Looking for where tp_clear() is being called, the only caller is line 713 in gmodule.c, which explicitly surrounds the call with an INCREF/DECREF pair. Perhaps that's the clue you're looking for?
--Guido On 2/12/07, Jeremy Hylton <[EMAIL PROTECTED]> wrote: > I was wondering today how I could convince myself that a sequence of > Py_CLEAR() calls in a tp_clear function was safe. Take for example a > really trivial sequence of code on frame_clear(): > > Py_CLEAR(f->f_exc_type); > Py_CLEAR(f->f_exc_value); > Py_CLEAR(f->f_exc_traceback); > Py_CLEAR(f->f_trace); > > We use Py_CLEAR() so that multiple calls to frame_clear() are safe. > The first time this runs it sets exc_type to NULL before calling > DECREF. This guarantees that a subsequent frame_clear() or > frame_dealloc() won't attempt to DECREF it a second time. I think I > understand why that's desireable and why it works. The primary risk > is that via DECREF we may execute an arbitrary amount of Python code > via weakref callbacks, finalizers, and code in other threads that gets > resumed while the callbacks and finalizers are running. > > My question, specifically, then: Why it is safe to assume that f > doesn't point to trash after a particular call to Py_CLEAR()? Any > particular call to Py_CLEAR() could break the cycle that the object is > involved in an lead to a call to frame_dealloc(). The frame could get > returned to an obmalloc pool, returned to the OS, or just re-used by > another object before we get back to Py_CLEAR(). It seems like such > behavior would be exceedingly unlikely, but I can't convince myself > that it is impossible. Which is it: improbable or impossible? If it > is only improbable, should we figure out how to write code that is > safe against such an improbable event? > > Jeremy > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > http://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: > http://mail.python.org/mailman/options/python-dev/guido%40python.org > -- --Guido van Rossum (home page: http://www.python.org/~guido/) _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com