Fix a deadlock with VertexOnlyMesh'es created not on COMM_WORLD (#3461)

* Fix a deadlock with VertexOnlyMesh'es created not on COMM_WORLD

* Add a test that checks we can make VertexOnlyMeshes on things other than COMM_WORLD

---------

Co-authored-by: Patrick Farrell <[email protected]>

Author: pefarrell

firedrake/mesh.py

@@ -2325,7 +2325,8 @@ def _c_locator(self, tolerance=None):
                     str(libspatialindex_so),
                     f"-Wl,-rpath,{sys.prefix}/lib",
                     lsi_runpath
-                ]
+                ],
+                comm=self.comm
             )
 
             locator.argtypes = [ctypes.POINTER(function._CFunction),

tests/vertexonly/test_different_comms.py

@@ -0,0 +1,13 @@
+from firedrake import *
+import pytest
+
+
+@pytest.mark.parallel(nprocs=2)
+def test_different_comms():
+
+    rank = COMM_WORLD.rank
+
+    if rank == 0:
+        mesh = UnitIntervalMesh(1, comm=COMM_SELF)
+        vom = VertexOnlyMesh(mesh, [[0.5]])
+        assert vom is not None