BUG: alter column type crash

[dentiny]

Hi team, the following python script crashes

import tempfile
import shutil

import pyarrow as pa
import lance

def main() -> None:
    tmpdir = tempfile.mkdtemp(prefix="lance_nested_cast_")
    try:
        # Schema: { b: struct<c: int32> }, b non-nullable, c non-nullable.
        schema = pa.schema(
            [
                pa.field(
                    "b",
                    pa.struct([pa.field("c", pa.int32(), nullable=False)]),
                    nullable=False,
                )
            ]
        )

        table = pa.table(
            {
                "b": pa.StructArray.from_arrays(
                    [pa.array([1, 2, 3], type=pa.int32())],
                    fields=[pa.field("c", pa.int32(), nullable=False)],
                )
            },
            schema=schema,
        )

        ds = lance.write_dataset(table, tmpdir)
        print("initial schema:", ds.schema)

        ds.alter_columns(
            {"path": "b.c", "data_type": pa.int64()},
        )
        print("after alter schema:", ds.schema)
        print("data:", ds.to_table().to_pylist())
    finally:
        shutil.rmtree(tmpdir, ignore_errors=True)


if __name__ == "__main__":
    main()

error message

hjiang$ python3 bug.py
initial schema: b: struct<c: int32 not null> not null
child 0, c: int32 not null

thread 'lance_background_thread' (25255780) panicked at /Users/runner/work/lance/lance/rust/lance/src/dataset/schema_evolution.rs:637:39:
called Option::unwrap() on a None value
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
Traceback (most recent call last):
File "/Users/hjiang/Desktop/bug.py", line 51, in
main()
~~~~^^
File "/Users/hjiang/Desktop/bug.py", line 41, in main
ds.alter_columns(
~~~~~~~~~~~~~~~~^
{"path": "b.c", "data_type": pa.int64()},
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
)
^
File "/opt/homebrew/lib/python3.14/site-packages/lance/dataset.py", line 1818, in alter_columns
self._ds.alter_columns(list(alterations))
~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^
RuntimeError: Task was aborted