From 60eeb4a4ff217e3abbfcb6b4de94f0a6ef6d437d Mon Sep 17 00:00:00 2001
From: David Kastrup <dak@gnu.org>
Date: Wed, 18 Feb 2015 16:40:49 +0100
Subject: [PATCH] Issue 4293/3: smobs.tcc: robustify {mark,print}_trampoline in
 connection with free_smob

Guilev2 has little qualms calling mark/print even after freeing a smob.
So we clean out the pointer to smob data when freeing/unregistering it,
and let the mark/print trampolines detect this condition and punt.
---
 lily/include/smobs.tcc | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/lily/include/smobs.tcc b/lily/include/smobs.tcc
index c0a62535f5..b4dacacaa3 100644
--- a/lily/include/smobs.tcc
+++ b/lily/include/smobs.tcc
@@ -32,14 +32,20 @@ template <class Super>
 SCM
 Smob_base<Super>::mark_trampoline (SCM arg)
 {
-  return (Super::unsmob (arg))->mark_smob ();
+  Super *ptr = Super::unsmob (arg);
+  if (ptr)
+    return ptr->mark_smob ();
+  return SCM_UNDEFINED;
 }
 
 template <class Super>
 int
 Smob_base<Super>::print_trampoline (SCM arg, SCM port, scm_print_state *p)
 {
-  return (Super::unsmob (arg))->print_smob (port, p);
+  Super *ptr = Super::unsmob (arg);
+  if (ptr)
+    return ptr->print_smob (port, p);
+  return 0;
 }
 
 template <class Super>
@@ -96,6 +102,7 @@ Smob_base<Super>::unregister_ptr (SCM obj)
 {
   Super *p = Super::unchecked_unsmob (obj);
   scm_gc_unregister_collectable_memory (p, sizeof (*p), smob_name_.c_str ());
+  SCM_SET_SMOB_DATA (obj, static_cast<Super *> (0));
   return p;
 }
 
-- 
2.39.2