[Python-Dev] The Amazing Unreferenced Weakref

[Gregory P. Smith]

This code appears to have been added to fix
https://bugs.python.org/issue3100 - A crash involving a weakref subclass.

-gps

On Tue, Sep 6, 2016 at 3:51 PM Larry Hastings <larry at hastings.org> wrote:

>
> This is all about current (3.6) trunk.
>
> In Objects/weakrefobject.c, we have the function
> PyObject_ClearWeakRefs().  This is called when a generic object that
> supports weakrefs is destroyed; this is the code that calls the callbacks.
> Here's a little paragraph of code from the center:
>
> for (i = 0; i < count; ++i) {
>     PyWeakReference *next = current->wr_next;
>
>     if (((PyObject *)current)->ob_refcnt > 0)
>     {
>         Py_INCREF(current);
>         PyTuple_SET_ITEM(tuple, i * 2, (PyObject *) current);
>         PyTuple_SET_ITEM(tuple, i * 2 + 1, current->wr_callback);
>     }
>     else {
>         Py_DECREF(current->wr_callback);
>     }
>     current->wr_callback = NULL;
>     clear_weakref(current);
>     current = next;
> }
>
> "current" is the doubly-linked list of PyWeakReference objects stored
> inside the object that's getting destroyed.
>
> My question: under what circumstances would ob_refcnt ever be 0?  The
> tp_dealloc handler for PyWeakReference * objects removes it from this list
> and frees the memory.  How could the reference count reach 0 without
> tp_dealloc being called and it being removed from the list?
>
> Scratching my head like crazy,
>
>
> */arry*
>
> p.s. If you're thinking "why does he care?", understanding this would
> maybe help with the Gilectomy.  So yes there's a point to this question.
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