From fc0fe26a7d286480c1bb25f57e469ece575bb68d Mon Sep 17 00:00:00 2001
From: David Mitchell <davem@iabyn.com>
Date: Thu, 7 Jul 2016 17:03:29 +0100
Subject: [PATCH] SEGV in "Subroutine redefined" warning
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RT #128257

The following SEGVed:

    sub P::f{}
    undef *P::;
    *P::f =sub{};

due to the code which generates the "Subroutine STASH::NAME redefined"
warning assuming that the GV always has a stash. Make it so that if it
hasn't, the message changes to  "Subroutine NAME redefined" rather than
just crashing.

Signed-off-by: Petr Písař <ppisar@redhat.com>
---
 sv.c              | 18 +++++++++++-------
 t/lib/warnings/sv |  8 ++++++++
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/sv.c b/sv.c
index 1b7a283..0cbe371 100644
--- a/sv.c
+++ b/sv.c
@@ -4074,14 +4074,18 @@ Perl_gv_setref(pTHX_ SV *const dstr, SV *const sstr)
 			    CvCONST((const CV *)sref)
 				 ? cv_const_sv((const CV *)sref)
 				 : NULL;
+                        HV * const stash = GvSTASH((const GV *)dstr);
 			report_redefined_cv(
-			   sv_2mortal(Perl_newSVpvf(aTHX_
-				"%"HEKf"::%"HEKf,
-				HEKfARG(
-				 HvNAME_HEK(GvSTASH((const GV *)dstr))
-				),
-				HEKfARG(GvENAME_HEK(MUTABLE_GV(dstr)))
-			   )),
+			   sv_2mortal(
+                             stash
+                               ? Perl_newSVpvf(aTHX_
+				    "%"HEKf"::%"HEKf,
+				    HEKfARG(HvNAME_HEK(stash)),
+				    HEKfARG(GvENAME_HEK(MUTABLE_GV(dstr))))
+                               : Perl_newSVpvf(aTHX_
+				    "%"HEKf,
+				    HEKfARG(GvENAME_HEK(MUTABLE_GV(dstr))))
+			   ),
 			   cv,
 			   CvCONST((const CV *)sref) ? &new_const_sv : NULL
 			);
diff --git a/t/lib/warnings/sv b/t/lib/warnings/sv
index 5ddd4fe..c8e0e62 100644
--- a/t/lib/warnings/sv
+++ b/t/lib/warnings/sv
@@ -413,3 +413,11 @@ Argument "a_c" isn't numeric in preincrement (++) at - line 5.
 Argument "(?^:abc)" isn't numeric in preincrement (++) at - line 6.
 Argument "123x" isn't numeric in preincrement (++) at - line 7.
 Argument "123e" isn't numeric in preincrement (++) at - line 8.
+########
+# RT #128257 This used to SEGV
+use warnings;
+sub Foo::f {}
+undef *Foo::;
+*Foo::f =sub {};
+EXPECT
+Subroutine f redefined at - line 5.
-- 
2.5.5