[BUG] lists::detail::sort_lists returns wrong result for sliced lists column containing structs entries

[ttnghia]

During working on drop_list_duplicates for structs entries, a bug shows up indicating that sliced columns of structs are not properly handled in lists::detail::sort_lists:

• With cudf::test::print(lists_original):

List<Struct<int8_t,cudf::string_view,>>:
Length : 3
Offsets : 0, 8, 16, 24
   Struct<int8_t,cudf::string_view,>:
   Length : 24:
      1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 2, 2, 2, 2, 2, 2, 3, 2, 3, 3
      Banana, Mango, Apple, Cherry, Kiwi, Banana, Cherry, Kiwi, Bear, Duck, Cat, Dog, Panda, Bear, Cat, Panda, ÁÁÁ, ÉÉÉÉÉ, ÍÍÍÍÍ, ÁBC, XYZ, ÁÁÁ, ÁBC, XYZ

• With auto const lists = cudf::slice(lists_original, {1, 3})[0];:

List<Struct<int8_t,cudf::string_view,>>(sliced):
Length : 2
Offsets : 0, 8, 16
   Struct<int8_t,cudf::string_view,>:
   Length : 16:
      1, 1, 1, 1, 2, 1, 2, 2, 2, 2, 2, 2, 3, 2, 3, 3
      Bear, Duck, Cat, Dog, Panda, Bear, Cat, Panda, ÁÁÁ, ÉÉÉÉÉ, ÍÍÍÍÍ, ÁBC, XYZ, ÁÁÁ, ÁBC, XYZ

• When calling detail::sort_lists(lists_column_view(lists), order::ASCENDING, null_order::AFTER);:

List<Struct<int8_t,cudf::string_view,>>:
Length : 2
Offsets : 0, 8, 16
   Struct<int8_t,cudf::string_view,>:
   Length : 16:
      1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2
      Banana, Banana, Apple, Cherry, Mango, Cherry, Kiwi, Kiwi, Dog, Bear, Bear, Duck, Cat, Panda, Panda, Cat

Note that sort_lists works just fine with sliced columns of basic types. I quickly look at the implementation of sort_lists and see that it calls segmented_sorted_order and gather so maybe the bug is related to one of these APIs.

[ttnghia]

Today I did a quick test:

    auto gather_map = IntsCol{0}.release();
    auto const gathered =
      cudf::gather(cudf::table_view{std::vector<cudf::column_view>{lists}}, gather_map->view())
        ->get_column(0);
    cudf::test::print(gathered.view());

And the result clearly shows that the bug is due to gather (it is the first list of lists_original column, not of lists column that gather is operating on):

List<Struct<int8_t,cudf::string_view,>>:
Length : 1
Offsets : 0, 8
   Struct<int8_t,cudf::string_view,>:
   Length : 8:
      1, 1, 1, 1, 1, 1, 1, 1
      Banana, Mango, Apple, Cherry, Kiwi, Banana, Cherry, Kiwi