[Pyrex] Pyrex 0.9: Compiled code produces segmentation fault

[Paul Prescod]

Matthias Baas wrote:
> Hi,
> 
> I've come across a weird bug that can lead to a crash when Pyrex 
> generated code is executed.
> I have an extension type "vec3" that represents vectors with 3 float 
> components and an accompanying iterator class "vec3iter" to iterate over 
> the components. But using the iterator can lead to a segmentation fault. 
>  However, it doesn't always happen, sometimes the iterator works as 
> expected but suddenly it crashes.
> I've seen it crash on both, Windows (XP, MSVC6) and Linux. And it only 
> occurs when using Python 2.2.2. All tests have been ok so far when using 
> Python 2.3.
> The problem also does not occur when using Pyrex 0.8.2 instead of 0.9.
> 
> Now here's an example showing the problem:

It seems to me that by the time Python calls the iterator object's 
tp_dealloc function, the object has already been freed. I don't know why 
that is. But I can observe it happening because on my platform the 
memory manager is complaining about a double free of an object when the 
tp_dealloc tries to free the object. I'm not setup to debug into Python 
so that's all I can figure out right now. You can see the effect in 
action with this instrumented dealloc function:

static void __pyx_tp_dealloc_3vec_vec3iter(PyObject *o){
   struct __pyx_obj_3vec_vec3iter *p = (struct __pyx_obj_3vec_vec3iter *)o;
   printf("Deallocating iterator @ %p with refcnt %d\n", p, p->ob_refcnt);
   Py_XDECREF(p->v);
   (*o->ob_type->tp_free)(o);
   printf("freed %p\n", o);
}

I get:

Deallocating iterator @ 0x2aeb10 with refcnt 0
*** malloc[1944]: Deallocation of a pointer not malloced: 0x2aeb10; This 
could be a double free(), or free() called with the middle of an 
allocated block; Try setting environment variable MallocHelp to see 
tools to help debug
freed 0x2aeb10

I don't know why the iterator would have already been freed when its 
deallocator is called. It should be the job of the deallocator to do the 
freeing.

  Paul Prescod