#11339: Illegal use of __deallocate__ in cython (pyx) code
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Reporter: gagern | Owner: drkirkby
Type: defect | Status: new
Priority: major | Milestone: sage-4.7
Component: porting | Keywords:
Work_issues: | Upstream: N/A
Reviewer: | Author:
Merged: | Dependencies:
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As [https://github.com/cschwan/sage-on-
gentoo/issues/51#issuecomment-1181259 described for the Sage on Gentoo
project], there is a problem in how parts of sage use `__deallocate__` in
their Cython code. I've actually traced an instance of
[http://hg.sagemath.org/sage-
main/file/361a4ad7d52c/sage/libs/singular/groebner_strategy.pyx#l134
groebner_strategy.pyx] causing segmentation faults under Python 2.7.
What happens is that the garbage collector invokes the
[http://docs.python.org/c-api/typeobj.html#tp_clear tp_clear] function for
the object. Its implementation is provided by Cython, and one of its
effects is that every python reference will be set to `None`. A bit later
on, [http://docs.python.org/c-api/typeobj.html#tp_dealloc tp_dealloc] is
called and invokes the `__deallocate__` method. By that time, `_parent` is
`None`, so accessing `_parent._ring` is a bad idea, and in this case it
turns out to be null.
Others have had similar problems before. There are
[http://stackoverflow.com/questions/4501938/how-can-c-object-lifetimes-be-
correctly-managed-in-cython crude workarounds] floating around. I guess a
proper solution would be twaking cython to allow custom code in the
tp_clear function. In other words, have a "magic" mehod `__clear__`
similar to the magic `__deallocate__`. But I'll wait for comments here
first, before taking this to cython upstream. Perhaps people with more
experience have better solutions to offer. And I won't mind if you decide
to take this to Cython devs yourselves.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/11339>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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