From: Eric Wong <normalperson@...>
Date: 2011-07-25T23:13:06+09:00
Subject: [ruby-core:38498] Re: [Ruby 1.9 - Bug #5047] Segfault (most likely involving require)

Yusuke ENDOH <mame@tsg.ne.jp> wrote:
> 2011/7/25 Eric Wong <normalperson@yhbt.net>:
> > sidenote: I didn't realize xmalloc() (via rb_unlink_method_entry) is
> > safe/allowed inside bm_free(); but apparently it's only rb_new_obj()
> > that's prevented inside GC.
> 
> Good point.  I was not aware of this.
> Indeed, GC does not occur recursively even if vm_xmalloc is called
> during GC.  But NoMemoryError may be raised from bm_free()...  It
> will probably make the interpreter state inconsistent, so I guess
> it is not possible to sanely continue the execution by rescue'ing
> the exception.
> But I have no solution about the case.  Frankly speaking, I think
> we can do nothing if just few dozens byte allocation is failed.

We can pre-allocate the unlinked_method_entry_list_entry struct
in the METHOD struct and then push it into unlinked_method_entry_list
in bm_free (untested patch below):

--- a/proc.c
+++ b/proc.c
@@ -19,6 +19,7 @@ struct METHOD {
     VALUE rclass;
     ID id;
     rb_method_entry_t *me;
+    struct unlinked_method_entry_list_entry *ume;
 };
 
 VALUE rb_cUnboundMethod;
@@ -868,7 +869,11 @@ static void
 bm_free(void *ptr)
 {
     struct METHOD *data = ptr;
-    rb_unlink_method_entry(data->me);
+    struct unlinked_method_entry_list_entry *ume = data->ume;
+
+    ume->me = data->me;
+    ume->next = GET_VM()->unlinked_method_entry_list;
+    GET_VM()->unlinked_method_entry_list = ume;
     xfree(ptr);
 }
 
@@ -978,6 +983,7 @@ mnew(VALUE klass, VALUE obj, ID id, VALUE mclass, int scope)
     data->me = me2;
     *data->me = *me;
     data->me->def->alias_count++;
+    data->ume = ALLOC(struct unlinked_method_entry_list_entry);
 
     OBJ_INFECT(method, klass);
 
@@ -1088,6 +1094,7 @@ method_unbind(VALUE obj)
     *data->me = *orig->me;
     if (orig->me->def) orig->me->def->alias_count++;
     data->rclass = orig->rclass;
+    data->ume = ALLOC(struct unlinked_method_entry_list_entry);
     OBJ_INFECT(method, obj);
 
     return method;
-- 
Eric Wong