A combination eval, DESTROY method and $@ - dangerous - bug

[p5pRT]

Migrated from rt.perl.org#38034 (status was 'resolved')

Searchable as RT38034$

[p5pRT]

From root@localhost.localdomain

Created by root@localhost.localdomain

Brief​:
If an some object is created in eval block under my operator and this object has a DESTROY method (for example, disconnect database method) then any threw exceptions in eval block are wiped and lost. I think that this is incorrect behaviour of perl. The perl should keep $@​ if in eval there is some DESTROY methods...

To see example below

// Perlover (perlover@​perlover.com)
Example​:

eval

{
my $alex = Alex->new();
$alex->alex();
};

my $error = $@​;

print "error = $error\n";

{
package Alex;

sub new
{
return bless {}, shift;
}

sub DESTROY
{
  $@​ = undef;
}

sub alex
{
die "Vasya cool";
}
}

Perl Info

Flags:
    category=core
    severity=medium

This perlbug was built using Perl v5.8.0 in the Red Hat build system.
It is being executed now by Perl v5.8.0 - Sat Dec 11 01:51:59 GMT 2004.

Site configuration information for perl v5.8.0:

Configured by root at Sat Dec 11 01:51:59 GMT 2004.

Summary of my perl5 (revision 5.0 version 8 subversion 0) configuration:
  Platform:
    osname=linux, osvers=2.4.21-20.0.1.el, archname=i386-linux-thread-multi
    uname='linux centos-build 2.4.21-20.0.1.el #1 fri dec 3 02:17:26 gmt 2004 i686 athlon i386 gnulinux '
    config_args='-des -Doptimize=-O2 -g -pipe -march=i386 -mcpu=i686 -Dmyhostname=localhost -Dperladmin=root@localhost -Dcc=gcc -Dcf_by=Red Hat, Inc. -Dinstallprefix=/usr -Dprefix=/usr -Darchname=i386-linux -Dvendorprefix=/usr -Dsiteprefix=/usr -Dotherlibdirs=/usr/lib/perl5/5.8.0 -Duseshrplib -Dusethreads -Duseithreads -Duselargefiles -Dd_dosuid -Dd_semctl_semun -Di_db -Ui_ndbm -Di_gdbm -Di_shadow -Di_syslog -Dman3ext=3pm -Duseperlio -Dinstallusrbinperl -Ubincompat5005 -Uversiononly -Dpager=/usr/bin/less -isr'
    hint=recommended, useposix=true, d_sigaction=define
    usethreads=define use5005threads=undef useithreads=define usemultiplicity=define
    useperlio=define d_sfio=undef uselargefiles=define usesocks=undef
    use64bitint=undef use64bitall=undef uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='gcc', ccflags ='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -DDEBUGGING -fno-strict-aliasing -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -I/usr/include/gdbm',
    optimize='-O2 -g -pipe -march=i386 -mcpu=i686',
    cppflags='-D_REENTRANT -D_GNU_SOURCE -DTHREADS_HAVE_PIDS -DDEBUGGING -fno-strict-aliasing -I/usr/local/include -I/usr/include/gdbm'
    ccversion='', gccversion='3.2.3 20030502 (Red Hat Linux 3.2.3-42)', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=12
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=4, prototype=define
  Linker and Libraries:
    ld='gcc', ldflags =' -L/usr/local/lib'
    libpth=/usr/local/lib /lib /usr/lib
    libs=-lnsl -lgdbm -ldb -ldl -lm -lpthread -lc -lcrypt -lutil
    perllibs=-lnsl -ldl -lm -lpthread -lc -lcrypt -lutil
    libc=/lib/libc-2.3.2.so, so=so, useshrplib=true, libperl=libperl.so
    gnulibc_version='2.3.2'
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=so, d_dlsymun=undef, ccdlflags='-rdynamic -Wl,-rpath,/usr/lib/perl5/5.8.0/i386-linux-thread-multi/CORE'
    cccdlflags='-fPIC', lddlflags='-shared -L/usr/local/lib'

