Re: Update on CPython Extension Module -fanalyzer plugin development

[Eric Feng <ef2648@columbia.edu>]

On Wed, Aug 23, 2023 at 7:16 PM David Malcolm <dmalcolm@redhat.com> wrote:
>
> On Wed, 2023-08-23 at 17:15 -0400, Eric Feng wrote:
> > On Mon, Aug 21, 2023 at 11:04 AM David Malcolm <dmalcolm@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.
I see; sounds good!
>
> 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)
Will post a WIP soon. Thanks!
>
> Dave
>