BoundaryMesh unittest hangs in parallel when using MPICH

[Johannes Ring]

The mpich buildbot fails to run the test BoundaryMesh.py (in test/unit/mesh/python) in parallel:

  http://fenicsproject.org:8010/builders/dolfin-trunk-full-mpich/builds/150/steps/make%20run_unittests/logs/stdio

The test makes the buildbot time out, and when run manually, it hangs and I get the error below.

The test pass when using SCOTCH instead of ParMETIS:

  parameters["mesh_partitioner"] = "SCOTCH"

Details about the mpich buildslave are available from http://fenicsproject.org:8010/buildslaves/mpich.

$ mpirun -np 3 python BoundaryMesh.py
Process 0: Number of global vertices: 21
Process 0: Number of global cells: 20
Process 1: *** Warning: UnitInterval is deprecated. Use UnitIntervalMesh.Process 0: *** Warning: UnitInterval is deprecated. Use UnitIntervalMesh.
Process 2: *** Warning: UnitInterval is deprecated. Use UnitIntervalMesh.

...Process 0: Number of global vertices: 25
Process 0: Number of global cells: 32
Process 0: *** Warning: UnitSquare is deprecated. Use UnitSquareMesh.Process 1: *** Warning: UnitSquare is deprecated. Use UnitSquareMesh.
Process 2: *** Warning: UnitSquare is deprecated. Use UnitSquareMesh.

...Process 0: Number of global vertices: 8
Process 0: Number of global cells: 6
Process 1: *** Warning: Mesh is empty, unable to create connectivity 2 --> 3.
Process 1: *** Warning: Mesh is empty, unable to create entities of dimension 2.
Process 1: *** Warning: Mesh is empty, unable to create entities of dimension 2.
Process 1: *** Warning: Mesh is empty, unable to create entities of dimension 2.
E
======================================================================
ERROR: test_3D_mesh (__main__.BoundaryMeshConstruction)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "BoundaryMesh.py", line 48, in test_3D_mesh
    mesh = UnitCube(1, 1, 1)
  File "/home/buildbot/fenicsbbot/trunk/dolfin-full/lib/python2.7/site-packages/dolfin/cpp/mesh.py", line 8099, in __init__
    _mesh.UnitCube_swiginit(self,_mesh.new_UnitCube(*args))
RuntimeError:

*** -------------------------------------------------------------------------
*** DOLFIN encountered an error. If you are not able to resolve this issue
*** using the information listed below, you can ask for help at
***
*** https://answers.launchpad.net/dolfin
***
*** Remember to include the error message listed below and, if possible,
*** include a *minimal* running example to reproduce the error.
***
*** -------------------------------------------------------------------------
*** Error: Unable to create mesh entity.
*** Reason: Mesh entity index 0 out of range [0, 0] for entity of dimension 2.
*** Where: This error was encountered inside MeshEntity.cpp.
*** Process: 1
*** -------------------------------------------------------------------------

----------------------------------------------------------------------
Ran 3 tests in 0.011s

FAILED (errors=1)
^CCtrl-C caught... cleaning up processes