Fix stream copy on scoped host variable in json reduce_to_column_tree

cpp/src/io/json/column_tree_construction.cu

@@ -221,8 +221,12 @@ std::tuple<compressed_sparse_row, column_tree_properties> reduce_to_column_tree(
         }),
         cuda::std::plus<NodeIndexT>{});
     } else {
-      auto single_node = 1;
-      row_idx.set_element_async(1, single_node, stream);
+      // Uses thrust::fill instead of device_uvector::set_element_async to prevent the case where
+      // single_node goes out of scope before the memcpy-async(stream) completes. This is also
+      // allows us to make the copy without incurring a stream synchronize.
+      auto single_node = NodeIndexT{1};
+      auto exec_policy = rmm::exec_policy_nosync(stream, cudf::get_current_device_resource_ref());
+      thrust::fill(exec_policy, row_idx.begin() + 1, row_idx.end(), single_node);
     }
 
 #ifdef CSR_DEBUG_PRINT

cpp/tests/io/json/json_tree_csr.cu

@@ -364,3 +364,11 @@ TEST_F(JsonColumnTreeTests, JSONL_CornerCase_EmptyListOfLists)
   std::string json_string = R"([[]])";
   run_test(json_string, true);
 }
+
+TEST_F(JsonColumnTreeTests, JSONL_CornerCase_SingleColumn)
+{
+  std::string json_string = R"({}
+{}
+{})";
+  run_test(json_string, true);
+}