gc assertion failure

[Tim Peters]

[Todd Miller]
> I recently discovered an assertion failure in the Python garbage
> collection system when scripts using our C extension (numarray) exit.
> The assertion is activated for Pythons configured using
>   --with-pydebug. I have a feeling I may be doing something wrong
> with garbage collection support for some of our c types,  but I'm not
> sure exactly what.
>
> Here is the assertion output:
>
> python: Modules/gcmodule.c:231: visit_decref: Assertion
> `gc->gc.gc_refs != 0' failed.
> Abort (core dumped)

Looking at the source code should clarify:

	assert(gc->gc.gc_refs != 0); /* else refcount was too small */

That is, gc found more pointers to an object than that object's refcount
believes exists.  A missing Py_INCREF or an extra Py_DECREF are plausible
causes; so is a bad tp_traverse function that passes a single containee
multiple times (although I've only see that once in real life).  A missing
Py_INCREF is (IME) the most common cause for this assertion.

> ...
> #5  0x080e9222 in visit_decref (op=0x405adc74, data=0x0) at
> Modules/gcmodule.c:231
> #6  0x0808cebf in tupletraverse (o=0x40a62f74, visit=0x80e9194
> <visit_decref>, arg=0x0) at Objects/tupleobject.c:398

So it's complaing about an object that happens to be in a tuple.  Displaying
more info about op would tell you more about the kind of object it's
complaining about.