BUG in SWIG refcouting

[Grzegorz Makarewicz]

Every SWIG version, which generates python code, has a bug.
This bug is in function which takes Python's self variable.
For SWIG-883 it is a SWIG_GetPtrObj function.

Below is the patched version:

SWIGSTATICRUNTIME(char *)
SWIG_GetPtrObj(PyObject *obj, void **ptr, char *type) {
  PyObject *sobj = obj;
  char     *str;
  if (!PyString_Check(obj)) {
    sobj = PyObject_GetAttrString(obj,"this");
    if (!sobj) return "";
//test by commenting NEXT line!
    Py_DECREF(sobj);
  }
  str = PyString_AsString(sobj);
  return SWIG_GetPtr(str,ptr,type);
}


How to check this?

Create the files:

1. swigbug.i which contains:
//
%module swigbug

%{
#include "swigbug.h"
%}

%include "swigbug.h"
// EOF

2. swigbug.h which contains:
class SwigBug {
public:
	SwigBug(){}
	~SwigBug(){}
};
//EOF

3. d_xrange.py which contains:

import sys
import swigbug
def demo():
	bug=swigbug.SwigBug()
def main():
	print 'xrange:'
	rc0=sys.gettotalrefcount()
	rc1=sys.gettotalrefcount()
	for i in xrange(100):
		demo()
		rc1=sys.gettotalrefcount()
		if rc1-rc0:
			print 'loop:',i,rc0,'+',rc1-rc0
			rc0=rc1
main()
# EOF

4. Compile the debug version of module swigbug and execute in the debug
version of Python
	command line for swig: swig -python -dnone -c++ -shadow -o swigbugc.cpp
swigbug.i

5. Have you patched the file generated by SWIG?
	NO - at the end of every loop you should see the message, that reference
counter is increased
	YES - there are messages about increasing the reference counter for numbers
in range 86-99 (WHY???)

6. For curious: you could check what will you get if you change the loop
from:
	for i in xrange(100)
to:
	for i in [0 , 1 , ..., 100]:
or:
	for i in range(100):
or:
	while i<101:

Grzegorz Makarewicz