[C++-sig] Boost.Python and garbage collection

[Gustavo Carneiro]

On 04/11/2007, john haddon <theboyhaddon at hotmail.com> wrote:
>
>
> Hi,
>
> I've run into a problem with cyclic references between instances of
> objects bounds with boost::python - the garbage collector doesn't seem to be
> able to collect them. This is illustrated by the python code below :
>
>
>
> import weakref
> import gc
>
> # make a cyclic reference between instances of a bound class
> o1 = BoostPythonClass()
> o2 = BoostPythonClass()
> o1.r = o2
> o2.r = o1
>
> # make weak references to track their lifetime
> w1 = weakref.ref( o1 )
> w2 = weakref.ref( o2 )
>
> # delete the instances and run the garbage collector
> del o1
> del o2
> gc.collect()


Maybe this doesn't invalidate your conclusions, but just a heads up: don't
simply call "gc.collect()", instead do "while gc.collect(): pass".  That is
because sometimes multiple GC iterations are required to collect all
objects.

# check that the objects have been collected
> assert( w1() is None )
> assert( w2() is None )
>
>
>
> I dug a bit deeper into this and had a go at fixing things by implementing
> garbage collection support for instances in src/object/class.cpp. By
> implementing a traversal to instance::dict the problem seems to be fixed and
> the asserts in the test above pass. I've also run the unit tests for a
> largish project using boost python with this modified code and they pass ok.
> The changes are pretty slight - here's the output from diff :
>
>
>
> 354a355,373
> >
> >       static int instance_traverse( PyObject* op, visitproc visit, void*
> arg )
> >       {
> >          instance<>* inst = downcast<instance<>>(op);
> >          if( inst->dict )
> >          {
> >             return visit( inst->dict, arg );
> >          }
> >          return 0;
> >       }
> >
> >       static int instance_clear( PyObject* op )
> >       {
> >          instance<>* inst = downcast<instance<>>(op);
> >          PyObject* tmp = inst->dict;
> >          inst->dict = 0;
> >          python::xdecref( tmp );
> >          return 0;
> >       }
> 391c410
> <       Py_TPFLAGS_DEFAULT // | Py_TPFLAGS_HAVE_GC
> ---
> >       Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC
> 394,395c413,414
> <       0,                                      /* tp_traverse */
> <       0,                                      /* tp_clear */
> ---
> >       instance_traverse,                      /* tp_traverse */
> >       instance_clear,                         /* tp_clear */
>
>
> The internals of boost::python are a bit of a mystery to me, so I don't
> know if I've missed something or if there's a good reason this isn't
> implemented already. I'm posting to the list in the hope that it might be
> useful and if I haven't messed up could get merged into the source.
>
> Cheers...
> John
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-- 
Gustavo J. A. M. Carneiro
INESC Porto, Telecommunications and Multimedia Unit
"The universe is always one step beyond logic." -- Frank Herbert
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