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List:       perl5-porters
Subject:    Re: [perl #124097] Attempt to free unreferenced scalar: SV 0x5eb03f8 (memory corruption)
From:       Dave Mitchell <davem () iabyn ! com>
Date:       2015-11-30 11:41:42
Message-ID: 20151130114142.GS2958 () iabyn ! com
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On Mon, Nov 30, 2015 at 10:14:31AM +1100, Tony Cook wrote:
> On Fri, Nov 27, 2015 at 01:42:30PM +0000, Dave Mitchell wrote:
> > On Wed, Nov 25, 2015 at 08:24:22PM -0800, Tony Cook via RT wrote:
> > > Like the attached?
> > 
> > > +                /* make sure the GP isn't removed out from under us */
> > > +                first_copy = (GV*)newSVsv(MUTABLE_SV(PL_firstgv));
> > > +                second_copy = (GV*)newSVsv(MUTABLE_SV(PL_secondgv));
> > > +                SAVEFREESV(first_copy);
> > > +                SAVEFREESV(second_copy);
> > 
> > Wouldn't save_gp(PL_firstgv, 0) be more efficient?
> 
> It would, but it also restores the GP to the GV, and the user has
> chosen to replace it - should we be restoring it?
> 
> With save_gp:
> 
> $ ./perl -e 'sub a { print "hello\n" } a(); @x = sort { *a = sub { print \
> "goodbye\n" }; 1 } 0 .. 1; a();' hello
> hello
> 
> With the patch I supplied:
> 
> $ ./perl -e 'sub a { print "hello\n" } a(); @x = sort { *a = sub { print \
> "goodbye\n" }; 1 } 0 .. 1; a();' hello
> goodbye

I'd expect sort {block} 1, 2  to have the same semantics as:

    {
        local *a = \1;
        local *b = \2;
        { block }
    }


'local *a = ...' does a combination of

    save_gp(gv,0) via rv2gv

and

    SAVEGENERICSV(SvGV(gv)) via pp_sassign and gv_setref,

so I'd expect some combination of those to Do The Right Thing.

And local does seem to Do The Right Thing:

    sub a { print "hello\n" }
    a();
    {
        local *a = \1;
        {
            *a = sub { print "goodbye\n" };
            1;
        }
    }
    a()

prints:

    hello
    goodbye

So maybe there's a way to achieve this with save_gp?

-- 
In England there is a special word which means the last sunshine
of the summer. That word is "spring".


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