Random failure in src/sage/schemes/elliptic_curves/ell_field.py

[vbraun]

File "src/sage/schemes/elliptic_curves/ell_field.py", line 988, in sage.schemes.elliptic_curves.ell_field.EllipticCurve_field.isogenies_prime_degree
Failed example:
    E.isogenies_prime_degree(max_l=3)
Exception raised:
    Traceback (most recent call last):
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/doctest/forker.py", line 681, in _run
        self.compile_and_execute(example, compiler, test.globs)
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/doctest/forker.py", line 1105, in compile_and_execute
        exec(compiled, globs)
      File "<doctest sage.schemes.elliptic_curves.ell_field.EllipticCurve_field.isogenies_prime_degree[15]>", line 1, in <module>
        E.isogenies_prime_degree(max_l=Integer(3))
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/elliptic_curves/ell_field.py", line 1091, in isogenies_prime_degree
        return sum([isogenies_prime_degree(self, d) for d in L], [])
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/elliptic_curves/isogeny_small_degree.py", line 2340, in isogenies_prime_degree
        return isogenies_2(E, minimal_models=minimal_models)
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/elliptic_curves/isogeny_small_degree.py", line 708, in isogenies_2
        isogs = [E.isogeny(f, model=model) for f in ff]
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/elliptic_curves/ell_field.py", line 878, in isogeny
        return EllipticCurveIsogeny(self, kernel, codomain, degree, model, check=check)
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/elliptic_curves/ell_curve_isogeny.py", line 983, in __init__
        if not isinstance(kernel, list) and kernel in E :
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/elliptic_curves/ell_generic.py", line 423, in __contains__
        P = self(P)
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/elliptic_curves/ell_generic.py", line 549, in __call__
        return plane_curve.ProjectivePlaneCurve.__call__(self, *args, **kwds)
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/generic/scheme.py", line 266, in __call__
        return self.point(args)
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/projective/projective_subscheme.py", line 120, in point
        return self._point(self, v, check=check)
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/schemes/elliptic_curves/ell_point.py", line 275, in __init__
        raise TypeError("v (=%s) must have %s components" % (v, d))
      File "sage/structure/sage_object.pyx", line 191, in sage.structure.sage_object.SageObject.__repr__ (build/cythonized/sage/structure/sage_object.c:2428)
        result = reprfunc()
      File "sage/rings/polynomial/polynomial_element.pyx", line 2589, in sage.rings.polynomial.polynomial_element.Polynomial._repr_ (build/cythonized/sage/rings/polynomial/polynomial_element.c:26822)
        return self._repr()
      File "sage/rings/polynomial/polynomial_element.pyx", line 2541, in sage.rings.polynomial.polynomial_element.Polynomial._repr (build/cythonized/sage/rings/polynomial/polynomial_element.c:26025)
        coeffs = self.list(copy=False)
      File "sage/rings/polynomial/polynomial_zz_pex.pyx", line 191, in sage.rings.polynomial.polynomial_zz_pex.Polynomial_ZZ_pEX.list (build/cythonized/sage/rings/polynomial/polynomial_zz_pex.cpp:15301)
        return [K(ZZ_pE_c_to_list(ZZ_pEX_coeff(self.x, i)))
      File "sage/structure/parent.pyx", line 900, in sage.structure.parent.Parent.__call__ (build/cythonized/sage/structure/parent.c:9197)
        return mor._call_(x)
      File "sage/structure/coerce_maps.pyx", line 162, in sage.structure.coerce_maps.DefaultConvertMap_unique._call_ (build/cythonized/sage/structure/coerce_maps.c:4556)
        raise
      File "sage/structure/coerce_maps.pyx", line 157, in sage.structure.coerce_maps.DefaultConvertMap_unique._call_ (build/cythonized/sage/structure/coerce_maps.c:4448)
        return C._element_constructor(x)
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/categories/sets_cat.py", line 994, in _element_constructor_from_element_class
        return self.element_class(self, *args, **keywords)
      File "sage/rings/finite_rings/element_pari_ffelt.pyx", line 336, in sage.rings.finite_rings.element_pari_ffelt.FiniteFieldElement_pari_ffelt.__init__ (build/cythonized/sage/rings/finite_rings/element_pari_ffelt.c:3414)
        self.construct_from(x)
      File "sage/rings/finite_rings/element_pari_ffelt.pyx", line 504, in sage.rings.finite_rings.element_pari_ffelt.FiniteFieldElement_pari_ffelt.construct_from (build/cythonized/sage/rings/finite_rings/element_pari_ffelt.c:5493)
        self.construct_from(self._parent.polynomial_ring()([Fp(y) for y in x]))
      File "sage/rings/finite_rings/element_pari_ffelt.pyx", line 494, in sage.rings.finite_rings.element_pari_ffelt.FiniteFieldElement_pari_ffelt.construct_from (build/cythonized/sage/rings/finite_rings/element_pari_ffelt.c:5172)
        self.construct_from(x.substitute(self._parent.gen()))
      File "sage/rings/polynomial/polynomial_element.pyx", line 452, in sage.rings.polynomial.polynomial_element.Polynomial.subs (build/cythonized/sage/rings/polynomial/polynomial_element.c:8543)
        return self(*x, **kwds)
      File "sage/rings/polynomial/polynomial_zmod_flint.pyx", line 321, in sage.rings.polynomial.polynomial_zmod_flint.Polynomial_zmod_flint.__call__ (build/cythonized/sage/rings/polynomial/polynomial_zmod_flint.cpp:15655)
        return Polynomial.__call__(self, *x, **kwds)
      File "sage/rings/polynomial/polynomial_element.pyx", line 775, in sage.rings.polynomial.polynomial_element.Polynomial.__call__ (build/cythonized/sage/rings/polynomial/polynomial_element.c:9340)
        cst, aa = coercion_model.canonical_coercion(cst, a)
      File "sage/structure/coerce.pyx", line 1270, in sage.structure.coerce.CoercionModel.canonical_coercion (build/cythonized/sage/structure/coerce.c:11302)
        x_elt = (<Map>x_map)._call_(x)
      File "sage/rings/finite_rings/hom_prime_finite_field.pyx", line 95, in sage.rings.finite_rings.hom_prime_finite_field.FiniteFieldHomomorphism_prime._call_ (build/cythonized/sage/rings/finite_rings/hom_prime_finite_field.c:3590)
        return self._codomain._element_constructor(x)
      File "/home/buildbot/slave/sage_git/build/local/lib/python2.7/site-packages/sage/categories/sets_cat.py", line 994, in _element_constructor_from_element_class
        return self.element_class(self, *args, **keywords)
      File "sage/rings/finite_rings/element_pari_ffelt.pyx", line 336, in sage.rings.finite_rings.element_pari_ffelt.FiniteFieldElement_pari_ffelt.__init__ (build/cythonized/sage/rings/finite_rings/element_pari_ffelt.c:3414)
        self.construct_from(x)
      File "sage/rings/finite_rings/element_pari_ffelt.pyx", line 410, in sage.rings.finite_rings.element_pari_ffelt.FiniteFieldElement_pari_ffelt.construct_from (build/cythonized/sage/rings/finite_rings/element_pari_ffelt.c:3985)
        sig_on()
    SystemError: calling remove_from_pari_stack() inside sig_on()
**********************************************************************
1 item had failures:
   1 of  43 in sage.schemes.elliptic_curves.ell_field.EllipticCurve_field.isogenies_prime_degree
    [219 tests, 1 failure, 3.54 s]
----------------------------------------------------------------------
sage -t --long src/sage/schemes/elliptic_curves/ell_field.py  # 1 doctest failed
----------------------------------------------------------------------

