mesh.write() crashes for a mesh loaded directly from a .msh file (CoordinateSystemType is None)

[lmoresi]

Found while building the #269 reproducer matrix (2026-07-23).

uw.discretisation.Mesh("file.msh") — loading a gmsh file directly, without passing coordinate_system_type= — leaves mesh.CoordinateSystemType = None. Any subsequent mesh.write() / mesh.write_timestep() then crashes writing the metadata block in Mesh.write:

coordinates_type_dict = {
    "name": self.CoordinateSystemType.name,   # AttributeError: 'NoneType' object has no attribute 'name'
    "value": self.CoordinateSystemType.value,
}

Reproducer:

import underworld3 as uw
tmp = uw.meshing.UnstructuredSimplexBox(minCoords=(0,0), maxCoords=(1,1), cellSize=0.2, filename="/tmp/box.msh")
del tmp
mesh = uw.discretisation.Mesh("/tmp/box.msh")
mesh.write("/tmp/box.h5")   # AttributeError

Workaround: load via _from_gmsh + Mesh(plex, coordinate_system_type=uw.coordinates.CoordinateSystemType.CARTESIAN, ...) (the Spiegelman-benchmark pattern), which sets the type explicitly.

Suggested fix: either default the coordinate system to CARTESIAN when loading a plain gmsh file (matching what the uw.meshing.* constructors do), or make Mesh.write tolerate/refuse-legibly a missing coordinate system rather than tracebacking in the metadata writer.

Underworld development team with AI support from Claude Code

[lmoresi]

Fixed by PR #401 (coordinate system defaults to CARTESIAN when none is given, with a rank-0 warning when an h5 checkpoint is missing its metadata), merged to development on 2026-07-23 after a public adversarial review round. (Manual close — PRs target development, so auto-close does not fire.)

Underworld development team with AI support from Claude Code