VOM: doc meshes with shifted coordinate fields

They won't work!
Also add tests.
CF issue #2185

firedrake/mesh.py

@@ -1964,6 +1964,14 @@ def VertexOnlyMesh(mesh, vertexcoords, missing_points_behaviour=None):
 
         Meshes created from a coordinates `firedrake.Function` and immersed
         manifold meshes are not yet supported.
+
+    .. note::
+
+        This should also only be used for meshes which have not had their
+        coordinates field modified as, at present, this does not update the
+        coordinates field of the underlying DMPlex. Such meshes may cause
+        unexpected behavioir or hangs when running in parallel.
+
     """
 
     import firedrake.functionspace as functionspace

tests/vertexonly/test_interpolation_from_parent.py

@@ -16,7 +16,8 @@
                         "cube",
                         "tetrahedron",
                         pytest.param("immersedsphere", marks=pytest.mark.xfail(reason="immersed parent meshes not supported")),
-                        pytest.param("periodicrectangle", marks=pytest.mark.xfail(reason="meshes made from coordinate fields are not supported"))],
+                        pytest.param("periodicrectangle", marks=pytest.mark.xfail(reason="meshes made from coordinate fields are not supported")),
+                        pytest.param("shiftedmesh", marks=pytest.mark.skip(reason="meshes with modified coordinate fields are not supported"))],
                 ids=lambda x: f"{x}-mesh")
 def parentmesh(request):
     if request.param == "interval":
@@ -35,6 +36,10 @@ def parentmesh(request):
         return UnitIcosahedralSphereMesh()
     elif request.param == "periodicrectangle":
         return PeriodicRectangleMesh(3, 3, 1, 1)
+    elif request.param == "shiftedmesh":
+        m = UnitSquareMesh(1, 1)
+        m.coordinates.dat.data[:] -= 0.5
+        return m
 
 
 @pytest.fixture(params=[0, 1, 100], ids=lambda x: f"{x}-coords")

tests/vertexonly/test_swarm.py

@@ -42,7 +42,8 @@ def cell_midpoints(m):
                         "cube",
                         "tetrahedron",
                         pytest.param("immersedsphere", marks=pytest.mark.skip(reason="immersed parent meshes not supported and will segfault PETSc when creating the DMSwarm")),
-                        pytest.param("periodicrectangle", marks=pytest.mark.skip(reason="periodic meshes do not work properly with swarm creation"))])
+                        pytest.param("periodicrectangle", marks=pytest.mark.skip(reason="meshes made from coordinate fields are not supported")),
+                        pytest.param("shiftedmesh", marks=pytest.mark.skip(reason="meshes with modified coordinate fields are not supported"))])
 def parentmesh(request):
     if request.param == "interval":
         return UnitIntervalMesh(1)
@@ -58,6 +59,10 @@ def parentmesh(request):
         return UnitIcosahedralSphereMesh()
     elif request.param == "periodicrectangle":
         return PeriodicRectangleMesh(3, 3, 1, 1)
+    elif request.param == "shiftedmesh":
+        m = UnitSquareMesh(1, 1)
+        m.coordinates.dat.data[:] -= 0.5
+        return m
 
 
 # pic swarm tests

tests/vertexonly/test_vertex_only_fs.py

@@ -12,7 +12,8 @@
                         "cube",
                         "tetrahedron",
                         pytest.param("immersedsphere", marks=pytest.mark.xfail(reason="immersed parent meshes not supported")),
-                        pytest.param("periodicrectangle", marks=pytest.mark.xfail(reason="meshes made from coordinate fields are not supported"))])
+                        pytest.param("periodicrectangle", marks=pytest.mark.xfail(reason="meshes made from coordinate fields are not supported")),
+                        pytest.param("shiftedmesh", marks=pytest.mark.skip(reason="meshes with modified coordinate fields are not supported"))])
 def parentmesh(request):
     if request.param == "interval":
         return UnitIntervalMesh(1)
@@ -28,6 +29,10 @@ def parentmesh(request):
         return UnitIcosahedralSphereMesh()
     elif request.param == "periodicrectangle":
         return PeriodicRectangleMesh(3, 3, 1, 1)
+    elif request.param == "shiftedmesh":
+        m = UnitSquareMesh(1, 1)
+        m.coordinates.dat.data[:] -= 0.5
+        return m
 
 
 @pytest.fixture(params=[0, 1, 100], ids=lambda x: f"{x}-coords")

tests/vertexonly/test_vertex_only_mesh_generation.py

@@ -42,7 +42,8 @@ def cell_midpoints(m):
                         "cube",
                         "tetrahedron",
                         pytest.param("immersedsphere", marks=pytest.mark.xfail(reason="immersed parent meshes not supported")),
-                        pytest.param("periodicrectangle", marks=pytest.mark.xfail(reason="meshes made from coordinate fields are not supported"))])
+                        pytest.param("periodicrectangle", marks=pytest.mark.xfail(reason="meshes made from coordinate fields are not supported")),
+                        pytest.param("shiftedmesh", marks=pytest.mark.skip(reason="meshes with modified coordinate fields are not supported"))])
 def parentmesh(request):
     if request.param == "interval":
         return UnitIntervalMesh(1)
@@ -58,6 +59,10 @@ def parentmesh(request):
         return UnitIcosahedralSphereMesh()
     elif request.param == "periodicrectangle":
         return PeriodicRectangleMesh(3, 3, 1, 1)
+    elif request.param == "shiftedmesh":
+        m = UnitSquareMesh(1, 1)
+        m.coordinates.dat.data[:] -= 0.5
+        return m
 
 
 @pytest.fixture(params=[0, 1, 100], ids=lambda x: f"{x}-coords")