fix(bigquery): replace unbounded Flux BUFFER with Flux.fromStream in Query.storeResult

[fdelbrayelle]

Fix #604

Root cause

Query.storeResult used Flux.create(..., FluxSink.OverflowStrategy.BUFFER) to drive FileSerde.writeAll:

Flux<Object> flowable = Flux.create(
    s -> {
        StreamSupport.stream(result.iterateAll().spliterator(), false)
            .forEach(fv -> s.next(this.convertRows(result, fv)));
        s.complete();
    },
    FluxSink.OverflowStrategy.BUFFER
);

Flux.create is push-based: the producer (forEach) runs synchronously to completion before the subscriber drains, pumping every converted row into Reactor's internal unbounded queue. iterateAll() pages lazily from BigQuery, but that backpressure is invisible to Reactor here — every fetched row gets materialised as a LinkedHashMap and queued in JVM heap before any byte hits disk.

Customer had two parallel tasks:

• 19,681 rows × 12 cols (some REPEATED STRING) → 75.5 MiB ION → ~4 KB/row in memory
• 3,324 rows × 10 cols (same) → 60.1 MiB ION → ~18 KB/row in memory

With N parallel executions, heap spiked to ~2.5 GiB (visible in Grafana) and triggered an OOM kill at ~01:00 CEST on 2026-04-27.

Fix

Replace Flux.create(..., BUFFER) with Flux.fromStream, which is pull-based (synchronous-fuseable source). Reactor's writeAll chain pulls one row at a time via doOnNext, so only one LinkedHashMap is live during each write cycle:

Flux<Object> flowable = Flux.fromStream(
    StreamSupport.stream(result.iterateAll().spliterator(), false)
        .map(fieldValues -> (Object) this.convertRows(result, fieldValues))
);
Mono<Long> longMono = FileSerde.writeAll(output, flowable);

This matches the pattern already used in firestore/Query.java (Flux.fromIterable + FileSerde.writeAll).

The FluxSink import is removed as it is no longer referenced.

Out of scope

fetchResult (used by FETCH / FETCH_ONE) also collects to a List — same memory class of issue, separate follow-up.

[github-actions]

📦 Artifacts

Name	Size	Updated	Expiration
	108.49 MB	Apr 28, 26, 1:39:22 PM UTC	May 5, 26, 1:39:17 PM UTC

🛡 Trivy

Vulnerability in: Java

Vulnerability	Severity	Package	Installed Version	Fixed Version
GHSA-72hv-8253-57qq	MEDIUM	com.fasterxml.jackson.core:jackson-core	2.20.1	2.21.1, 2.18.6

🧪 Java Unit Tests

	Tests	Passed ✅	Skipped ⚠️	Failed	Time ⏱
Java Tests Report	117 ran	107 ✅	10 ⚠️	0 ❌	7m 14s 803ms

🔁 Unreleased Commits

3 commits since v2.2.0

SHA	Title	Author	Date
4fa4ff3	docs: add/update AGENTS.md with clear Why/What	François Delbrayelle	Apr 18, 26, 7:07:31 AM UTC
044d7b4	docs: rewrite AGENTS.md Why section	François Delbrayelle	Apr 19, 26, 7:46:28 AM UTC
8eebb99	docs: normalize README with Why and What	François Delbrayelle	Apr 19, 26, 8:06:11 AM UTC