Fix JSON parsing memory corruption - Fix Mixed types nested children …

…removal (#15798)

Fixes #15750
The references of deleted child columns are not removed, which caused segfault, and also memory errors (found with valgrind). This fix removes references of child columns and deletes them recursively.

Authors:
  - Karthikeyan (https://github.com/karthikeyann)

Approvers:
  - David Wendt (https://github.com/davidwendt)
  - Bradley Dice (https://github.com/bdice)

URL: #15798

cpp/src/io/json/json_column.cu

@@ -594,8 +594,7 @@ void make_device_json_column(device_span<SymbolT const> input,
     col.validity =
       cudf::detail::create_null_mask(col.num_rows, cudf::mask_state::ALL_NULL, stream, mr);
     col.type = json_col_t::StringColumn;
-    col.child_columns.clear();  // their references should be deleted too.
-    col.column_order.clear();
+    // destroy references of all child columns after this step, by calling remove_child_columns
   };
 
   path_from_tree tree_path{column_categories,
@@ -628,6 +627,19 @@ void make_device_json_column(device_span<SymbolT const> input,
   std::vector<uint8_t> is_pruned(num_columns, 0);
   columns.try_emplace(parent_node_sentinel, std::ref(root));
 
+  std::function<void(NodeIndexT, device_json_column&)> remove_child_columns =
+    [&](NodeIndexT this_col_id, device_json_column& col) {
+      for (auto col_name : col.column_order) {
+        auto child_id                  = mapped_columns[{this_col_id, col_name}];
+        is_mixed_type_column[child_id] = 1;
+        remove_child_columns(child_id, col.child_columns.at(col_name));
+        mapped_columns.erase({this_col_id, col_name});
+        columns.erase(child_id);
+      }
+      col.child_columns.clear();  // their references are deleted above.
+      col.column_order.clear();
+    };
+
   auto name_and_parent_index = [&is_array_of_arrays,
                                 &row_array_parent_col_id,
                                 &column_parent_ids,
@@ -721,6 +733,7 @@ void make_device_json_column(device_span<SymbolT const> input,
           auto& col = columns.at(old_col_id).get();
           if (col.type == json_col_t::ListColumn or col.type == json_col_t::StructColumn) {
             reinitialize_as_string(old_col_id, col);
+            remove_child_columns(old_col_id, col);
             // all its children (which are already inserted) are ignored later.
           }
           col.forced_as_string_column = true;

cpp/tests/io/json_test.cpp

@@ -2679,4 +2679,40 @@ TEST_F(JsonReaderTest, JsonNestedDtypeFilter)
   }
 }
 
+TEST_F(JsonReaderTest, JSONMixedTypeChildren)
+{
+  std::string const json_str = R"(
+{ "Root": { "Key": [ { "EE": "A" } ] } }
+{ "Root": { "Key": {  } } }
+{ "Root": { "Key": [{ "YY": 1}] } }
+)";
+  // Column "EE" is created and destroyed
+  // Column "YY" should not be created
+
+  cudf::io::json_reader_options options =
+    cudf::io::json_reader_options::builder(cudf::io::source_info{json_str.c_str(), json_str.size()})
+      .lines(true)
+      .recovery_mode(cudf::io::json_recovery_mode_t::RECOVER_WITH_NULL)
+      .normalize_single_quotes(true)
+      .normalize_whitespace(false)
+      .mixed_types_as_string(true)
+      .keep_quotes(true);
+
+  auto result = cudf::io::read_json(options);
+
+  ASSERT_EQ(result.tbl->num_columns(), 1);
+  ASSERT_EQ(result.metadata.schema_info.size(), 1);
+  EXPECT_EQ(result.metadata.schema_info[0].name, "Root");
+  ASSERT_EQ(result.metadata.schema_info[0].children.size(), 1);
+  EXPECT_EQ(result.metadata.schema_info[0].children[0].name, "Key");
+  ASSERT_EQ(result.metadata.schema_info[0].children[0].children.size(), 2);
+  EXPECT_EQ(result.metadata.schema_info[0].children[0].children[0].name, "offsets");
+  // types
+  EXPECT_EQ(result.tbl->get_column(0).type().id(), cudf::type_id::STRUCT);
+  EXPECT_EQ(result.tbl->get_column(0).child(0).type().id(), cudf::type_id::STRING);
+  cudf::test::strings_column_wrapper expected({R"([ { "EE": "A" } ])", "{  }", R"([{ "YY": 1}])"});
+
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected, result.tbl->get_column(0).child(0));
+}
+
 CUDF_TEST_PROGRAM_MAIN()