code: def _set_missing_values(
      protocol_col,
      cudf_col: cudf.core.column.ColumnBase,
      allow_copy: bool,
  ) -> cudf.core.column.ColumnBase:
      valid_mask = protocol_col.get_buffers()["validity"]
      if valid_mask is not None:
          null, invalid = protocol_col.describe_null
          if null == _MaskKind.BYTEMASK:
              valid_mask = _ensure_gpu_buffer(
                  valid_mask[0], valid_mask[1], allow_copy
              )
              boolmask = as_column(valid_mask._buf, dtype="bool")
              bitmask = cudf._lib.transform.bools_to_mask(boolmask)
              return cudf_col.set_mask(bitmask)
          elif null == _MaskKind.BITMASK:
              valid_mask = _ensure_gpu_buffer(
                  valid_mask[0], valid_mask[1], allow_copy
              )
              bitmask = valid_mask._buf
              return cudf_col.set_mask(bitmask)
      return cudf_col
  
imports: import enum
  from collections import abc
  from typing import Any, Dict, Iterable, Mapping, Optional, Sequence, Tuple, cast
  import cupy
  import numpy
  from numba.cuda import as_cuda_array
  import rmm
  import cudf
  from cudf.core.buffer import Buffer, as_buffer
  from cudf.core.column import as_column, build_categorical_column, build_column
label_desc: Use image node_id 5 for calling a global function with example usage: _set_missing_values(protocol_col, cudf_col, allow_copy) and returns: cudf_col, cudf_col, cudf_col