SCOTCH mesh partitioner has side effects

Subsequent calls to UnitSquareMesh(4, 4) produce different meshes (with SCOTCH partitioner). On the other hand dofmap reordering (using SCOTCH) seems to be fine. Following script demonstrates the problem:

from __future__ import print_function
from dolfin import *

parameters.mesh_partitioner = 'SCOTCH'

comm = mpi_comm_world()
rank = MPI.rank(comm)

def create_mesh():
    mesh = UnitSquareMesh(4, 4)
    return mesh.num_vertices(), mesh.num_cells()

def create_dofmap(mesh=None):
    if not mesh:
        mesh = UnitSquareMesh(4, 4)
    V = FunctionSpace(mesh, "P", 1)
    return (mesh.num_vertices(), mesh.num_cells(),
            mesh.topology().hash(), mesh.geometry().hash(),
            V.dofmap().ownership_range())

def test_determinism(func):
    result0 = func()
    while True:
        result1 = func()
        diff = int(result0 != result1)
        if MPI.max(comm, diff) > 0:
            print(rank, result0, result1)
            break

def main():
    print(rank, "Test mesh build")
    test_determinism(create_mesh)
    print()
    MPI.barrier(comm)

    print(rank, "Test mesh and dofmap build")
    test_determinism(create_dofmap)
    print()
    MPI.barrier(comm)

    print(rank, "Test dofmap build with static mesh")
    mesh = UnitSquareMesh(4, 4)
    test_determinism(lambda: create_dofmap(mesh=mesh))
    print()
    MPI.barrier(comm)

if __name__ == '__main__':
    main()

Output on my system:

1 Test mesh build
0 Test mesh build
2 Test mesh build
0 (11, 10) (11, 10)

2 (12, 11) (13, 11)
1 (12, 11) (11, 11)


2 Test mesh and dofmap build
1 Test mesh and dofmap build
0 Test mesh and dofmap build
2 (12, 11, 16345232928620356316L, 8317758475742265463, (16, 25)) (12, 11, 15156240723649878619L, 7318551913629007991, (16, 25))
1 (12, 11, 8462581566930303203L, 6060639746735424119, (8, 16)) (12, 11, 13550905085782729940L, 4625243689556973335, (7, 16))

0 (11, 10, 4607311713341590251L, 2348913127836622316, (0, 8)) (11, 10, 482017684130753681L, 4193610146276079084, (0, 7))


0 Test dofmap build with static mesh
2 Test dofmap build with static mesh
1 Test dofmap build with static mesh

and the program hangs (the last test never fails).

With parameters.mesh_partitioner = 'ParMETIS' the problem disappers (the program hangs in the first test). Dof reordering library does not play a role here.

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