Tim.Bunce at pobox
Nov 10, 2009, 8:16 AM
Post #30 of 30
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Re: [perl #60374] Safe.pm sort {} bug with -Dusethreads
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On Tue, Nov 10, 2009 at 02:56:36PM +0000, Nicholas Clark wrote:
> >
> > http://rt.perl.org/rt3/Public/Bug/Display.html?id=60374#txn-491794 says
> > "pp_sort uses CopSTASH(PL_curcop) to get the package stash (symbol
> > table) in order to locate the $a and $b variables in it."
> >
> > While pp_sort does call CopSTASH(PL_curcop) directly, all it does with
> > it is store the value in PL_sortstash which, as far as I can tell, isn't
> > used or documented anywhere at all. (It could possibly be deleted.)
> >
> > The code to lookup the $a and $b uses:
> > PL_firstgv = gv_fetchpvs("a", GV_ADD|GV_NOTQUAL, SVt_PV);
> > PL_secondgv = gv_fetchpvs("b", GV_ADD|GV_NOTQUAL, SVt_PV);
> > and it's Perl_gv_fetchpvn_flags that ends up calling CopSTASH(PL_curcop).
> >
> > If there was a way to get the stash pointer (not name) that was in
> > effect at the time the code was compiled we could use that to lookup $a
> > and $b.
>
> The logical way to do this would be to indirect via an entry in the
> subroutine's pad, as the reference would be correctly translated by the
> ithreads clone code. This probably isn't *trivial* to implement, as a
> subroutine can contain code compiled in more than one package, but I think*
> it would be viable at compile time to loop up/store if not there yet, the
> stash pointer, under the stash's name.
>
> > There are stash pointers, er, stashed in lots of places.
> > One approach might be CvSTASH(find_runcv(...))
>
> I think that that doesn't cope with
>
> package foo;
> sub bar {
> # here package is foo
> package baz;
> # here package is baz
> }
I couldn't get it to work even without that (but trying to follow the
inside-safe vs outside-safe and compile-time vs run-time intricacies
made my rusty old head spin, so I could have done something silly).
Alex's approach, of arranging to execute the closure 'inside' the
compartment, seems like a good idea from a 'safety' point of view
anyway. The fact it works around the bug is a bonus.
On the other hand, the fact it costs two extra sub calls is unfortunate.
I tried to look for a way to the PL_defstash switch into pp_entersub but
couldn't see a viable approach.
> * I think. I don't know this stuff well.
Me neither. It's been many years since I helped develop Opcode and Safe.
(I recall refering to Safe as a "failed experiment" more than once.)
Tim.
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