perf: sign matching variations on the worker pool, not the event loop - #372
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nGervasyuk merged 2 commits intoSep 1, 2026
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📝 WalkthroughWalkthroughThe change-signature computation moved to a worker-safe module. ChangesChange signature comparison
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🟡 Moderate · up to Signature processing now decodes uploaded PNGs concurrently without enforcing size and pixel limits before decoding, so an authenticated oversized upload could amplify memory use and affect service availability. Merge should wait for those bounds to be enforced or for explicit security-owner acceptance. Sequence Diagram(s)sequenceDiagram
participant TestRunsService
participant StaticService
participant CompareService
participant DiffWorkerPool
TestRunsService->>TestRunsService: filter siblings by diffPercent
TestRunsService->>StaticService: fetch baseline and image buffers
StaticService-->>TestRunsService: return PNG buffers
TestRunsService->>CompareService: request signatures concurrently
CompareService->>DiffWorkerPool: run signature jobs
DiffWorkerPool-->>CompareService: return signatures
CompareService-->>TestRunsService: return signatures
TestRunsService->>TestRunsService: compare signatures and classify siblings
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Opening "Approve variations" on a build with many locales took ~20s and blocked every other request while it ran. Finding the siblings that carry the same change signed the reviewed run and each sibling one after another, and each signature synchronously decoded two full-size PNGs on the main thread — 66 decodes back to back for a screen with 33 variations. The signature now runs as a job on the existing diff worker pool, next to the pixelmatch diff it already carried: the pool's job type became a union discriminated on `kind`, and both the worker and its inline fallback go through one dispatcher so the two paths cannot answer differently. The undecoded bytes travel to the worker, so no screenshot is decoded on the event loop any more. On top of that: - The reviewed run and its siblings are signed in one fan-out, bounded to 8 in flight so a 100-locale screen cannot flood the queue that build ingestion shares. - The free checks run first. A sibling whose change is far larger, or that has no change at all, is answered from its diffPercent and never costs a decode. The skipped list is put back in the siblings' own order so the dialog does not reorder by which check rejected what. - Signatures are memoized per image pair, ignore areas and diff config, so reopening the dialog or stepping back to a screen is free. Bounded to 2000 entries; a failed read is not remembered. The histogram, downscale and cosine similarity moved to signature.core.ts unchanged, and are now covered directly — position independence, ignore areas and dimension mismatch included.
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Inline comments:
In `@src/compare/libs/pixelmatch/signature.core.ts`:
- Around line 87-88: Update computeChangeSignature to catch PNG.sync.read
failures for both baselineImage and checkpointImage decoding, and return {
signature: null } when either input is corrupt or truncated. Preserve normal
signature computation for successfully decoded images so unreadable candidates
are skipped without rejecting the overall matching operation.
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src/compare/compare.service.tssrc/compare/diff-worker-pool.spec.tssrc/compare/diff-worker-pool.tssrc/compare/libs/pixelmatch/pixelmatch.core.tssrc/compare/libs/pixelmatch/pixelmatch.service.tssrc/compare/libs/pixelmatch/pixelmatch.worker.tssrc/compare/libs/pixelmatch/signature.core.spec.tssrc/compare/libs/pixelmatch/signature.core.tssrc/compare/libs/pixelmatch/worker-job.tssrc/test-runs/test-runs.service.spec.tssrc/test-runs/test-runs.service.ts
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Moving the signature onto the worker pool sent raw PNG bytes rather than going through staticService.getImage, which had been catching decode failures and answering undefined — a corrupt or truncated screenshot used to leave its sibling reported as "no diff to match". Undecodable bytes now throw out of the worker instead, and because the siblings are signed in a single fan-out, one bad candidate rejects the whole Approve variations request rather than being left out of it. Guard both decodes and answer with no signature, as the doc comment above already claimed. Found by CodeRabbit on Visual-Regression-Tracker#372.
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Inline comments:
In `@src/compare/libs/pixelmatch/signature.core.ts`:
- Around line 93-94: Update the image ingestion and signature path around
toBuffer and PNG.sync.read to enforce compressed-byte and decoded-pixel limits
before persisting uploads and before decoding stored baselineImage and image
data. Validate PNG dimensions from the encoded bytes without allocating
full-resolution buffers, reject inputs exceeding SIGNATURE_MAX_DIMENSION or the
configured pixel limit, and preserve normal processing for compliant images.
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What
Opening Approve variations on a build with many locales took ~20 s, and blocked every other request for the whole time. On a 9588-run iOS build, a screen with 33 variations spent that entire window on a spinner.
findMatchingSiblingssigned the reviewed run and then each sibling one after another, and each signature calledstaticService.getImage→PNG.sync.read— a synchronous full-resolution PNG decode. Two decodes per run, 66 back to back on the event loop.The diff worker pool added in #368 was right there, but only
compareService.getDiffcould reach it.How
kind('diff' | 'signature'). Both the worker thread and the pool's inline fallback go through onerunWorkerJobdispatcher, so the two paths cannot answer a job differently.getImageBuffer), so no screenshot is decoded on the event loop any more.diffPercentratio > 2), or that has no change at all, is answered from its row and never costs a decode. Previously the cheap magnitude test ran after the expensive signature. The skipped list is restored to the siblings' own order, so the dialog does not reorder by which check rejected what.The histogram, downscale and cosine similarity moved to
signature.core.tsunchanged, and are now covered directly rather than only through the service.Behaviour
Unchanged, with one deliberate exception: a sibling with no diff at all used to be reported as
no diff to matchby the signature pass; now the cheap pass reaches the same verdict fromdiffPercentand reports the same reason. Without that, it would have been mislabelleddifferent change size.Tests
signature.core.spec.ts— position independence, ignore areas, dimension mismatch, colour discrimination, nothing-changed.diff-worker-pool.spec.ts— both job kinds route correctly.test-runs.service.spec.ts— concurrency (peak in flight = siblings + 1), the bound, the cheap-check ordering, memoization, and cache invalidation on an ignore-area change.Every one was watched failing first; the memoization and ordering tests were also re-checked by reverting each change in isolation.
All 36 backend spec files pass. (The full parallel run trips over a missing Prisma engine binary in the sandbox, identically on
master, so suites were run serially.)Summary by CodeRabbit
New Features
Performance
Bug Fixes