segfault in perl_clone under ithreads and custom PerlIO layer

[Stas Bekman]

We have yet another chicken-n-egg problem with perl_clone. If you do a
simple:
use threads;
my $thread = threads->create(sub {}, undef)
and STDOUT is opened to some PerlIO layer, an attempt to clone it will be 
performed. The problem is that this cloning happens too early:

perl-5.8.6/sv.c:10859:
#ifdef PERLIO_LAYERS
    /* Clone PerlIO tables as soon as we can handle general xx_dup() */
    PerlIO_clone(aTHX_ proto_perl, param);
#endif
xx_dup() works for the core perl, but it doesn't
take into account that a custom PerlIO layer may need other bits set
in my_perl. In the above case I get a segfault because PL_defstash is
bogus. it's set only after PerlIO_clone is called. (line 10954)
I get this segfault:
#0  0x404c8a04 in Perl_gv_fetchpv (my_perl=0x85e6540,
    nambeg=0x404939d0 "Apache::RequestRec", add=1, sv_type=11) at gv.c:670
#1  0x40479f37 in PerlIOApache_getarg (my_perl=0x85e6540, f=0x828634c,
    param=0xbfffe9c0, flags=1) at modperl_io_apache.c:87
#2  0x405c55b5 in PerlIOBase_dup (my_perl=0x85e6540, f=0x83576c4, o=0x828634c,
    param=0xbfffe9c0, flags=1) at perlio.c:2187
#3  0x405c1e50 in PerlIO_fdupopen (my_perl=0x85e6540, f=0x828634c,
    param=0xbfffe9c0, flags=1) at perlio.c:542
#4  0x40555faf in Perl_fp_dup (my_perl=0x85e6540, fp=0x828634c, type=0 '\0',
    param=0xbfffe9c0) at sv.c:9505
#5  0x405c22a5 in PerlIO_clone (my_perl=0x85e6540, proto=0x8abfb10,
    param=0xbfffe9c0) at perlio.c:650
#6  0x4055a6fe in perl_clone (proto_perl=0x8abfb10, flags=2) at sv.c:10859
#7  0x4078aa50 in Perl_ithread_create (my_perl=0x8abfb10, obj=0x0,
    classname=0x82dc948 "threads", init_function=0x8153728, params=0x815377c)
    at threads.xs:426
#8  0x4078b3f7 in XS_threads_new (my_perl=0x8abfb10, cv=0x82f0698)
    at threads.xs:687
the segfault happens here:
            if (!stash)
                stash = PL_defstash;
            if (!stash || !SvREFCNT(stash)) /* symbol table under destruction */
PL_defstash == 0xabababab here
PerlIOApache_getarg happens to call gv_fetchpv which needs a valid
PL_defstash.
How do we resolve this problem?
This is not the first time we hit a segfault, due to
PerlIO+clone. Some of those are still unresolved (e.g. perl -m still
segfaults under threads)
--
__________________________________________________________________
Stas Bekman            JAm_pH ------> Just Another mod_perl Hacker
http://stason.org/     mod_perl Guide ---> http://perl.apache.org
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