Error when loading mesh from statepoint

[shimwell]

Bug Description

I think the changes introduced by #3221 broke some of the mesh reading from a statepoint file. Not sure how it got through the CI but this is the error I'm seeing from this commit or newer in the develop branch

It looks like the unstructured mesh is somehow getting read in with routines designed for regularmesh. It is being assumed to be a regular mesh due to this line in mesh.py

mesh_type = 'regular' if 'type' not in group.attrs else group.attrs['type'].decode()

Steps to Reproduce

git checkout 775c41512283d178a49c787dc5fdfe617bac4938
pip install .

 import openmc
 sp=openmc.StatePoint('statepoint.4.h5')  # contains a unstructured mesh, file attached to issue
 sp.meshes

    um = sp.meshes[1] 
         ^^^^^^^^^
  File "/home/jon/openmc/openmc/statepoint.py", line 311, in meshes
    mesh = openmc.MeshBase.from_hdf5(group)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jon/openmc/openmc/mesh.py", line 107, in from_hdf5
    return RegularMesh.from_hdf5(group, mesh_id, mesh_name)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/jon/openmc/openmc/mesh.py", line 819, in from_hdf5
    mesh.dimension = group['dimension'][()]
                     ~~~~~^^^^^^^^^^^^^
  File "h5py/_objects.pyx", line 54, in h5py._objects.with_phil.wrapper
  File "h5py/_objects.pyx", line 55, in h5py._objects.with_phil.wrapper
  File "/home/jon/miniforge3/envs/openmc-dev/lib/python3.12/site-packages/h5py/_hl/group.py", line 357, in __getitem__
    oid = h5o.open(self.id, self._e(name), lapl=self._lapl)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "h5py/_objects.pyx", line 54, in h5py._objects.with_phil.wrapper
  File "h5py/_objects.pyx", line 55, in h5py._objects.with_phil.wrapper
  File "h5py/h5o.pyx", line 257, in h5py.h5o.open
KeyError: "Unable to synchronously open object (object 'dimension' doesn't exist)"

Environment

openmc develop
ubuntu 24.04
python 3.12
statepoint.4.zip

[shimwell]

a h5dump of the statepoint file shows that the type is "unstructured"

            DATASET "type" {
               DATATYPE  H5T_STRING {
                  STRSIZE 12;
                  STRPAD H5T_STR_NULLTERM;
                  CSET H5T_CSET_ASCII;
                  CTYPE H5T_C_S1;
               }
               DATASPACE  SCALAR
               DATA {
               (0): "unstructured"
               }
            }

So I think the fix is to change this line

mesh_type = 'regular' if 'type' not in group.attrs else group.attrs['type'].decode()

to

        if 'type' in group.keys():
            mesh_type = group['type'][()].decode()
        else:
            mesh_type = 'regular'

[shimwell]

Ah perhaps this is just due to me reading a statepoint file produced with the version of openmc before the #3221 and reading it with a newer version

[shimwell]

Confirmed this is just that the statepoint mesh reading part of the python API is not compatible with statepoints written with earlier versions API.

[pshriwise]

Hrmmm yeah before that PR the type was written as a dataset not an attribute, that's going to cause a compatibility issue for sure. I think we should revert that and update the Mesh.from_hdf5 code accordingly. Thanks for catching this quickly @shimwell. I'll try to find a convenient place to add a test for this as well.

[pshriwise]

OR we should increment the statepoint version, but I think in this case I'd rather revert the file structure change.