#13447: Make libsingular multivariate polynomial rings collectable
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Reporter: nbruin |
Owner:
Type: defect |
Status: needs_work
Priority: major |
Milestone: sage-5.4
Component: memleak |
Resolution:
Keywords: | Work
issues: Understand why sometimes `new_RingWrap` needs an incref and sometimes
not
Report Upstream: None of the above - read trac for reasoning. |
Reviewers: Simon King
Authors: Nils Bruin, Simon King | Merged
in:
Dependencies: #11521 |
Stopgaps:
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Comment (by jpflori):
Replying to [comment:29 SimonKing]:
> Replying to [comment:27 nbruin]:
> > We have:
> > {{{
> > def __dealloc__(self):
> > celement_destruct(&self.x,
get_cparent((<Polynomial_template>self)._parent))
> > }}}
> > and for us:
> > {{{
> > get_cparent(parent) == <ntl_ZZ_pEContext_class>(parent._modulus)
> > }}}
> > The `_parent` attribute is a cython slot.
>
> Interestingly, there is no complaint about a missing attribute
`_parent`. It is `_modulus` that is missing.
>
> > However, it holds a reference to a python-managed object, so I think
cython ensures it's properly taken into account in GC cycle counting. But
that would suggest to me python could clear this slot to break cycles! So
in that case, `Polynomial_template` is never safe. It could be I'm wrong,
however.
>
> I think you are right. The `__dealloc__` of `Polynomial_template` is
unsafe, unless polynomial rings will stay in memory forever. But I'd love
to hear that we are wrong, because otherwise each polynomial would need a
pointer to the c-data expected to be returned by
`get_cparent((<Polynomial_template>self)._parent)`, and we'd need to take
into account reference counting for the c-parent during creation and
deletion of polynomials.
>
> Or perhaps there is a way out. We have a polynomial ring R and we have
some elements a,b,c,... Each element points to R, and R points to some of
its elements, namely to its generators. The problem is that deallocation
of the elements is only possible as long as R is alive.
>
> If we'd manually incref R upon creation of an element x, decrefing R
when x gets deallocated, then we would ensure that R will survive until
the last of its elements is deleted. Or would that mean that the elements
will survive as well, because of the reference from R to its generators?
'''Edit:''' Yes it would.
FYI, the cdefed pointer _parent stuff was added in #12313.
Have a look at http://trac.sagemath.org/sage_trac/ticket/12313#comment:13
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13447#comment:88>
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