According to the docs here:

  http://docs.python.org/c-api/gcsupport.html

Any object that uses PyObject_GC_Track in its constructor must call PyObject_GC_UnTrack in its deallocator.  The CThunkObject in _ctypes does the former but not the later.  Attached patch adds a call to PyObject_GC_UnTrack.

This doesn't fix any particular bug I was seeing, I just happened to be going through the _ctypes sources (you know, for fun...) and noticed this discrepancy.

This patch seems reasonable, is consistent with the GC docs, and passed all regression tests.  Can someone apply?

I was going to say that the patch has no visible effect, since PyObject_GC_Del() calls something which has the same effect as PyObject_GC_Untrack...
But the following code crashes the interpreter! And of course the patch fixes it...


import ctypes, gc
class Nasty:
    def __del__(self):
        gc.collect()

ctypes.CFUNCTYPE(None)(lambda x=Nasty(): None)
print("OK")

New changeset eb9f566fd8db by Amaury Forgeot d'Arc in branch '2.7':
Issue #12483: ctypes: Fix a crash when the destruction of a callback
http://hg.python.org/cpython/rev/eb9f566fd8db

New changeset eae8e4ab0455 by Amaury Forgeot d'Arc in branch '3.2':
Issue #12483: ctypes: Fix a crash when the destruction of a callback
http://hg.python.org/cpython/rev/eae8e4ab0455

New changeset fe125a3fda54 by Amaury Forgeot d'Arc in branch 'default':
Merge 3.2: Issue #12483: ctypes: Fix a crash when the destruction of a callback
http://hg.python.org/cpython/rev/fe125a3fda54

Heh, I was just about to upload another patch with your test case.  Thanks for committing this Amaury.

Thanks for your help! I fear they are many other places like this one in CPython code.