Re: [xml] [bnwest rice edu: xml mailing list ...]

On Thu, Sep 21, 2006 at 05:52:21PM -0500, Ross J. Reedstrom wrote:
Forwarding for a colleague who can't seem to get his email subscription
to work.

  I didn't see any bounce, very strange, try again !

He's starting with a quote of an email from Daniel earlier up
the list, when we ran over the missing python implemention of the range


----- Forwarded message from "Brian N. West" <bnwest rice edu> -----

Delivery-date: Thu, 21 Sep 2006 15:51:52 -0500
Date: Thu, 21 Sep 2006 15:44:46 -0500
From: "Brian N. West" <bnwest rice edu>
To: "Ross J. Reedstrom" <reedstrm rice edu>
Subject: xml mailing list ...

Daniel Veillard said:
  Hum, yeah, that must be the binding missing support from the extra
nobody tried to really use them from Python, you're in unknown land.
  Probably libxml_xmlXPathObjectPtrWrap() should be augmented to
handle them (python/types.c around line 397), and convert them to Python
type. I don't know what would be the best way to represent a point,
maybe a tuple made of a node and an index.
  I would be glad to get patches and some example to run the regression 
from python/test/ !

  Thanks in advance :-)

We made the following changes to libxml_xmlXPathObjectPtrWrap():


We create a 'point' tuple with contains the node and an index:

node = libxml_xmlNodePtrWrap(obj->user);
indexIntoNode = PyInt_FromLong((long) obj->index);

ret = PyTuple_New(2);
PyTuple_SetItem(ret, 0, node);
PyTuple_SetItem(ret, 1, indexIntoNode);


We create a list (PyList) with two point tuples (PyTuples),
or one point tuple if range is collapsed.


We create a list (PyList) with an entry per XPath Object contained in the 
location set.  We call libxml_xmlXPathObjectPtrWrap() recursively for each
XPath Object.

for our python test of interest:


libxml_xmlXPathObjectPtrWrap() gets called with a Location Set with one 
XPath Object which is a Range. We build :

a PyList (Location Set) which contain one PyList (Range)
which contains two point PyTuples.

  Okay, that sound fine.

At some point after libxml_xmlXPathObjectPtrWrap() is returned, a call is 
made to libxml_type() where we experience a memory access error.  Within a 
call is made to

PyArg_ParseTuple(args, (char *) "O:last", &obj)

followed by

cur = PyxmlNode_Get(obj)

which leaves us with a 'cur' of value 1, instead of a valid memory pointer.
Deferencing cur causes the memory access error.

  Hum, you need to chase where this is really coming from, it's hard to infer
from the stack trace since whatever called libxml_type() is not shown.
It is likely to be in the wrapper taking the low level python object 
to convert it to a instance of the classes in . Check
xmlCore.get_type, add debug there if necessary. Check the wrapper when
doing the reentrancy in the .py code when returning from C.

It is a mystery to us why libxml_type() is being called.

Any thoughts?

  Something is being seen as an xmlCore (i.e. a node) and that is used
when building a node instance from a low level wrapper object. Add
debug around get_type()


Red Hat Virtualization group
Daniel Veillard      | virtualization library
veillard redhat com  | libxml GNOME XML XSLT toolkit | Rpmfind RPM search engine

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