BUG: Newton's method + R space + Fieldsplit throws PyOP2 error

[ABaierReinio]

When using Newton's method on a form that involves R spaces, and doing a fieldsplit, a PyOP2 error is thrown.

MWE below:

from firedrake import *

M = UnitSquareMesh(5, 5)

V = FunctionSpace(M, "CG", 1)
R = FunctionSpace(M, "R", 0)
Z = V * R

func = Function(Z)
u, l = split(func)
v, w = TestFunctions(Z)

F = (
    (u + u**2 - 8) * v * dx
    + l * w * dx
)

newton_solver_parameters = {"snes_type" : "newtonls",
                            "snes_monitor" : "",
                            "snes_converged_reason" : "",
                            "mat_type" : "nest",
                            "ksp_type" : "fgmres",
                            "ksp_monitor" : "",
                            "pc_type": "fieldsplit",
                            "pc_fieldsplit_type": "schur",
                            "pc_fieldsplit_schur_fact_type": "full",
                            "pc_fieldsplit_0_fields": "0",
                            "pc_fieldsplit_1_fields" : "1",
                            "fieldsplit_0_ksp_type" : "preonly",
                            "fieldsplit_0_pc_type" : "lu",
                            "fieldsplit_0_pc_factor_mat_solver_type" : "mumps",
                            "fieldsplit_1_ksp_type" : "gmres",
                            "fieldsplit_1_max_it" : 1,
                            "fieldsplit_1_convergence_test" : "skip"
                            }

solve(F == 0, func, solver_parameters=newton_solver_parameters)

The script successfully does an iteration of Newton's method, but it then throws the error

  File "/firedrake/src/PyOP2/pyop2/parloop.py", line 421, in reduction_begin
    requests.append(self.comm.Iallreduce(glob._data, glob._buf, op=mpi_op))
AttributeError: 'Mat' object has no attribute '_data'