fix(js): stream one record batch per result-iterator next()

[fornwall]

Problem

AdbcResultIteratorCore::next (javascript/src/client.rs) buffered the entire result set into a single contiguous IPC buffer on its first call and then marked itself exhausted:

self.exhausted = true;
let mut writer = StreamWriter::try_new(&mut output, &schema)?;
for batch in self.reader.by_ref() {   // drains the ENTIRE reader
    writer.write(&batch?)?;
}

For large result sets this materializes the whole result in memory at once — an OOM risk — and defeats the streaming abstraction the iterator name implies.

Fix

next() now yields the bytes for one record batch per call and returns Ok(None) once the reader is exhausted.

A persistent StreamWriter<Vec<u8>> is created lazily on the first call so the schema message is written exactly once at the front of the first buffer. Each subsequent buffer contains only its record batch; the underlying Vec is drained after each write. An empty result set still emits a schema-only stream (finished with the end-of-stream marker) so the consumer observes the result schema, exactly as before. Error conversion is unchanged: an arrow_schema::ArrowError propagates through ? as ClientError::Arrow.

JS-consumer compatibility (verified)

iteratorToReader in lib/index.ts feeds every per-next() chunk into a single RecordBatchReader.from(asyncIterable), which concatenates the chunks into one logical byte stream. The concatenation of the drained buffers here is byte-for-byte a single valid IPC stream (schema header followed by each batch), so the existing decoder reads every batch — no JS changes required.

This was confirmed empirically against apache-arrow 21:

• The naive "self-contained IPC stream per batch" variant would stop at the first stream's end-of-stream marker, reading only the first batch (2 of 6 rows in a 3-batch test).
• The continuous-stream approach used here reads all batches (6 of 6), and an empty-result stream still decodes to a schema with zero rows.

Testing

cd javascript && cargo build passes.

🤖 Generated with Claude Code

https://claude.ai/code/session_01XNCrC87g9MkppGpL4MDgh5