#10100: Segfault calling change_ring(RR) on a symbolic polynomial
-----------------------------+----------------------------------------------
Reporter: davidloeffler | Owner: burcin
Type: defect | Status: positive_review
Priority: major | Milestone: sage-4.7.1
Component: symbolics | Keywords:
Work_issues: | Upstream: N/A
Reviewer: Volker Braun | Author: Mike Hansen
Merged: | Dependencies:
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Changes (by vbraun):
* status: needs_review => positive_review
* reviewer: => Volker Braun
* milestone: => sage-4.7.1
Old description:
> The following code seems to crash Sage on some (but not all) systems:
> {{{
> sage: a = var('a')
> sage: R.<x> = a.parent()[]
> sage: (x - a).change_ring(RR) # boom!
> }}}
>
> This bug was originally spotted when calculating char polys of symbolic
> matrices:
> {{{
> sage: a,b,c= var('a,b,c')
> sage: mprime = matrix([[01,b+1,2],[a+1,2,1],[2,1,c+1]])
> sage: cp = mprime.charpoly()
> sage: cp.real_roots() # boom!
> }}}
> See [http://groups.google.co.uk/group/sage-
> devel/browse_thread/thread/0ea6c973a56191ab/e91868cd98f7fb74 this sage-
> devel thread].
New description:
The following code seems to crash Sage on some (but not all) systems:
{{{
sage: a = var('a')
sage: R.<x> = a.parent()[]
sage: (x - a).change_ring(RR) # boom!
}}}
This bug was originally spotted when calculating char polys of symbolic
matrices:
{{{
sage: a,b,c= var('a,b,c')
sage: mprime = matrix([[01,b+1,2],[a+1,2,1],[2,1,c+1]])
sage: cp = mprime.charpoly()
sage: cp.real_roots() # boom!
}}}
See [http://groups.google.co.uk/group/sage-
devel/browse_thread/thread/0ea6c973a56191ab/e91868cd98f7fb74 this sage-
devel thread].
Apply [attachment:trac_10100.patch]
--
Comment:
For the record, the bug is that Python might throw an exception in a loop,
so there is no guarantee that the whole loop is evaluated. Which is why
Mike's fix is the correct one, first loop over all values and
`mpfr_init2()` them, then loop again over all values and set their values.
It would be nicer to apply RAII and keep resource allocation strictly to
the `__cinit__()` constructor, but in this case that is challenging
because the degree of the polynomial must be computed first. Could be done
by factoring out that part into a factory function. But thats already a
significant rewrite...
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/10100#comment:4>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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