Fix Submesh.submesh_parent (#4207)

* Fix Submesh.parent

Author: pbrubeck

firedrake/mesh.py

@@ -4676,6 +4676,6 @@ def Submesh(mesh, subdim, subdomain_id, label_name=None, name=None):
     submesh = Mesh(subplex, name=name, distribution_parameters={"partition": False,
                                                                 "overlap_type": (DistributedMeshOverlapType.NONE, 0)})
     submesh.topology.submesh_parent = mesh.topology
-    submesh.submesh_parent = mesh
     submesh.init()
+    submesh.submesh_parent = mesh
     return submesh

tests/firedrake/submesh/test_submesh_basics.py

@@ -0,0 +1,16 @@
+from firedrake import *
+
+
+def test_submesh_parent():
+    mesh = UnitIntervalMesh(2)
+
+    M = FunctionSpace(mesh, "DG", 0)
+    m = Function(M)
+    m.dat.data[0] = 1
+
+    cell_marker = 100
+    parent = RelabeledMesh(mesh, [m], [cell_marker])
+
+    submesh = Submesh(parent, parent.topological_dimension(), cell_marker)
+    assert submesh.topology.submesh_parent is parent.topology
+    assert submesh.submesh_parent is parent