Locally applied patches:
    MAINT18379


@INC for perl v5.8.0:
    /usr/lib/perl5/5.8.0/i386-linux-thread-multi
    /usr/lib/perl5/5.8.0
    /usr/lib/perl5/site_perl/5.8.0/i386-linux-thread-multi
    /usr/lib/perl5/site_perl/5.8.0
    /usr/lib/perl5/site_perl
    /usr/lib/perl5/vendor_perl/5.8.0/i386-linux-thread-multi
    /usr/lib/perl5/vendor_perl/5.8.0
    /usr/lib/perl5/vendor_perl
    /usr/lib/perl5/5.8.0/i386-linux-thread-multi
    /usr/lib/perl5/5.8.0
    .


Environment for perl v5.8.0:
    HOME=/root
    LANG=en_US.iso885915
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/usr/kerberos/sbin:/usr/kerberos/bin:/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/usr/X11R6/bin:/root/bin
    PERL_BADLANG (unset)
    SHELL=/bin/bash

[p5pRT]

From @smpeters

[root@​localhost.localdomain - Sun Dec 25 12​:15​:57 2005]​:

This is a bug report for perl from root@​localhost.localdomain,
generated with the help of perlbug 1.34 running under perl v5.8.0.

-----------------------------------------------------------------
[Please enter your report here]

Brief​:
If an some object is created in eval block under my operator and this
object has a DESTROY method (for example, disconnect database
method) then any threw exceptions in eval block are wiped and lost.
I think that this is incorrect behaviour of perl. The perl should
keep $@​ if in eval there is some DESTROY methods...

To see example below

// Perlover (perlover@​perlover.com)
Example​:

eval

{
my $alex = Alex->new();
$alex->alex();
};

my $error = $@​;

print "error = $error\n";

{
package Alex;

sub new
{
return bless {}, shift;
}

sub DESTROY
{
$@​ = undef;
}

sub alex
{
die "Vasya cool";
}
}

But what if the scenario you show above is exactly what I want? It may
not be the wisest choice in writing a Perl module, but I do not see
where it is a bug. Anyone else have any comments?

[p5pRT]

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

[p5pRT]

From @ysth

On Mon, Jan 02, 2006 at 07​:43​:32AM -0800, Steve Peters via RT wrote​:

[root@​localhost.localdomain - Sun Dec 25 12​:15​:57 2005]​:

This is a bug report for perl from root@​localhost.localdomain,
generated with the help of perlbug 1.34 running under perl v5.8.0.

-----------------------------------------------------------------
[Please enter your report here]

Brief​:
If an some object is created in eval block under my operator and this
object has a DESTROY method (for example, disconnect database
method) then any threw exceptions in eval block are wiped and lost.
I think that this is incorrect behaviour of perl. The perl should
keep $@​ if in eval there is some DESTROY methods...

To see example below

// Perlover (perlover@​perlover.com)
Example​:

eval

{
my $alex = Alex->new();
$alex->alex();
};

my $error = $@​;

print "error = $error\n";

{
package Alex;

sub new
{
return bless {}, shift;
}

sub DESTROY
{
$@​ = undef;
}

sub alex
{
die "Vasya cool";
}
}

But what if the scenario you show above is exactly what I want? It may
not be the wisest choice in writing a Perl module, but I do not see
where it is a bug. Anyone else have any comments?

You can prevent DESTROY methods from affecting $@​ by starting them with
local $@​;

[p5pRT]

@smpeters - Status changed from 'open' to 'resolved'

[p5pRT]

From mjtg@cam.ac.uk

Yitzchak Scott-Thoennes <sthoenna@​efn.org> wrote

You can prevent DESTROY methods from affecting $@​ by starting them with
local $@​;

This fact is notoriously underdocumented; I suggest the attached.

