On Wed, 2023-08-23 at 17:15 -0400, Eric Feng wrote:
> On Mon, Aug 21, 2023 at 11:04 AM David Malcolm <dmalc...@redhat.com>
> wrote:
> >
> > On Mon, 2023-08-21 at 10:05 -0400, Eric Feng wrote:
> > > Hi Dave,
> > >
> > > Just wanted to give you and everyone else a short update on how
> > > reference count checking is going — we can now observe the refcnt
> > > diagnostic being emitted:
> > >
> > > rc3.c:22:10: warning: REF COUNT PROBLEM
> > > 22 | return list;
> > > | ^~~~
> > > ‘create_py_object’: events 1-4
> > > |
> > > | 4 | PyObject* item = PyLong_FromLong(3);
> > > | | ^~~~~~~~~~~~~~~~~~
> > > | | |
> > > | | (1) when ‘PyLong_FromLong’
> > > succeeds
> > > | 5 | PyObject* list = PyList_New(1);
> > > | | ~~~~~~~~~~~~~
> > > | | |
> > > | | (2) when ‘PyList_New’ succeeds
> > > |......
> > > | 14 | PyList_Append(list, item);
> > > | | ~~~~~~~~~~~~~~~~~~~~~~~~~
> > > | | |
> > > | | (3) when ‘PyList_Append’ fails
> > > |......
> > > | 22 | return list;
> > > | | ~~~~
> > > | | |
> > > | | (4) here
> > > |
> > >
> > > I will fix up and refactor the logic for counting the actual ref
> > > count
> > > before coming back and refining the diagnostic to give much more
> > > detailed information.
> >
> > Excellent! Thanks for the update.
> >
> > Dave
> >
>
> Hi Dave,
>
> I've since fixed up the logic to count the actual reference counts of
> the PyObject* instances.
Sounds promising.
> Now, I'm contemplating the specific
> diagnostics we'd want to issue and the appropriate conditions for
> emitting them. With this in mind, I wanted to check in with you on
> the
> appropriate approach:
>
> To start, I'm adopting the same assumptions as cpychecker for
> functions returning a PyObject*. That is, I'm operating under the
> premise that by default such functions return a new reference upon
> success rather than, for example, a borrowed reference (which we can
> tackle later on). Given this, it's my understanding that the
> reference
> count of the returned object should be 1 if the object is newly
> created within the function body and incremented by 1 from what it
> was
> previously if not newly created (e.g passed in as an argument).
> Furthermore, the reference count for any PyObject* instances created
> within the function should be 0, barring situations where we're
> returning a collection, like a list, that includes references to
> these
> objects.
>
> Let me know what you think; thanks!
This sounds like a good approach for v1 of the implementation.
It's probably best to focus on getting a simple version of the patch
into trunk, and leave any polish of it to followups.
In terms of deciding what the reference count of a returned PyObject *
ought to be, cpychecker had logic to try to detect callbacks used by
PyMethodDef tables, so that e.g. in:
static PyMethodDef widget_methods[] = {
{"display",
(PyCFunction)widget_display,
(METH_VARARGS | METH_KEYWORDS), /* ml_flags */
NULL},
{NULL, NULL, 0, NULL} /* terminator */
};
...we'd know that the callback function "widget_display" follows the
standard rules for a PyCFunction (e.g. returns a new reference).
But that's for later; don't bother trying to implement that until we
have the basics working.
Is it worth posting a work-in-progress patch of what you have so far?
(you don't need to bother with a ChangeLog for that)
Dave
