Bug#816280: Binary incompatibility between debugperl and perl

[Nick Wellnhofer]

Package: perl-debug
Version: 5.20.2-2

The layout of interpreter variables is different in the debug and normal 
version of the perl binary. This means that XS extensions might read from or 
write to the wrong area of the interpreter variable struct, causing crashes 
and other strange behavior.

Here's an example, originally reported by me at 
https://rt.cpan.org/Public/Bug/Display.html?id=111211

     $ PERL_DESTRUCT_LEVEL=2 debugperl -MList::Util=shuffle -e shuffle
     Segmentation fault

This segfaults because `shuffle` calls `seedDrand01` which writes to the 
`random_state` interpreter variable. If you have a look at `intrpvar.h` in the 
Perl source, you'll see that `random_state` is at the very end with some 
variables before that are only enabled in debug builds. This causes 
`random_state` to be at a different offset in the debug build. The XS module 
`List::Util` uses the non-debug offset, so any writes to `random_state` cause 
memory corruption.

I'd propose that all interpreter variables that are only used in the debug 
build should be moved to very end in `intrpvar.h`.