[C++] SparseUnionArray::Make cannot build a valid array from a sliced type_ids array #54907

Describe the bug, including details regarding any error messages, version, and platform.

SparseUnionArray::Make uses type_ids.offset() as the offset of the new union array, but it checks the children's length against type_ids.length() only. As a result, there is no way to build a valid sparse union from a sliced type_ids array:

• Children of length type_ids.length() are accepted. However, the resulting array is invalid: union slot i refers to child position type_ids.offset() + i, which these children do not have.
• Children of length type_ids.offset() + type_ids.length(), which the resulting array would need, are rejected.

Reproducer (pyarrow 26.0.0 from PyPI, Python 3.12, Windows 11). UnionArray.from_sparse calls SparseUnionArray::Make and then Validate():

import pyarrow as pa

type_ids = pa.array([0, 1, 0], pa.int8()).slice(1)   # offset 1, length 2
for n in (2, 3):
    children = [pa.array(range(n), pa.int64()), pa.array(["s"] * n)]
    try:
        pa.UnionArray.from_sparse(type_ids, children)
        print(n, "accepted")
    except Exception as e:
        print(n, e)
# 2 Sparse union child array #0 has length smaller than expected for union array (2 < 3)
# 3 Sparse UnionArray must have len(child) == len(type_ids) for all children

Without the Validate() call that pyarrow adds, the first case silently produces an array that maps its slots to the wrong child positions.

Code: array_nested.cc.

One possible fix is to slice the type ids buffer and build the array with offset 0. Children would then keep corresponding to the logical positions of type_ids, as the existing error message ("len(child) == len(type_ids)") suggests. Another option is to require len(child) == type_ids.offset() + type_ids.length().

This was found with an AI-assisted analysis of the C++ code against the columnar format specification. The reproducer above was run against pyarrow 26.0.0.

Component(s)

C++