Fix empty mask behavior of apply_deletion_mask

cpp/include/cudf/detail/stream_compaction.hpp

@@ -48,6 +48,9 @@ enum class mask_type : bool {
 /**
  * @brief Filters @p input using @p boolean_mask.
  *
+ * @note An empty mask returns an empty table for @p mask_kind `RETENTION` and a copy of @p input
+ * for @p mask_kind `DELETION`.
+ *
  * @param input The input table to filter
  * @param boolean_mask A nullable BOOL8 column used to filter @p input
  * @param mask_kind Specifies how the boolean mask is treated (retentions or deletions)

cpp/include/cudf/stream_compaction.hpp

@@ -244,7 +244,8 @@ std::unique_ptr<table> apply_retention_mask(
  * if the corresponding element `i` in the mask is non-null and `false`.
  * This operation is stable: the input order is preserved.
  *
- * @note If @p deletion_mask is empty, or @p input has zero rows, an empty table is returned.
+ * @note If @p deletion_mask is empty, a copy of @p input is returned. If @p input has zero rows,
+ * an empty table is returned.
  *
  * @throws cudf::logic_error if non-empty @p input has different number of rows than @p
  * deletion_mask.

cpp/src/stream_compaction/apply_mask.cu

@@ -67,15 +67,18 @@ std::unique_ptr<table> apply_mask(table_view const& input,
                                   cuda::stream_ref stream,
                                   rmm::device_async_resource_ref mr)
 {
-  if (boolean_mask.is_empty()) { return empty_like(input); }
+  auto const is_retention = (mask_kind == mask_type::RETENTION);
+
+  if (boolean_mask.is_empty()) {
+    return is_retention ? empty_like(input) : std::make_unique<table>(input, stream, mr);
+  }
 
   CUDF_EXPECTS(boolean_mask.type().id() == type_id::BOOL8, "Mask must be Boolean type");
   CUDF_EXPECTS(input.num_rows() == 0 || input.num_rows() == boolean_mask.size(),
                "Column size mismatch");
 
   auto device_boolean_mask = cudf::column_device_view::create(boolean_mask, stream);
 
-  auto const is_retention = (mask_kind == mask_type::RETENTION);
   if (boolean_mask.has_nulls()) {
     if (is_retention) {
       return detail::copy_if(input, retention_mask_filter<true>{*device_boolean_mask}, stream, mr);

cpp/tests/stream_compaction/apply_mask_tests.cpp

@@ -415,14 +415,10 @@ TEST_F(ApplyDeletionMask, EmptyMask)
   cudf::test::fixed_width_column_wrapper<double> col3{{10, 40, 70, 5, 2, 10}, {1, 1, 0, 1, 1, 0}};
   cudf::table_view input{{col1, col2, col3}};
   cudf::test::fixed_width_column_wrapper<bool> deletion_mask{};
-  cudf::test::fixed_width_column_wrapper<int16_t> col1_expected{};
-  cudf::test::fixed_width_column_wrapper<int32_t> col2_expected{};
-  cudf::test::fixed_width_column_wrapper<double> col3_expected{};
-  cudf::table_view expected{{col1_expected, col2_expected, col3_expected}};
 
   auto got = cudf::apply_deletion_mask(input, deletion_mask);
 
-  CUDF_TEST_EXPECT_TABLES_EQUAL(expected, got->view());
+  CUDF_TEST_EXPECT_TABLES_EQUAL(input, got->view());
 }
 
 TEST_F(ApplyDeletionMask, WrongMaskType)

python/pylibcudf/pylibcudf/lists.pyx

@@ -860,14 +860,14 @@ cpdef Column apply_retention_mask(
     input : Column
         The input column.
     retention_mask : Column
-        The boolean mask whose true values retain input elements.
+        A lists-of-bools column used as a retention mask.
     stream : Stream | None
         CUDA stream on which to perform the operation.
 
     Returns
     -------
     Column
-        A Column of filtered elements based upon the retention mask.
+        Lists column with elements kept where retention mask is valid and true.
     """
     cdef unique_ptr[column] c_result
     cdef ListsColumnView list_view = input.list_view()
@@ -924,7 +924,7 @@ cpdef Column apply_deletion_mask(
     Returns
     -------
     Column
-        Lists column with elements removed where deletion_mask is true.
+        Lists column with elements removed where deletion mask is valid and true.
     """
     cdef unique_ptr[column] c_result
     cdef ListsColumnView list_view = input.list_view()

python/pylibcudf/pylibcudf/stream_compaction.pyx

@@ -143,12 +143,12 @@ cpdef Table apply_retention_mask(
     source_table : Table
         The input table to filter.
     retention_mask : Column
-        The boolean mask whose true values retain input rows.
+        A boolean column used as a retention mask.
 
     Returns
     -------
     Table
-        A new table with rows removed based on the boolean mask.
+        A new table with rows kept where retention mask is valid and true.
     """
     cdef unique_ptr[table] c_result
 
@@ -200,7 +200,7 @@ cpdef Table apply_deletion_mask(
     Returns
     -------
     Table
-        Table with rows removed where deletion_mask is true.
+        Table with rows removed where deletion mask is valid and true.
     """
     cdef unique_ptr[table] c_result