assemble_system fails (for pure facet integrals)

[Max Jensen]

This report is a follow up of

DOLFIN Questions Question #170899
https://answers.launchpad.net/dolfin/+question/170899

The following script does not run:

====
from dolfin import *
import numpy

#parameters ["linear_algebra_backend"] = "uBLAS"
#parameters ["linear_algebra_backend"] = "Epetra"
parameters ["linear_algebra_backend"] = "PETSc"

mesh = UnitCube(6, 6, 6)
U = FunctionSpace(mesh, 'CG', 1)
g = Expression('0.0')

class DirichletBoundaryU(SubDomain):
    def inside(self, x, on_boundary):
        return on_boundary
Du = DirichletBoundaryU()
BCu = DirichletBC(U, g, Du)

u = TrialFunction(U)
v = TestFunction(U)

Au = inner(grad(u), grad(v))*dx + u*v*dx
Lu = g*v*ds

#Mu = assemble(Au)
#Bu = assemble(Lu)
#BCu.apply(Mu, Bu)
Mu, Bu = assemble_system(Au, Lu, BCu)
===

Similary Epetra and uBlas backends cause difficulties. The above code runs smoothly with the last line removed and the three above that uncommented.

Johan Hake suggested that the problem is that the right-hand side consists purely of facet terms.