@DB::args inconsistently reflects changes to @_

[p5pRT]

Migrated from rt.perl.org#120959 (status was 'open')

Searchable as RT120959$

[p5pRT]

From @jimav

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

[p5pRT]

From @iabyn

On Thu, Jan 09, 2014 at 03​:35​:24PM -0800, Jim Avera wrote​:

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?

I think it's intentional behaviour. The code in caller() specifically
handles the case of a shifted @​_, and has been there since at least 5.000.
Also, some code tests fail without it.

I think its designed around the common idiom of

  sub foo {
  my $a = shift;
  my $b = shift;
  ...

which allows debuggers etc to see the args that that foo() was called
with, rather than just arg 3 onwards.

In particular, arrays that are shifted retain their earlier elements; its
just that the pointer to the start of the array is incremented - this is
the internal implementation detail that allows caller() to recover
shifted elements.

But yes, its a bit messy and inconsistent (but no more inconsistent than
it needs to be, given the constraints).

--
But Pity stayed his hand. "It's a pity I've run out of bullets",
he thought. -- "Bored of the Rings"

[p5pRT]

The RT System itself - Status changed from 'new' to 'open'