Fail in parallel when we've confused NetGen

This might be useful in letting application code recover from such
errors, and it's definitely useful in debugging unit tests where I've
screwed something up.

Author: roystgnr

src/mesh/mesh_netgen_interface.C

@@ -128,6 +128,12 @@ void NetGenMeshInterface::triangulate ()
       // Receive the mesh data rank 0 will send later, then fix it up
       // together
       MeshCommunication().broadcast(this->_mesh);
+
+      // If we got an empty mesh here then our tetrahedralization
+      // failed.
+      libmesh_error_msg_if (!this->_mesh.n_elem(),
+                            "NetGen failed to generate any tetrahedra");
+
       this->_mesh.prepare_for_use();
       return;
     }
@@ -326,8 +332,17 @@ void NetGenMeshInterface::triangulate ()
 
   const int n_elem = Ng_GetNE(ngmesh);
 
-  libmesh_error_msg_if (n_elem <= 0,
-                        "NetGen failed to generate any tetrahedra");
+  // If Netgen fails us, we're likely to get n_elem <= 0.  This is a
+  // common enough failure from bad setups that I want to make sure
+  // it's thrown in parallel so as to not desynchronize any unit tests
+  // that trigger it.  So we'll broadcast the empty mesh to indicate
+  // the problem and enable throwing exceptions in parallel.
+  if (n_elem <= 0)
+    {
+      this->_mesh.clear();
+      MeshCommunication().broadcast(this->_mesh);
+      libmesh_error_msg ("NetGen failed to generate any tetrahedra");
+    }
 
   const dof_id_type n_points = Ng_GetNP(ngmesh);
   const dof_id_type old_nodes = this->_mesh.n_nodes();