perf: buffer json file output

[He-Pin]

Motivation:

The Native stdout buffering follow-up showed that downstream buffering can materially reduce large-output write overhead. JSON -o output still sent ByteRenderer chunks directly to the file output stream, relying only on ByteBuilder's internal flush threshold.

Key Design Decision:

Keep the change local to the JSON output-file fast path. Rather than changing ByteBuilder thresholds globally, wrap the file output stream in a BufferedOutputStream with the same 256 KiB output buffer size used for the Native stdout buffering follow-up. YAML, expect-string, stdout, and renderer semantics stay unchanged.

Modification:

• Add OutputBufferSize = 256 * 1024 in SjsonnetMainBase.
• Wrap JSON output-file ByteRenderer targets in BufferedOutputStream(out, OutputBufferSize).
• Flush the buffered stream at the same completion boundary before closing the underlying file output stream.

Benchmark Results:

Workload: jrsonnet/tests/realworld/entry-kube-prometheus.jsonnet -J vendor -o /tmp/fileout-*.json

Candidate was benchmarked on the Scala Native 0.5.12 stacked exploration branch after the Native stdout buffering commit.

Order	Clean	Candidate	Result
Forward mean	217.372 ms	205.062 ms	-5.7%
Forward median	196.625 ms	183.491 ms	-6.7%
Reverse mean	210.517 ms	177.174 ms	-15.8%
Reverse median	193.394 ms	175.878 ms	-9.1%

Output equality matched by cmp.

Validation:

• ./mill --no-server --ticker false --color false __.reformat
• ./mill --no-server --ticker false --color false -j 1 __.test — 444 passed, 0 failed
• ./mill --no-server --ticker false --color false bench.runRegressions

Analysis:

This preserves the existing rendering pipeline and only changes the buffering layer for file output. It avoids global ByteBuilder threshold changes, keeps stdout behavior separate, and does not affect YAML or expect-string paths.

References:

• Native stdout buffering PR: perf: buffer native stdout writes #869
• Scala Native 0.5.12 migration PR: chore: upgrade Scala Native to 0.5.12 #867
• Related performance stack context: perf(stdlib): skip getBytes(UTF_8) for ASCII-safe base64 inputs #863, perf: bulk-write safe runs in BaseRenderer.escape #864, perf: hand-rolled YAML quote scanner removes fastparse allocations in Native #865, perf: skip UTF-8 encode for clean-ASCII long strings in renderer #866, fix: sort ASCII-safe strings by runtime kind #868

Result:

Large JSON file output writes are buffered more effectively while preserving byte-identical output and the existing flush/close contract.