Fix assertion failure with undef &my_sub/&anon

$ ./perl -Ilib -le 'use experimental lexical_subs; my sub x; undef &x;'
Assertion failed: (isGV_with_GP(_gvname_hek)), function Perl_leave_scope, file scope.c, line 1035.
Abort trap: 6

pp_undef undefines a subroutine via cv_undef, which wipes out the
name, and then restores the name again afterwards.

For subs with GVs, it would call CvGV_set afterwards with the same gv.
But cv_undef could have freed the GV, if the CV held the only refer-
ence count.

I caused this for lexical subs a few commits ago in ae77754ae (because
CvGV will always return non-null; in fact the CvNAME_HEK code in
pp_undef is no longer exercised, but I will address that soon).

For anonymous subs it is older:

$ perl5.14.4 -e '$_ = sub{}; delete $::{__ANON__}; undef &$_; use Devel::Peek; Dump $_'
SV = IV(0x7fed9982f9c0) at 0x7fed9982f9d0
  REFCNT = 1
  FLAGS = (ROK)
  RV = 0x7fed9982f9e8
  SV = PVCV(0x7fed9982e290) at 0x7fed9982f9e8
    REFCNT = 2
    FLAGS = (PADMY,WEAKOUTSIDE,CVGV_RC)
    COMP_STASH = 0x7fed99806b68	"main"
    ROOT = 0x0
    GVGV::GV = 0x7fed9982fa48Assertion failed: (isGV_with_GP(_gvname_hek)), function Perl_do_gvgv_dump, file dump.c, line 1477.
Abort trap: 6

(Probably commit 803f2748.)

Presumably that could be made to crash in other ways than introspec-
tion, but it is much harder.

This commit fixes the problem by fiddling with reference counts.  But
this is only a temporary fix.  I think I plan to stop cv_undef from
removing the name (gv/hek) when called from pp_undef.

1 files changed, 9 insertions, 1 deletions

diff --git a/t/op/lexsub.t b/t/op/lexsub.t
index 1efcf1c336..d2edb79bab 100644
--- a/t/op/lexsub.t
+++ b/t/op/lexsub.t
@@ -7,7 +7,7 @@ BEGIN {
     *bar::is = *is;
     *bar::like = *like;
 }
-plan 133;
+plan 135;
 
 # -------------------- Errors with feature disabled -------------------- #
 
@@ -376,6 +376,10 @@ like runperl(
     'state subs and DB::sub under -d'
   );
 }
+# This used to fail an assertion, but only as a standalone script
+is runperl(switches => ['-lXMfeature=:all'],
+           prog     => 'state sub x {}; undef &x; print defined &x',
+           stderr   => 1), "\n", 'undefining state sub';
 
 # -------------------- my -------------------- #
 
@@ -719,6 +723,10 @@ pass "pad taking ownership once more of packagified my-sub";
     'my subs and DB::sub under -d'
   );
 }
+# This used to fail an assertion, but only as a standalone script
+is runperl(switches => ['-lXMfeature=:all'],
+           prog     => 'my sub x {}; undef &x; print defined &x',
+           stderr   => 1), "\n", 'undefining my sub';
 
 # -------------------- Interactions (and misc tests) -------------------- #