Correctly set bitmask size in from_column_view

An empty struct column (dtype of StructDtype({})) has no children, and
hence a base_size of zero. However, it may still have a non-zero size
and non-empty null mask. When slicing such a column, the mask size
must be transferred over correctly by inspecting the size and offset
of the owning column. Previously, we incorrectly determined the sliced
column to have a mask buffer of zero bytes in this case.

Closes #13305.

python/cudf/cudf/_lib/column.pyx

@@ -155,7 +155,11 @@ cdef class Column:
             )
 
         if value is not None:
-            required_size = bitmask_allocation_size_bytes(self.base_size)
+            # base_size may be zero for a struct column with no
+            # children, but the column may have non-zero length.
+            required_size = bitmask_allocation_size_bytes(
+                max(self.base_size, self._size)
+            )
             if value.size < required_size:
                 error_msg = (
                     "The Buffer for mask is smaller than expected, "
@@ -605,7 +609,9 @@ cdef class Column:
             else:
                 mask = as_buffer(
                     data=mask_ptr,
-                    size=bitmask_allocation_size_bytes(base_size),
+                    size=bitmask_allocation_size_bytes(
+                        max(size+offset, base_size)
+                    ),
                     owner=mask_owner,
                     exposed=True
                 )

python/cudf/cudf/tests/test_struct.py

@@ -394,18 +394,7 @@ def test_struct_with_null_memory_usage():
     assert s.memory_usage() == 272
 
 
-@pytest.mark.parametrize(
-    "zlice",
-    [
-        pytest.param(
-            slice(0, 3),
-            marks=pytest.mark.xfail(
-                reason="https://github.com/rapidsai/cudf/issues/13305"
-            ),
-        ),
-        slice(1, 4),
-    ],
-)
+@pytest.mark.parametrize("zlice", [slice(0, 3), slice(1, 4)])
 def test_struct_empty_children_nulls_slice(zlice):
     values = [None, {}, {}, None]