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nrobi144
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Phase 1 part B. The core re-encode + downscale pipeline:
- AmethystTempDir resolves ~/.amethyst/tmp/ at mode 0700 with a
boot-time sweep of amethyst_* files > 24h. Defends against
/tmp tmpfs OOM on Linux VMs and against multi-user temp races
on shared systems. Overridable via -Damethyst.tmp.dir=.
- ImageReencoder.reencode(File, CompressionQuality) returns a
sealed ReencodeResult (Reencoded(file) | PassThrough(reason))
and throws CompressionException for fatal cases.
* Format sniffer first → pass-through for animated GIF /
animated WebP / SVG; refuse AVIF / HEIC with UnsupportedFormat.
* Pre-decode pixel guard: stream header dims via
ImageReader.getWidth(0)/getHeight(0), refuse > 50 MP before
any pixel buffer is allocated.
* Subsampled decode (floor stride) so a 4032×3024 source decodes
to ~2016×1512 in heap before Thumbnailator's final resize.
* Never upscale: Thumbnails.of(...).size() is only called when
the source actually exceeds the target box.
* CPU-bound work runs on Dispatchers.Default.limitedParallelism(1)
with ensureActive() between stages.
* Cleanup on cancellation/failure runs in NonCancellable.
12 unit tests cover preset routing, never-upscale, InputTooLarge,
AVIF/HEIC refused, animated-GIF pass-through, SVG pass-through,
JPEG SOI verification, and temp-file placement under
AmethystTempDir.
ICC profile preservation and the wide-gamut warning path land in
the next commit alongside UploadOrchestrator wiring.