Mike Guy

Inline Patch

--- ./pod/perlobj.pod.orig	2003-07-10 07:12:41.000000000 +0100
+++ ./pod/perlobj.pod	2006-01-03 18:48:55.479193000 +0000
@@ -433,6 +433,11 @@
 the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>, 
 C<%{$_[0]}> etc.) is not similarly constrained.
 
+Since DESTROY methods can be called at unpredictable times, it is
+important that you localise any global variables that the method may
+update.  In particular, localise C<$@> if you use C<exec {} and
+localise C<$?> if you use C<system> or backticks.
+
 If you arrange to re-bless the reference before the destructor returns,
 perl will again call the DESTROY method for the re-blessed object after
 the current one returns.  This can be used for clean delegation of

End of patch

[p5pRT]

From mjtg@cam.ac.uk

Ooops - typo. Try again.

Mike Guy

Inline Patch

--- ./pod/perlobj.pod.orig	2003-07-10 07:12:41.000000000 +0100
+++ ./pod/perlobj.pod	2006-01-03 18:48:55.479193000 +0000
@@ -433,6 +433,11 @@
 the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>, 
 C<%{$_[0]}> etc.) is not similarly constrained.
 
+Since DESTROY methods can be called at unpredictable times, it is
+important that you localise any global variables that the method may
+update.  In particular, localise C<$@> if you use C<exec {}> and
+localise C<$?> if you use C<system> or backticks.
+
 If you arrange to re-bless the reference before the destructor returns,
 perl will again call the DESTROY method for the re-blessed object after
 the current one returns.  This can be used for clean delegation of

End of patch

[p5pRT]

From mjtg@cam.ac.uk

Joshua ben Jore <twists@​gmail.com> wrote

Didn't you mean eval {}?

Bah! Haven't recovered from New Year's Eve yet. Try 3​:

Mike Guy

Inline Patch

--- ./pod/perlobj.pod.orig	2003-07-10 07:12:41.000000000 +0100
+++ ./pod/perlobj.pod	2006-01-03 18:48:55.479193000 +0000
@@ -433,6 +433,11 @@
 the thingy the reference points to, namely C<${$_[0]}>, C<@{$_[0]}>, 
 C<%{$_[0]}> etc.) is not similarly constrained.
 
+Since DESTROY methods can be called at unpredictable times, it is
+important that you localise any global variables that the method may
+update.  In particular, localise C<$@> if you use C<eval {}> and
+localise C<$?> if you use C<system> or backticks.
+
 If you arrange to re-bless the reference before the destructor returns,
 perl will again call the DESTROY method for the re-blessed object after
 the current one returns.  This can be used for clean delegation of

End of patch

[p5pRT]

From @smpeters

On Tue, Jan 03, 2006 at 07​:18​:41PM +0000, Mike Guy wrote​:

Joshua ben Jore <twists@​gmail.com> wrote

Didn't you mean eval {}?

Bah! Haven't recovered from New Year's Eve yet. Try 3​:

Try 3 added as change #26625.

Thanks!

Steve Peters
steve@​fisharerojo.org

[p5pRT]

From nick@ing-simmons.net

Mike Guy <mjtg@​cam.ac.uk> writes​:

Ooops - typo. Try again.

Mike Guy

--- ./pod/perlobj.pod.orig 2003-07-10 07​:12​:41.000000000 +0100
+++ ./pod/perlobj.pod 2006-01-03 18​:48​:55.479193000 +0000
@​@​ -433,6 +433,11 @​@​
the thingy the reference points to, namely C<${$_[0]}>, C<@​{$_[0]}>,
C<%{$_[0]}> etc.) is not similarly constrained.

+Since DESTROY methods can be called at unpredictable times, it is
+important that you localise any global variables that the method may
+update. In particular, localise C<$@​> if you use C<exec {}> and

Did you mean C<eval> there?

+localise C<$?> if you use C<system> or backticks.
+
If you arrange to re-bless the reference before the destructor returns,
perl will again call the DESTROY method for the re-blessed object after
the current one returns. This can be used for clean delegation of

End of patch