[Python-Dev] Big trouble in CVS Python

Chad Netzer cnetzer@mail.arc.nasa.gov
14 Apr 2003 13:09:04 -0700

On Sun, 2003-04-13 at 13:28, Tim Peters wrote:

> It appears to be a refcount error in recently-added C code that tries to
> generalize the builtin range() function, specifically here:
>   Fail:
> 	Py_XDECREF(curnum);
> 	Py_XDECREF(istep);  <- here
> 	Py_XDECREF(zero);
> Word to the wise:  don't ever try to reuse a variable whose address is
> passed to PyArg_ParseTuple for anything other than holding what
> PyArg_ParseTuple does or doesn't store into it.

Hmm, then this is my fault.  I did exactly that.  My approach was to
Py_INCREF an optional argument it if it was given (ie. not NULL),
otherwise to create it from scratch, and Py_DECREF when I was done.  I
believe this was a not uncommon idiom (I can't recal the specifics, but
being my first submitted patch, I made sure to try to look for existing
idioms for argument and error handling).  I apologize if I erred.

  I assume a better approach, then is to get the optional istep
argument, and copy it into a variable for your own use (or create it if
it didn't exist)?  ie. Never increment or decrement the optional
argument object, returned from PyArg_ParseTuple, at all?

>   You'll never get the
> decrefs straight (and even if you manage to at first, the next person to
> modify your code will break it).

Bingo!   Guido took a slightly different approach (and ultimately a
better one, I think), in adapting my patch.  Perhaps I unknowingly left
a time bomb for him.

I'll submit a patch to fix this all up tonight, if it hasn't already
been addressed by then.

> only-consumed-eight-hours-this-time<wink>-ly y'rs  - tim

Oh, ow!  Now that pains me.  I am very sorry to hear this wasted so much

Chad Netzer