Zero-column RecordBatch loses numRows after IPC deserialization

[rustyconover]

RecordBatch.numRows returns 0 for zero-column batches deserialized from IPC, even when the IPC message header specifies a non-zero length. Zero-column RecordBatches are valid Arrow payloads and should preserve their row count through serialization round-trips.

Current behavior

1. An IPC stream contains a zero-column RecordBatch with length: 100
2. The IPC reader correctly passes header.length to makeData, and visitStruct preserves it
3. But new RecordBatch(schema, data) calls ensureSameLengthData which recomputes length as chunks.reduce((max, col) => Math.max(max, col.length), 0) — with zero children, this returns 0

Expected behavior

RecordBatch.numRows should be 100 after deserializing a zero-column batch with length: 100.

Reproducer

import { makeData, RecordBatch, Schema, Struct } from 'apache-arrow';

const schema = new Schema([]);
const data = makeData({ type: new Struct([]), length: 100, nullCount: 0, children: [] });
const batch = new RecordBatch(schema, data);

console.log(batch.numRows); // 0 — expected 100

Root cause

In src/recordbatch.ts line 84, the 2-arg constructor path calls:

[this.schema, this.data] = ensureSameLengthData<T>(this.schema, this.data.children as Data<T[keyof T]>[]);

ensureSameLengthData (line 323) defaults maxLength to chunks.reduce(...), which returns 0 for an empty array.

The 1-arg constructor path (line 102) already passes length explicitly and does not have this bug.

Fix

Pass this.data.length as the third argument to ensureSameLengthData, which already accepts an optional maxLength parameter.