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Sep 1, 2012, 4:29 PM
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[perl #49115] $_[0] seems to get reused inappropriately
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On Wed Dec 26 12:09:18 2007, schmorp [at] schmorp wrote:
>
> This is a bug report for perl from schmorp [at] schmorp,
> generated with the help of perlbug 1.35 running under perl v5.8.8.
>
>
> -----------------------------------------------------------------
> [Please enter your report here]
>
> Hi!
>
> I cannot provide a testcase for this bug. I can only provide a
> description
> and hope that somebody with enough experience might spot the problem
> quickly.
>
> If you can't, do not hesitate to just close this bug.
>
> (I might re-try with 5.10 which I plan to install soon, but I will
> likely
> not find the time).
>
> In any case, in a fairly complex (perl-wise) program, I added a
> Carp::cluck call, and got an exception:
>
> Bizarre copy of ARRAY in sassign at
> /opt/perl/lib/perl5/Carp/Heavy.pm line 104.
>
> I prepended this code to the cluck call:
>
> for(1..10) {
> package DB;
> @DB::args=();
> print join " ", $_, caller $_; print "\n";
> print join ":", " ", @DB::args; print "\n";#d#
> @DB::args=();
> }
> Carp::cluck;#d#
>
> And got the following output:
> http://ue.tst.eu/7b8f67644b1cfc209f013991fac35a30.txt
>
> Note the double :: in "4". Digging with gdb showed that carp indeed
> tried
> to copy this very value. When dumped, I get:
>
> SV = NULL(0x0) at 0x202e570
> REFCNT = 2
> FLAGS = (PADBUSY,PADMY)
>
> (which is not an array..., so maybe I was wrong - unfortunately gcc
> optimised most local variables away)
>
> Now, this is during a hide method call. The hide method in question
> looks like this (including the dump call):
>
> # sv_dump simply calls sv_dump (SvROK (arg) ? SvRV (arg) : arg);
> use PApp::Util; PApp::Util::sv_dump $self->{parent};
>
> $self->{parent}->remove ($self)
> if $self->{parent};
>
> background: $self is a widget, $self->{parent} is a widget, and a side
> effect
> of remove is to:
>
> delete $child->{parent}; # $child == the above $self
>
> Also, $self->{parent} is a weak reference, so I do not get an unwanted
> reference cycle.
>
> When I use sv_dump on $self->{parent}, I get this:
> http://ue.tst.eu/2936444271424c67b6a4e8ab5bea6fd9.txt
>
> SV = PVLV(0x9c80f8) at 0x1951b50 ...
>
> This is the $self->{parent} scalar that later goes "missing".
>
> When I reformulate the hide method to:
>
> if (my $parent = $self->{parent}) {
> $parent->remove ($self);
> }
>
> then I no longer get any crash.
>
> My theory is that somehow, $_[0] is not correctly being refcounted, or
> for soem other reason gets freed (later reused and then replaced by
> e.g.
> an AV). When I increase the refcount artificially by copying it to a
> my
> variable, all is well as it never gets freed.
>
> I must admit my knowledge of weak references is not too good, and I
> cannot
> understand why I get a PVLV there at all.
>
> My hope is that somebody else can pinpoint the bug or at least come up
> with a likely explanation I could test. I am sorry that I cannot work
> on
> this more right now.
>
> And again, feel free to close this report if no solution is
> forthcoming.
>
Well, no solution has been forthcoming in the 4+ years since this ticket
was originally filed.
I recommend that we close this ticket unless we can get a report of a
reproducible bug within the next 30 days.
Thank you very much.
Jim Keenan
---
via perlbug: queue: perl5 status: new
https://rt.perl.org:443/rt3/Ticket/Display.html?id=49115
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