[Python-Dev] refcounting vs PyModule_AddObject

[Armin Rigo]

Hi Michael,

On Wed, Jun 15, 2005 at 01:35:35PM +0100, Michael Hudson wrote:
>         if (ProfilerError == NULL)
>             ProfilerError = PyErr_NewException("hotshot.ProfilerError",
>                                                NULL, NULL);
>         if (ProfilerError != NULL) {
>             Py_INCREF(ProfilerError);
>             PyModule_AddObject(module, "ProfilerError", ProfilerError);
>         }

I think the Py_INCREF is needed here.  The ProfilerError is a global
variable that needs the extra reference.  Otherwise, a malicious user
could do "del _hotshot.ProfilerError" and have it garbage-collected
under the feet of _hotshot.c which still uses it.  What I don't get is
how ProfilerError could fail to be NULL in the first 'if' above, but
that's a different matter.

While we're at strange refcounting problems, PyModule_AddObject() only
decrefs its last argument if no error occurs.  This is probably wrong.

In general I've found that the C modules' init code is fragile.  This
might be due to the idea that it runs only once anyway, and global
C-module objects are immortal anyway, so sloppiness sneaks in.  But for
example, the following is common:

        m = Py_InitModule3("xxx", NULL, module_doc);
        Py_INCREF(&Xxx_Type);
        PyModule_AddObject(m, "Xxx", (PyObject *)&Xxx_Type);

This generates a segfault if Py_InitModule3() returns NULL (however rare
that situation is).


A bientot,

Armin