[perl #85670] Copy magic to ary elems properly
On Tue Mar 08 07:26:35 2011, thospel wrote:
> #!/usr/bin/perl -l
> use Data::Dumper;
> use Scalar::Util qw(weaken);
> our @ISA;
>
> for (1..2) {
> @ISA = qw(Foo);
> weaken($a = \@ISA);
> weaken($a = \$ISA[0]);
> print STDERR Dumper(\@ISA);
> }
>
> This prints:
> $VAR1 = [
> 'Foo'
> ];
> $VAR1 = [
> 'Foo',
> \$VAR1->[0]
> ];
>
> So the first time it's the expected @ISA, but the second time round it
> automagically added a reference to to the first ISA element
>
> (bug also exists in blead)
Shorter:
#!/usr/bin/perl -l
use Scalar::Util qw(weaken);
weaken($a = \@ISA);
@ISA = qw(Foo);
use Devel::Peek; Dump \@ISA;
weaken($a = \$ISA[0]);
print scalar @ISA; # prints 2
The dump shows the problem. backref magic is being copied to the ele-
ment. Put the magic in a different order, and everything is fine:
#!/usr/bin/perl -l
use Scalar::Util qw(weaken);
weaken($a = $b = []);
*ISA = $a;
@ISA = qw(Foo);
use Devel::Peek; Dump \@ISA;
weaken($a = \$ISA[0]);
print scalar @ISA; # prints 2
This code in av_store is so wrong:
if (SvSMAGICAL(av)) {
const MAGIC* const mg = SvMAGIC(av);
if (val != &PL_sv_undef) {
sv_magic(val, MUTABLE_SV(av), toLOWER(mg->mg_type), 0, key);
}
if (PL_delaymagic && mg->mg_type == PERL_MAGIC_isa)
PL_delaymagic |= DM_ARRAY_ISA;
else
mg_set(MUTABLE_SV(av));
}
It doesn’t follow the magic chain at all. So anything magic could get
attached to the @ISA array, and that will be copied to the element
instead of isa magic.
Notice that MUTABLE_SV(av) is the second argument to sv_magic, so
mg->mg_obj for the element always points back to the array.
Since backref magic’s mg->mg_obj points to the backrefs array, @ISA
ends up being used as this element’s backrefs array.
What if arylen_p gets copied instead? Let’s see:
$#ISA = -1;
@ISA = qw(Foo);
$ISA[0] = "Bar";
main->ber;
sub Bar::ber { warn "shave" }
__END__
Can't locate object method "ber" via package "main" at - line 7.
I’ve fixed this by making av_store walk the magic chain, copying any
magic for which toLOWER(mg->mg_type) != mg->mg_type.