Fix has_nonempty_nulls ignoring column offset (#13647)

This fixes `has_nonempty_nulls` that always access the column offsets without considering the starting offset. As such, it may give wrong answer if the input column is sliced.

Closes #13638.

Authors:
  - Nghia Truong (https://github.com/ttnghia)

Approvers:
  - David Wendt (https://github.com/davidwendt)
  - Raza Jafri (https://github.com/razajafri)
  - Mike Wilson (https://github.com/hyperbolic2346)

URL: #13647

cpp/src/copying/purge_nonempty_nulls.cu

@@ -42,11 +42,10 @@ bool has_nonempty_null_rows(cudf::column_view const& input, rmm::cuda_stream_vie
 
   // Cross-reference nullmask and offsets.
   auto const type         = input.type().id();
-  auto const offsets      = (type == type_id::STRING) ? (strings_column_view{input}).offsets()
-                                                      : (lists_column_view{input}).offsets();
+  auto const offsets      = (type == type_id::STRING) ? (strings_column_view{input}).offsets_begin()
+                                                      : (lists_column_view{input}).offsets_begin();
   auto const d_input      = cudf::column_device_view::create(input, stream);
-  auto const is_dirty_row = [d_input = *d_input, offsets = offsets.begin<size_type>()] __device__(
-                              size_type const& row_idx) {
+  auto const is_dirty_row = [d_input = *d_input, offsets] __device__(size_type const& row_idx) {
     return d_input.is_null_nocheck(row_idx) && (offsets[row_idx] != offsets[row_idx + 1]);
   };
 

cpp/tests/copying/purge_nonempty_nulls_tests.cpp

@@ -40,6 +40,41 @@ using gather_map_t  = cudf::test::fixed_width_column_wrapper<cudf::size_type>;
 template <typename T>
 using LCW = cudf::test::lists_column_wrapper<T, int32_t>;
 
+struct HasNonEmptyNullsTest : public cudf::test::BaseFixture {};
+
+TEST_F(HasNonEmptyNullsTest, TrivialTest)
+{
+  auto const input = LCW<T>{{{{1, 2, 3, 4}, null_at(2)},
+                             {5},
+                             {6, 7},  // <--- Will be set to NULL. Unsanitized row.
+                             {8, 9, 10}},
+                            no_nulls()}
+                       .release();
+  EXPECT_FALSE(cudf::may_have_nonempty_nulls(*input));
+  EXPECT_FALSE(cudf::has_nonempty_nulls(*input));
+
+  // Set nullmask, post construction.
+  cudf::detail::set_null_mask(
+    input->mutable_view().null_mask(), 2, 3, false, cudf::get_default_stream());
+  input->set_null_count(1);
+  EXPECT_TRUE(cudf::may_have_nonempty_nulls(*input));
+  EXPECT_TRUE(cudf::has_nonempty_nulls(*input));
+}
+
+TEST_F(HasNonEmptyNullsTest, SlicedInputTest)
+{
+  auto const input = cudf::test::strings_column_wrapper{
+    {"" /*NULL*/, "111", "222", "333", "444", "" /*NULL*/, "", "777", "888", "" /*NULL*/, "101010"},
+    cudf::test::iterators::nulls_at({0, 5, 9})};
+
+  // Split into 2 columns from rows [0, 2) and [2, 10).
+  auto const result = cudf::split(input, {2});
+  for (auto const& col : result) {
+    EXPECT_TRUE(cudf::may_have_nonempty_nulls(col));
+    EXPECT_FALSE(cudf::has_nonempty_nulls(col));
+  }
+}
+
 struct PurgeNonEmptyNullsTest : public cudf::test::BaseFixture {
   /// Helper to run gather() on a single column, and extract the single column from the result.
   std::unique_ptr<cudf::column> gather(cudf::column_view const& input,
@@ -69,15 +104,11 @@ TEST_F(PurgeNonEmptyNullsTest, SingleLevelList)
                              {8, 9, 10}},
                             no_nulls()}
                        .release();
-  EXPECT_FALSE(cudf::may_have_nonempty_nulls(*input));
-  EXPECT_FALSE(cudf::has_nonempty_nulls(*input));
 
   // Set nullmask, post construction.
   cudf::detail::set_null_mask(
     input->mutable_view().null_mask(), 2, 3, false, cudf::get_default_stream());
   input->set_null_count(1);
-  EXPECT_TRUE(cudf::may_have_nonempty_nulls(*input));
-  EXPECT_TRUE(cudf::has_nonempty_nulls(*input));
 
   test_purge(*input);