[Python-Dev] The Amazing Unreferenced Weakref (original) (raw)

Gregory P. Smith greg at krypto.org
Tue Sep 6 20:19:36 EDT 2016

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 PyObjectClearWeakRefs(). 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->wrnext; if (((PyObject *)current)->obrefcnt > 0) { PyINCREF(current); PyTupleSETITEM(tuple, i * 2, (PyObject *) current); PyTupleSETITEM(tuple, i * 2 + 1, current->wrcallback); } else { PyDECREF(current->wrcallback); } current->wrcallback = NULL; clearweakref(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 obrefcnt ever be 0? The tpdealloc handler for PyWeakReference * objects removes it from this list and frees the memory. How could the reference count reach 0 without tpdealloc 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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