[taking this to cython-dev, as it's worth going into the archive]
Haoyu Bai, 20.06.2010 14:22:
>> Haoyu Bai, 20.06.2010 04:48:
>>> compiling (cpp) and running knuth_man_or_boy_test ... Doctest:
>>> knuth_man_or_boy_test.__test__.a (line 44) ... python:
>>> Modules/gcmodule.c:276: visit_decref: Assertion `gc->gc.gc_refs != 0'
>>> failed.
>>>
>>> This seems related to refcounting problem due to my work on nonlocal.
>>> I attempted to fix it but no success yet. In knuth_man_or_boy_test,
>>> the assertion failed on a binding_PyCFunction object (the function
>>> object b).
>
> I solved the problem. This is due to GC collecting may start inside a
> refnanny Pyx_INCREF or Pyx_DECREF.
Sure, DECREF can run arbitrary code. INCREF can't, though. Even in PyDebug
builds, it's pretty restricted.
> Thus a inconsistent state could be exposed to the GC.
>
> For example I was writing code like this:
>
> scope->v1 = arg1;
> scope->v2 = arg2;
> scope->v3 = arg3;
> Pyx_INCREF(scope->v1); Pyx_GIVEREF(scope->v1);
> Pyx_INCREF(scope->v2); Pyx_GIVEREF(scope->v2);
> Pyx_INCREF(scope->v3); Pyx_GIVEREF(scope->v3);
Don't ever do that!
The correct way to do this is to do the INCREF first, before the
assignment, i.e. in this order:
Pyx_INCREF(arg1);
Pyx_DECREF(scope->v1); (if required)
scope->v1 = arg1;
Pyx_GIVEREF(scope->v1); (or GIVEREF(arg1))
Actually, it would be even better to do this instead:
Pyx_INCREF(arg1);
temp = scope->v1;
scope->v1 = arg1;
Pyx_DECREF(temp);
Pyx_GIVEREF(arg1);
as DECREF(scope->v1) can run arbitrary code that may involve traversing the
references stored in "scope", including the reference in "scope->v1".
Read up on the Py_CLEAR() macro.
Stefan
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