Inappropriate use of dx causes segfault

[Kristian Ejlebjerg Jensen]

The below script produces a segfault because dx is redefined.

from dolfin import *
f = Expression("1",degree=2)
u = Expression("2*dx",dx=f,degree=2)
mesh = RectangleMesh(Point(0.,0.),Point(2.,1.),20,10)
V = FunctionSpace(mesh,'CG',1)
v = interpolate(u,V)

[Martin Sandve Alnæs]

Thanks. This is reproducable in dev.

Do you have some evidence for the diagnosis "because dx is redefined"? I don't even understand what that means.

Changing dx= to e.g. f= does avoid the segfault though, so something strange is going on here that involves the name.

[Kristian Ejlebjerg Jensen]

What I mean to say is that there is no segfault, if dx is replaced with something else.

I would suggest to throw an error, whenever a user tries to redefine dx. I just tried ds/dS and they do not cause segfaults.

[Jan Blechta]

The reason could be that SWIG does not correctly wrap dx due to a name clash because it is already imported. There is a lot of involved code, the dolfin SWIG interface files, function/expression.py, compilemodules/expressions.py, compilemodule/compilemodule.py, and it seemed to me that this is not that easy to debug.

Obvious fix would be to raise an error when creating such an expression, but due to a complexity of the code it is not that obvious which all the names will cause the same problem.

[Martin Sandve Alnæs]

It's not clear to me where there is a name clash. dx from ufl is in a completely different scope.

[Jan Blechta]

There's a method ufl.Expr.dx

>>> c = Constant(1.0)
>>> c.dx
<bound method Constant._dx of Coefficient(FunctionSpace(None, FiniteElement('Real', None, 0)), 3)>

Do we just put dx member to the blacklist?

[Martin Sandve Alnæs]

Good catch. Maybe just add all Expr attributes to the blacklist. I'm touching this code now so I can do it.

[Jan Blechta]

Fix suggested https://bitbucket.org/fenics-project/dolfin/branch/jan/fix-issue-807. Let's see if it breaks the tests or not.

[Jan Blechta]

@martinal, I see you're working on it much thoroughly. Maybe you could merge from jan/fix-issue-807 and take care of it so that I don't break your improvements (if you haven't fixed the issue already).

[Jan Blechta]

BTW the brach alone fixes the problem.

[Martin Sandve Alnæs]

Will do. I've done some other unrelated fixes as well in one branch.

[Prof Garth Wells]

Can this be closed after recent changes?