Transitioning from mesh.xml to mesh.xdmf, from dolfin-convert to meshio

mesh
49

Hi all.
I get an error trying to read a boundary XDMF file, created with pygmsh and meshio, into a dolfin MeshValueCollection.

*** Error:   Unable to find entity in map.
*** Reason:  Error reading MeshValueCollection.
*** Where:   This error was encountered inside HDF5File.cpp.

I am using dolfin master.

2D MWE:

import pygmsh
import meshio

geom = pygmsh.built_in.Geometry()
rect = geom.add_rectangle(0, 1, 0, 1, 0, lcar=0.1)
geom.add_physical(rect.surface, 0)

for i, line in enumerate(rect.lines):
    geom.add_physical(line, i + 1)

mesh = pygmsh.generate_mesh(geom)

# write to XDMF, strip 3rd dimension
meshio.write('test.xdmf',
             meshio.Mesh(points=mesh.points[:, 0:2],
                         cells={'triangle':
                                mesh.cells['triangle']}))

# write boundary markers to separate file

meshio.write(
    'test_bound.xdmf',
    meshio.Mesh(
        points=mesh.points[:, 0:2],
        cells={'line': mesh.cells['line']},
        cell_data={'line':
                   {'markers': mesh.cell_data['line']['gmsh:physical']}
                   }
    )
)


# read mesh and markers

from dolfin import *

mesh = Mesh()
XDMFFile('test.xdmf').read(mesh)

mvc = MeshValueCollection('size_t', mesh)
# gives an error
XDMFFile('test_bound.xdmf').read(mvc, 'markers')

Any help is much appreciated!