Just use maxdepth to avoid thread issues with the space.

src/space.c

@@ -1270,7 +1270,7 @@ void space_split_mapper(void *map_data, int num_elements, void *extra_data) {
     }
 
     /* If the depth is too large, we have a problem and should stop. */
-    if (s->maxdepth > space_cell_maxdepth) {
+    if (maxdepth > space_cell_maxdepth) {
       error("Exceeded maximum depth (%d) when splitting cells, aborting",
             space_cell_maxdepth);
     }