Small meshes fail in parallel

[connorjward]

Running the code

from firedrake import *
from gusto import Domain


m = PeriodicIntervalMesh(3, 10)  # increasing 3 to 5 here makes the error go away
mesh = ExtrudedMesh(m, layers=3, layer_height=10/3)
domain = Domain(mesh, 0.1, 'CG', 1)

with mpiexec -n 2 python myfile.py throws

Traceback (most recent call last):
  File "/home/connor/Code/firedrake-dev3/src/gusto/mydemo.py", line 7, in <module>
    domain = Domain(mesh, 0.1, 'CG', 1)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/connor/Code/firedrake-dev3/src/gusto/gusto/core/domain.py", line 152, in __init__
    self.set_height_above_surface()
  File "/home/connor/Code/firedrake-dev3/src/gusto/gusto/core/domain.py", line 179, in set_height_
above_surface
    columnwise_height, index_data = self.coords.get_column_data(CG1_height, self)
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/connor/Code/firedrake-dev3/src/gusto/gusto/core/coordinates.py", line 193, in get_co
lumn_data
    data_range = np.max(coords_X) - np.min(coords_X)
                 ^^^^^^^^^^^^^^^^
  File "/home/connor/Code/firedrake-dev3/lib/python3.12/site-packages/numpy/_core/fromnumeric.py",
 line 3199, in max
    return _wrapreduction(a, np.maximum, 'max', axis, None, out,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/connor/Code/firedrake-dev3/lib/python3.12/site-packages/numpy/_core/fromnumeric.py",
 line 86, in _wrapreduction
    return ufunc.reduce(obj, axis, dtype, out, **passkwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: zero-size array to reduction operation maximum which has no identity

which is because an array getting passed to np.max has zero size. I expect this is because one rank has no cells kept on it.