[embray]

Ticket retargeted after milestone closed

[mkoeppe]

Moving tickets to milestone sage-9.2 based on a review of last modification date, branch status, and severity.

[cremona]

I am not sure how to debug this at the Sage level. The test asks for all isogenies of the curve E of degree up to 3, and the error is raised when computing 2-isogenies, of which there are three. These are computed by constructing a EllipticCurveIsogeny? object using EllipticCurveIsogeny?(E, ker) where ker is a degree 1 polynomial of the form x-x0 over the base field. Since ker is a polynomial it should the 'kohel' algorithm, but the test in line 982 of ell_curve_isogeny tests whether ker is a point using "kernel in E", which attempts to construct a point via E(ker), and that is causing the error. There should be a better way to determine whether the caller has provided a point of E generating the kernel, and not a polynomial.

The file ell_curve_isogeny is over 4000 lines long, mostly not written by me but I have spent many hours in it debugging (though not recently). I would have written it *very* differently.

I can try replacing the test "kernel in E" with something like

sage: from sage.schemes.elliptic_curves.ell_point import EllipticCurvePoint_field                                               
sage: isinstance(P, EllipticCurvePoint_field) and P.curve() is E

but how to test whether the sporadic error has gone away?

[mkoeppe]

Moving to 9.4, as 9.3 has been released.