Switch the core MRO code over to HvENAME

This has the side-effect of fixing these one-liners:

$ perl5.13.5 -le' my $glob = \*foo::ISA; delete $::{"foo::"}; *$glob = *a'
Bus error
$ perl5.13.5 -le' my $glob = \*foo::ISA; delete $::{"foo::"}; *$glob = []'
Bus error
$ perl5.13.6 -le'sub baz; my $glob = \*foo::bar; delete $::{"foo::"}; *$glob = *baz;'
Bus error
$ perl5.13.6 -le'sub foo::bar; my $glob = \*foo::bar; delete $::{"foo::"}; *$glob = *baz;'
Bus error

In the first two cases the crash was inadvertently fixed (isn’t it
nice when that happens?) in 5.13.6 (by 6f86b615fa7), but there was
still a fatal error:
Can't call mro_isa_changed_in() on anonymous symbol table at -e line 1.

Because sv_clear calls ->DESTROY, if an object’s stash has been
detached from the symbol table, mro_get_linear_isa can be called on a
hash with no HvENAME. So HvNAME is used as a fallback for those cases.

1 files changed, 3 insertions, 1 deletions

diff --git a/mg.c b/mg.c
index 03ff000325..4a1a72b668 100644
--- a/mg.c
+++ b/mg.c
@@ -1630,7 +1630,9 @@ Perl_magic_clearisa(pTHX_ SV *sv, MAGIC *mg)
             : (const GV *)mg_find(mg->mg_obj, PERL_MAGIC_isa)->mg_obj
     );
 
-    if (stash)
+    /* The stash may have been detached from the symbol table, so check its
+       name before doing anything. */
+    if (stash && HvENAME_get(stash))
 	mro_isa_changed_in(stash);
 
     return 0;