Make inclusive scan safe for cases with leading nulls (#7432)

This PR includes a test that passes a column with leading nulls to `scan`, which is currently failing, and a fix for the failure.

Authors:
  - Wonchan Lee (@magnatelee)

Approvers:
  - Vukasin Milovanovic (@vuule)
  - Paul Taylor (@trxcllnt)

URL: #7432

cpp/src/bitmask/null_mask.cu

@@ -135,7 +135,8 @@ void set_null_mask(bitmask_type *bitmask,
 {
   CUDF_FUNC_RANGE();
   CUDF_EXPECTS(begin_bit >= 0, "Invalid range.");
-  CUDF_EXPECTS(begin_bit < end_bit, "Invalid bit range.");
+  CUDF_EXPECTS(begin_bit <= end_bit, "Invalid bit range.");
+  if (begin_bit == end_bit) return;
   if (bitmask != nullptr) {
     auto number_of_mask_words =
       num_bitmask_words(end_bit) - begin_bit / detail::size_in_bits<bitmask_type>();

cpp/tests/bitmask/set_nullmask_tests.cu

@@ -105,8 +105,7 @@ TEST_F(SetBitmaskTest, error_range)
   std::vector<size_pair> begin_end_fail{
     {-1, size},  // begin>=0
     {-2, -1},    // begin>=0
-    {8, 8},      // begin<end
-    {9, 8},      // begin<end
+    {9, 8},      // begin<=end
   };
   for (auto begin_end : begin_end_fail) {
     auto begin = begin_end.first, end = begin_end.second;
@@ -116,8 +115,9 @@ TEST_F(SetBitmaskTest, error_range)
   std::vector<size_pair> begin_end_pass{
     {0, size},         // begin>=0
     {0, 1},            // begin>=0
-    {8, 9},            // begin<end
-    {size - 1, size},  // begin<end
+    {8, 8},            // begin==end
+    {8, 9},            // begin<=end
+    {size - 1, size},  // begin<=end
   };
   for (auto begin_end : begin_end_pass) {
     auto begin = begin_end.first, end = begin_end.second;

cpp/tests/reductions/scan_tests.cpp

@@ -470,6 +470,27 @@ TYPED_TEST(ScanTest, EmptyColumnskip_nulls)
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected_col_out2, col_out->view());
 }
 
+TYPED_TEST(ScanTest, LeadingNulls)
+{
+  auto const v = cudf::test::make_type_param_vector<TypeParam>({100, 200, 300});
+  auto const b = std::vector<bool>{0, 1, 1};
+  cudf::test::fixed_width_column_wrapper<TypeParam> const col_in(v.begin(), v.end(), b.begin());
+  std::unique_ptr<cudf::column> col_out;
+
+  // expected outputs
+  std::vector<TypeParam> out_v(v.size());
+  std::vector<bool> out_b(v.size(), 0);
+
+  // skipna=false
+  CUDF_EXPECT_NO_THROW(
+    col_out =
+      cudf::scan(col_in, cudf::make_sum_aggregation(), scan_type::INCLUSIVE, null_policy::INCLUDE));
+  cudf::test::fixed_width_column_wrapper<TypeParam> expected_col_out(
+    out_v.begin(), out_v.end(), out_b.begin());
+  CUDF_TEST_EXPECT_COLUMN_PROPERTIES_EQUAL(expected_col_out, col_out->view());
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(expected_col_out, col_out->view());
+}
+
 template <typename T>
 struct FixedPointTestBothReps : public cudf::test::BaseFixture {
 };