use strict; use warnings;
sub f(@) {
   shift @_;
   my @a = &DB::test; print "After 'shift', DB::test returned (@a)\n";
}
sub g(@) {
   pop @_;
   my @a = &DB::test; print "After 'pop', DB::test returned (@a)\n";
}
sub h(@) {
   shift @_; unshift @_,"subst";
   my @a = &DB::test; print "After 'shift' then 'unshift', DB::test 
returned (@a)\n";
}
f(1,2,3);  # result is (1,2,3), i.e. shift alone not visible
g(1,2,3);  # result is (1,2)         pop is visible
h(1,2,3);  # result is ("subst",2,3) shift visible if followed by unshift

package DB;
sub test {
   (undef) = (caller(1));
   return @DB::args;
}

-----------------------------------------------------------------
---
Flags:
     category=core
     severity=low
---
Site configuration information for perl 5.14.2:

Configured by Debian Project at Thu Jul 18 22:04:35 UTC 2013.

Summary of my perl5 (revision 5 version 14 subversion 2) configuration:

   Platform:
     osname=linux, osvers=3.2.0-37-generic, 
archname=x86_64-linux-gnu-thread-multi
     uname='linux roseapple 3.2.0-37-generic #58-ubuntu smp thu jan 24 
15:28:10 utc 2013 x86_64 x86_64 x86_64 gnulinux '
     config_args='-Dusethreads -Duselargefiles -Dccflags=-DDEBIAN 
-D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector --param=ssp-buffer-size=4 
-Wformat -Werror=format-security -Dldflags= -Wl,-Bsymbolic-functions 
-Wl,-z,relro -Dlddlflags=-shared -Wl,-Bsymbolic-functions -Wl,-z,relro 
-Dcccdlflags=-fPIC -Darchname=x86_64-linux-gnu -Dprefix=/usr 
-Dprivlib=/usr/share/perl/5.14 -Darchlib=/usr/lib/perl/5.14 
-Dvendorprefix=/usr -Dvendorlib=/usr/share/perl5 
-Dvendorarch=/usr/lib/perl5 -Dsiteprefix=/usr/local 
-Dsitelib=/usr/local/share/perl/5.14.2 
-Dsitearch=/usr/local/lib/perl/5.14.2 -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 -DDEBUGGING=-g -Doptimize=-O2 -Duseshrplib 
-Dlibperl=libperl.so.5.14.2 -des'
     hint=recommended, useposix=true, d_sigaction=define
     useithreads=define, usemultiplicity=define
     useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
     use64bitint=define, use64bitall=define, uselongdouble=undef
     usemymalloc=n, bincompat5005=undef
   Compiler:
     cc='cc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DDEBIAN 
-fstack-protector -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 -fstack-protector 
-fno-strict-aliasing -pipe -I/usr/local/include'
     ccversion='', gccversion='4.8.1', gccosandvers=''
     intsize=4, longsize=8, ptrsize=8, doublesize=8, byteorder=12345678
     d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
     ivtype='long', ivsize=8, nvtype='double', nvsize=8, Off_t='off_t', 
lseeksize=8
     alignbytes=8, prototype=define
   Linker and Libraries:
     ld='cc', ldflags =' -fstack-protector -L/usr/local/lib'
     libpth=/usr/local/lib /lib/x86_64-linux-gnu /lib/../lib 
/usr/lib/x86_64-linux-gnu /usr/lib/../lib /lib /usr/lib
     libs=-lgdbm -lgdbm_compat -ldb -ldl -lm -lpthread -lc -lcrypt
     perllibs=-ldl -lm -lpthread -lc -lcrypt
     libc=, so=so, useshrplib=true, libperl=libperl.so.5.14.2
     gnulibc_version='2.17'
   Dynamic Linking:
     dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-Wl,-E'
     cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib 
-fstack-protector'

Locally applied patches:


---
@INC for perl 5.14.2:
     /home/jima/lib/perl
     /home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi
     /home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi
     /home/jima/perl5/lib/perl5
     /etc/perl
     /usr/local/lib/perl/5.14.2
     /usr/local/share/perl/5.14.2
     /usr/lib/perl5
     /usr/share/perl5
     /usr/lib/perl/5.14
     /usr/share/perl/5.14
     /usr/local/lib/site_perl
     .

---
Environment for perl 5.14.2:
     HOME=/home/jima
     LANG=en_US.UTF-8
     LANGUAGE=en_US
     LD_LIBRARY_PATH=/home/jima/local/lib
     LOGDIR (unset)
 
PATH=/home/jima/perl5/bin:/home/jima/bin:/home/jima/local/bin:/home/jima/jima_tools/x86_64/bin:/home/jima/jima_tools/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/bin/X11:/usr/local/bin:/usr/lib/lightdm/lightdm:/usr/local/sbin:/usr/games:/usr/local/games:/usr/lib/jvm/java-7-oracle/bin:/usr/lib/jvm/java-7-oracle/db/bin:/usr/lib/jvm/java-7-oracle/jre/bin:.
 
PERL5LIB=/home/jima/lib/perl:/home/jima/perl5/lib/perl5/x86_64-linux-gnu-thread-multi:/home/jima/perl5/lib/perl5
     PERL_BADLANG (unset)
     PERL_LOCAL_LIB_ROOT=/home/jima/perl5
     PERL_MB_OPT=--install_base /home/jima/perl5
     PERL_MM_OPT=INSTALL_BASE=/home/jima/perl5
     SHELL=/bin/bash

• [perl #120959] @DB::args inconsitently reflects changes to @_ by Jim Avera
• Re: [perl #120959] @DB::args inconsitently reflects changes to @_ by Dave Mitchell