SEGFAULTs in S_mg_findext_flags because obj_deref in line 110 is a broken SV.

gistfile1.c

#include <EXTERN.h>
#include <XSUB.h>
#include <perl.h>

static void xs_init (pTHX);
EXTERN_C void boot_DynaLoader (pTHX_ CV* cv);
EXTERN_C void boot_Socket (pTHX_ CV* cv);

XS(p5_call_p6_method);

EXTERN_C void xs_init(pTHX) {
    char *file = __FILE__;
    newXS("Perl6::Object::call_method", p5_call_p6_method, file);
}

PerlInterpreter *my_perl;

PerlInterpreter *p5_init_perl() {
    char *embedding[] = { "", "-e", "0" };
    PERL_SYS_INIT3(0, NULL, NULL);
    my_perl = perl_alloc();
    PERL_SET_CONTEXT(my_perl);
    PL_perl_destruct_level = 1;
    perl_construct( my_perl );
    perl_parse(my_perl, xs_init, 3, embedding, NULL);
    PL_exit_flags |= PERL_EXIT_DESTRUCT_END;
    perl_run(my_perl);
    return my_perl;
}

AV *p5_call_function(PerlInterpreter *my_perl, char *name, int len, SV *args[]) {
    dSP;
    int i;
    int count;
    AV * const retval = newAV();
    int flags = G_ARRAY | G_EVAL;

    PERL_SET_CONTEXT(my_perl);

    ENTER;
    SAVETMPS;

    PUSHMARK(SP);

    for (i = 0; i < len; i++) {
        XPUSHs(sv_2mortal(args[i]));
    }

    PUTBACK;

    count = call_pv(name, flags);

    return retval;
}

typedef struct {
    I32 key; /* to make sure it came from Inline */
    IV index;
    SV *(*call_p6_method)(int, char *, SV *, SV **);
} _perl6_magic;

#define PERL6_MAGIC_KEY 0x0DD515FE

SV *p5_wrap_p6_object(PerlInterpreter *my_perl, IV i, SV *p5obj, SV *(*call_p6_method)(int, char * , SV *, SV **)) {
    SV * inst;
    SV * inst_ptr;
    if (p5obj == NULL) {
        inst_ptr = newSViv(0);
        inst = newSVrv(inst_ptr, "Perl6::Object");
    }
    else {
        inst_ptr = p5obj;
        inst = SvRV(inst_ptr);
        SvREFCNT_inc(inst_ptr);
    }
    _perl6_magic priv;

    /* set up magic */
    priv.key = PERL6_MAGIC_KEY;
    priv.index = i;
    priv.call_p6_method = call_p6_method;
    sv_magic(inst, inst, PERL_MAGIC_ext, (char *) &priv, sizeof(priv));
    MAGIC * const mg = mg_find(inst, PERL_MAGIC_ext);

    return inst_ptr;
}

SV *call_p6_method(int obj, char *name, SV *i, SV **args) {
}

XS(p5_call_p6_method) {
    dXSARGS;
    SV * name = ST(0);
    SV * obj = ST(1);

    AV * args = newAV();
    av_extend(args, items - 2);
    int i;
    for (i = 0; i < items - 2; i++) {
        SV * const next = SvREFCNT_inc(ST(i + 2));
        if (av_store(args, i, next) == NULL)
            SvREFCNT_dec(next); /* see perlguts Working with AVs */
    }

    STRLEN len;
    char * const name_pv  = SvPV(name, len);
    char * const name_str = savepvn(name_pv, len);

    SV * const obj_deref = SvRV(obj);
    MAGIC * const mg = mg_find(obj_deref, '~');
    _perl6_magic* const p6mg = (_perl6_magic*)(mg->mg_ptr);
    SV *err = NULL;
    return newRV_inc((SV*)p5_call_function(my_perl, "zero", 0, NULL));
}

int main(int argc, char **argv) {
    PerlInterpreter *my_perl = p5_init_perl();
    eval_pv("\
        package Perl6::Object;\
\
        our $AUTOLOAD;\
        sub AUTOLOAD {\
            my ($self) = @_;\
            warn qq{AUTOLOAD $AUTOLOAD};\
            my $name = $AUTOLOAD =~ s/.*:://r;\
            Perl6::Object::call_method($name, @_);\
        }\
\
    ", 1);

    eval_pv("\
        sub zero {\
            warn 'zero';\
            return 1;\
        }\
        sub one {\
            my ($obj) = @_;\
            warn 'callback one';\
            return $obj->foo;\
        }\
    ", 1);

    SV *callme = p5_wrap_p6_object(my_perl, 0, NULL, &call_p6_method);
    p5_call_function(my_perl, "one", 1, &callme);
}