panic: attempt to copy freed scalar a7b9650 to ad20598

mje has asked for the wisdom of the Perl Monks concerning the following question:

I realise this is a tough one but I'm hoping someone might be to give me new ideas on how to track this error down. Background: a very large application written in Perl and happily running on 5.8 on another server. Moved to new server with 5.10.0 AND changed some code to use Try::Tiny and now we get a lot of these panics.

I've tried reproducing the problem in a small example but have failed. That example looks something like this:

use Try::Tiny;
use Log::Log4perl qw(get_logger :levels);
Log::Log4perl->init_and_watch('/etc/log4.conf', 60);
my $log = get_logger('Q::S');

fred();

sub fred
{
    my $x = try {
        dave();
        0;
    } catch {
        my $ev = $_;
        if (0) {
            eval {
                my $a = 1;
                1;
            } or $log->warn("failed $@");
        } else {
            $log->logwarn("test $_");
        }
        1;
   };
   print "$x\n";
   return if $x;  # <---- this would be q.pl line 1895
}

sub dave
{
   # the following line would be DB.pm line 1380
   my ($sth, $options, @args) = @_;
}
[download]

The big problem is we seem to have to run this code a while before it starts happening and then it happens all the time. I can run the code in debug and I get slightly more out of it:

perl -t -I /home/martin/modules/BET/lib q.pl -n1 -p 9999 -r -c /etc/se
+rvices.ini
Attempt to free unreferenced scalar: SV 0xa7b9650, Perl interpreter: 0
+x9ad0008 at q.pl line 1895, <GEN41> line 1.
Failed to insert into job_audit, panic: attempt to copy freed scalar a
+7b9650 to ad20598 at /home/martin/modules/X/lib/X/DB.pm line 1380, <G
+EN42> line 1.
[download]

Any ideas how to identify the freed scalar and track this down?

Comment on panic: attempt to copy freed scalar a7b9650 to ad20598 Select or Download Code

Replies are listed 'Best First'.
Re: panic: attempt to copy freed scalar a7b9650 to ad20598 by BrowserUk (Patriarch) on Dec 03, 2009 at 16:47 UTC
Suggestion: In the real code, inside its equivalent of dave(), put `sub dave { die; ... }` [download] Does that make the failure you are seeing occur? What I'm getting at is does the attempt to copy freed only occur when dave() fails? Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. RIP PCW It is as I've been saying!(Audio until 20090817)	[reply] [d/l]
Re^2: panic: attempt to copy freed scalar a7b9650 to ad20598 by mje (Curate) on Dec 03, 2009 at 17:01 UTC
The sample was not quite right. The sub "dave" is a generic sub which calls a procedure with DBD::Oracle. The call to "dave" in the example does fail but what is missing from the example is that the catch also may call "dave" but with a different procedure name. This is closer: use Try::Tiny; use Log::Log4perl qw(get_logger :levels); Log::Log4perl->init_and_watch('/etc/log4.conf', 60); my $log = get_logger('Q::S'); fred(); sub fred { my $x = try { # dave dies in the cases we get the panic dave('proc1', {}, 'arg1', 'arg2'); 0; } catch { my $ev = $_; if ($sometimes) { # sometimes goes this path and sometimes the + else eval { dave('proc2', {}, 'arg1', 'arg2'); 1; } or $log->warn("failed $@"); } else { $log->logwarn("test $_"); } 1; }; print "$x\n"; return if $x; # <---- this would be q.pl line 1895 } sub dave { # the following line would be DB.pm line 1380 my ($sth, $options, @args) = @_; } [download]	[reply] [d/l]
Re^3: panic: attempt to copy freed scalar a7b9650 to ad20598 by runrig (Abbot) on Dec 03, 2009 at 17:39 UTC
Re: trying to get a simple test case - After trying a simple die in dave(), try doing something simple in DBI, like selecting from a table that doesn't exist, with RaiseError set so that it dies. And with the variables scoped as closely as possible to that of your original program.	[reply]
Re^3: panic: attempt to copy freed scalar a7b9650 to ad20598 by BrowserUk (Patriarch) on Dec 03, 2009 at 17:05 UTC
What happens if you use `"test $ev"` instead?	[reply] [d/l]
Re^4: panic: attempt to copy freed scalar a7b9650 to ad20598 by mje (Curate) on Dec 03, 2009 at 17:09 UTC
Re^5: panic: attempt to copy freed scalar a7b9650 to ad20598 by BrowserUk (Patriarch) on Dec 03, 2009 at 17:13 UTC
Some notes below your chosen depth have not been shown here
Re: panic: attempt to copy freed scalar a7b9650 to ad20598 by gmargo (Hermit) on Dec 03, 2009 at 16:13 UTC
The obvious question is: Which change caused the problem, new perl or Try::Tiny?	[reply]
Re^2: panic: attempt to copy freed scalar a7b9650 to ad20598 by mje (Curate) on Dec 03, 2009 at 16:16 UTC
Problem goes away when I remove the Try::Tiny code and change it back to eval. I'd rather not do that, I'd rather find the problem.	[reply]
Re^3: panic: attempt to copy freed scalar a7b9650 to ad20598 by ikegami (Patriarch) on Dec 03, 2009 at 16:26 UTC
I doubt the problem is with Try::Tiny. This kind of problem is caused by bugs in Perl code or code that uses Perl as a library (XS code, B::, etc), and Try::Tiny doesn't have anything that. In fact, it's a tiny (living up to its name) and straightforward (nothing fancy) Perl module. Start by trying your code on 5.10.1. Lots of bug fixes there.	[reply]
Re^4: panic: attempt to copy freed scalar a7b9650 to ad20598 by mje (Curate) on Dec 03, 2009 at 18:10 UTC
Re^5: panic: attempt to copy freed scalar a7b9650 to ad20598 by ikegami (Patriarch) on Dec 03, 2009 at 20:07 UTC
Some notes below your chosen depth have not been shown here
Re^4: panic: attempt to copy freed scalar a7b9650 to ad20598 by mje (Curate) on Dec 03, 2009 at 16:46 UTC
Re: panic: attempt to copy freed scalar a7b9650 to ad20598 by markjugg (Curate) on Oct 22, 2010 at 23:24 UTC
We got this same symptom today, using Try::Tiny 0.04: `Attempt to free unreferenced scalar: SV 0x82d1fa0 at lib/Try/Tiny.pm l +ine 76. Attempt to free unreferenced scalar: SV 0x82d1fc0 at lib/Try/Tiny.pm l +ine 76. Attempt to free unreferenced scalar: SV 0x83d4060 at lib/Try/Tiny.pm l +ine 76. Attempt to free unreferenced scalar: SV 0x83d4080 at lib/Try/Tiny.pm l +ine 76. Segmentation fault (core dumped)` [download] We haven't found the root cause yet. Mark Stosberg	[reply] [d/l]
Re^2: panic: attempt to copy freed scalar a7b9650 to ad20598 by markjugg (Curate) on Oct 22, 2010 at 23:55 UTC
I should mention: We are using Perl 5.8.8 on that server. Also, here's a third case of it happening, in the Fennec test suite. Search through CPAN tester data via Google may find more:	[reply]
Re^2: panic: attempt to copy freed scalar a7b9650 to ad20598 by mje (Curate) on Oct 25, 2010 at 09:50 UTC
In my case there was no problem in Try::Tiny but I had declared a variable like this `my $thing if (some_test);` which is really bad. The changes I made to introduce Try::Tiny seemed to have triggered the error to be seen.	[reply] [d/l]

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