"Stephen Clouse" <stephenclouse@(protected):
> Under [mod_perl 2 and perl 5.10 on Fedora 9] I am getting some of the
> most bizarre and insidious perl core errors I've ever seen in my 15
> years of using perl.
>
> Attempt to free unreferenced scalar: SV 0xbd266be4, Perl interpreter:
> 0xba01c410 at /usr/lib/perl5/vendor_perl/5.10.0/HTML/Mason/Component.pm
> line
> 147.
> panic: attempt to copy value 20 to a freed scalar bd49dffc
> Use of freed value in iteration
> panic: free from wrong pool
>
> Not to mention countless application errors involving dereferencing.

Dereferencing demons, eh? Your error rang a bell with me. But it wasn't
until I saw List/Util/Util.so in your follow-up post that I remembered,
List::Util did that same scary crashing thing to me a while back while I
was debugging krang (which is mod_perl 1.3x-based).

It boiled down to the (IMO) senseless and unnecessary use of weak
referrences somewhere deep in the dark bowels of the Scalar-List-Utils
package. I'd read somewhere that weak references were not all that stable
in certain recent versions of perl, so when I saw all the weakening being
done in there (and all the XS bugs the module has had to fix, in its change
log:

http://search.cpan.org/src/GBARR/Scalar-List-Utils-1.19/Changes

At the time, upgrading the module to the latest release didn't stop my big
scary memory reference errors. So I read through the code that used the
module and was astonished at why the module was being used: the *one* line
of code in the entire module that I was troubleshooting that *depended* on
List::Util was a single, simple call to its exported method: first()

What does first() do? It invokes kilobytes of fast, compiled XS code to
...huh? Return to me the first element in a list I pass to it that returns
true for a code block I pass to it? Yup. Like this:

# from the POD doco at
#
http://search.cpan.org/~gbarr/Scalar-List-Utils-1.19/lib/List/Util.pm#DESCRIPTION

$foo = first { defined($_) } @list; # first defined value in @list


Who needs to install a CPAN module to do that? I personally would have
written it as:

$foo = (grep defined, @list)[0]; # first defined value in @list


But thats kind of non-obvious to the programmer who finds it later (if they
dont read the comment, that is). You could also simply replace "first"
with "grep" (and a pair of parens) and get the same effect:

($foo) = grep { defined($_) } @list; # first defined value in @list

(at the cost of one extra byte and some wasted cpu cycles if the compiler
doesn't notice that the lvalue is a list of length one, and lets grep
process the whole list anyway, even after it finds and assigns a defined
element in there).

IMO, even this would be better than loading List::Utils and calling
first():

my $foo;
for (@(protected)) {
  next unless defined($_);
  $foo = $_;
  last;
}

So I fixed my bug by writing a line or two of perl to in order to remove
one call to the List::Util module's first() function.


> My problem is that I don't even know where or how to start tracing or
> debugging such an issue to determine precisely what is scribbling on
> perl's
> memory.

Starting from your error message (line 147 of HTML::Mason::Component) it
appears HTML::Mason is calling a "dynamic method" method (one named in a
variable). No help there. Okay, what else is HTML::Mason::Component
using? What libraries has it loaded? Is List::Utils being used?

This looked suspicious to me:

use HTML::Mason::Tools qw(absolute_comp_path can_weaken);

Just because you "can_weaken" a reference, does that mean you should? Sure
enough HTML::Mason::Tools loads Scalar::Util apparently to do some
reference weakening.

I'd check my Mason component for use of the functions that the
Scalar-List-Utils modules provide:

List::Util

first max maxstr min minstr reduce shuffle sum

Scalar::Util

blessed dualvar isweak readonly refaddr reftype
tainted weaken isvstring looks_like_number set_prototype

And see if rewriting these in pure perl helps. Also, if it does, but would
require changing code that you'd rather not monkey with, it looks like
there is a pure-perl version of the module that can be installed by passing
a -pm option to Makefile.PL:

perl Makefile.PL -pm
make
make test
make install

Anyway, I hope somehow, some of this helps!

-dave