custodian=xxx memory problems

[Gustavo Carneiro]

--- example.hh ---
#include <iostream>

class A {
public:
  A(int val = 0): mVal(val) { std::cout << "A constructor." << std::endl; }
  virtual ~A() {}
  int val() const { return mVal; }
  void val(int x) { mVal = x; }
private:
  int mVal;
};

class B {
public:
  B(): mAptr(new A) { std::cout << "B constructor with " << mAptr << std::endl; }
  virtual ~B() {}
  A* Aptr() { return mAptr; }
private:
  A* mAptr;
};

--- example-gen.py ---
from pybindgen import *
import sys

mod = Module("example")
mod.add_include('"example.hh"')

a = mod.add_class("A", allow_subclassing=True)
a.add_constructor([param("int", "val", default_value="0")])
a.add_instance_attribute("val", "int", getter="val", setter="val")

b = mod.add_class("B", allow_subclassing=True)
b.add_constructor([])
#b.add_method("Aptr", retval("A*", caller_owns_return=False), [])
#b.add_method("Aptr", retval("A*", caller_owns_return=True), [])
b.add_method("Aptr", retval("A*", custodian=0), [])

f = open("example.C", "w")
mod.generate(FileCodeSink(f))
f.close()

--- test.py ---

import example
b = example.B()
a = b.Aptr()
print "Before changing: ", a.val, b.Aptr().val
a.val = 10
print "After changing: ", a.val, b.Aptr().val

--- valgrind ---

==10091== Invalid read of size 8
==10091== at 0x6058AFB: PyA__tp_clear(PyA*) (example.C:138)
==10091== by 0x6058B75: _wrap_PyA__tp_dealloc(PyA*) (example.C:154)
==10091== by 0x43DC71: list_dealloc (listobject.c:306)
==10091== by 0x44D832: dict_dealloc (dictobject.c:911)
==10091== by 0x6058F43: PyB__tp_clear(PyB*) (example.C:294)
==10091== by 0x6058FE1: _wrap_PyB__tp_dealloc(PyB*) (example.C:313)
==10091== by 0x44C096: insertdict (dictobject.c:459)
==10091== by 0x44E656: PyDict_SetItem (dictobject.c:701)
==10091== by 0x45010D: _PyModule_Clear (moduleobject.c:138)
==10091== by 0x4B53A9: PyImport_Cleanup (import.c:439)
==10091== by 0x4C2C34: Py_Finalize (pythonrun.c:434)
==10091== by 0x4185C1: Py_Main (main.c:625)
==10091== Address 0x602a660 is 0 bytes inside a block of size 16 free'd
==10091== at 0x4C24A7A: operator delete(void*) (vg_replace_malloc.c:346)
==10091== by 0x605933A: A::~A() (example.hh:8)
==10091== by 0x6058B0D: PyA__tp_clear(PyA*) (example.C:138)
==10091== by 0x6058B75: _wrap_PyA__tp_dealloc(PyA*) (example.C:154)
==10091== by 0x43DC71: list_dealloc (listobject.c:306)
==10091== by 0x44D832: dict_dealloc (dictobject.c:911)
==10091== by 0x6058F43: PyB__tp_clear(PyB*) (example.C:294)
==10091== by 0x6058FE1: _wrap_PyB__tp_dealloc(PyB*) (example.C:313)
==10091== by 0x44C096: insertdict (dictobject.c:459)
==10091== by 0x44E656: PyDict_SetItem (dictobject.c:701)
==10091== by 0x45010D: _PyModule_Clear (moduleobject.c:138)
==10091== by 0x4B53A9: PyImport_Cleanup (import.c:439)

[Gustavo Carneiro (gjc)]

The test program calls b.Aptr() multiple times, which creates multiple wrappers for class A. At the end of the simulation, each distinct wrapper will try to deallocate the same A instance, causing the double-free.

We need to add a option to create a wrapper which knows the C++ instance is shared and doesn't try to free it. Unfortunately that means adding bytes to the python wrapper structure, but that's life.

[Gustavo Carneiro (gjc)]

Workaround:

pybindgen.settings.wrapper_registry = pybindgen.settings.StdMapWrapperRegistry

[Gustavo Carneiro (gjc)]

All fixed in rev. 706.