[perl #112458] Undefining *^H causes hints to leak to outer scopes

# New Ticket Created by  Father Chrysostomos 
# Please include the string:  [perl #112458]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org:443/rt3/Ticket/Display.html?id=112458 >


We have an interesting comment (and following code) in scope.c:Perl_leave_scope:SAVEt_HINTS:

		/* Need to add a new one manually, else gv_fetchpv() can
		   add one in this code:
		   
		   if (SvTYPE(gv) == SVt_PVGV) {
		       if (add) {
		       GvMULTI_on(gv);
		       gv_init_sv(gv, sv_type);
		       if (*name=='!' && sv_type == SVt_PVHV && len==1)
			   require_errno(gv);
		       }
		       return gv;
		   }

		   and it won't have the magic set.  */

		HV *const hv = newHV();
		hv_magic(hv, NULL, PERL_MAGIC_hints);
		GvHV(PL_hintgv) = hv;

But any user code could make GvHV(PL_hintgv) null, triggering the bug that those three lines of code are trying to avoid.

I think the solution is simply to make gv_fetchpvn_flags add magic to %^H, instead of trying to work around the fact that it doesn’t.

This is the type of bug that results from this broken logic:

#!perl -X
{
    BEGIN {
	undef *^H;
	$^H{bar} = 'baz';
    }
}
BEGIN { warn $_ for keys %^H };  # bar at - line 8.
---
Flags:
    category=core
    severity=low
---
Site configuration information for perl 5.15.9:

Configured by sprout at Tue Apr 10 09:26:12 PDT 2012.

Summary of my perl5 (revision 5 version 15 subversion 9) configuration:
  Snapshot of: 2653c1e3b10f71430de1ab8ab0417ca5b048ab19
  Platform:
    osname=darwin, osvers=10.5.0, archname=darwin-2level
    uname='darwin pint.local 10.5.0 darwin kernel version 10.5.0: fri nov 5 23:20:39 pdt 2010; root:xnu-1504.9.17~1release_i386 i386 '
    config_args='-de -Dusedevel -DDEBUGGING'
    hint=recommended, useposix=true, d_sigaction=define
    useithreads=undef, usemultiplicity=undef
    useperlio=define, d_sfio=undef, uselargefiles=define, usesocks=undef
    use64bitint=undef, use64bitall=undef, uselongdouble=undef
    usemymalloc=n, bincompat5005=undef
  Compiler:
    cc='cc', ccflags ='-fno-common -DPERL_DARWIN -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include',
    optimize='-O3 -g',
    cppflags='-fno-common -DPERL_DARWIN -DDEBUGGING -fno-strict-aliasing -pipe -fstack-protector -I/usr/local/include'
    ccversion='', gccversion='4.2.1 (Apple Inc. build 5664)', gccosandvers=''
    intsize=4, longsize=4, ptrsize=4, doublesize=8, byteorder=1234
    d_longlong=define, longlongsize=8, d_longdbl=define, longdblsize=16
    ivtype='long', ivsize=4, nvtype='double', nvsize=8, Off_t='off_t', lseeksize=8
    alignbytes=8, prototype=define
  Linker and Libraries:
    ld='env MACOSX_DEPLOYMENT_TARGET=10.3 cc', ldflags =' -fstack-protector -L/usr/local/lib'
    libpth=/usr/local/lib /usr/lib
    libs=-ldbm -ldl -lm -lutil -lc
    perllibs=-ldl -lm -lutil -lc
    libc=, so=dylib, useshrplib=false, libperl=libperl.a
    gnulibc_version=''
  Dynamic Linking:
    dlsrc=dl_dlopen.xs, dlext=bundle, d_dlsymun=undef, ccdlflags=' '
    cccdlflags=' ', lddlflags=' -bundle -undefined dynamic_lookup -L/usr/local/lib -fstack-protector'

Locally applied patches:
    

---
@INC for perl 5.15.9:
    /usr/local/lib/perl5/site_perl/5.15.9/darwin-2level
    /usr/local/lib/perl5/site_perl/5.15.9
    /usr/local/lib/perl5/5.15.9/darwin-2level
    /usr/local/lib/perl5/5.15.9
    /usr/local/lib/perl5/site_perl
    .

---
Environment for perl 5.15.9:
    DYLD_LIBRARY_PATH (unset)
    HOME=/Users/sprout
    LANG=en_US.UTF-8
    LANGUAGE (unset)
    LD_LIBRARY_PATH (unset)
    LOGDIR (unset)
    PATH=/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin:/usr/local/bin
    PERL_BADLANG (unset)
    SHELL=/bin/bash

• [perl #112458] Undefining *^H causes hints to leak to outer scopes by Father Chrysostomos