jobtrackingapp/docs/_archive/performance/ROOT_CAUSE_ANALYSIS.md

Root Cause Analysis — Job Tracker resource audit

Companion to: MEMORY_LEAK_REPORT.md

Summary

There is no memory leak to root-cause. The investigation surfaced exactly one defect — an over-eager blob-URL revocation in the CV PDF carousel — which is a release-too-early bug, the inverse of a leak. This document root-causes that defect and explains why the "app memory grows" symptom does not indicate a leak here.

The one defect — over-revoked preview URLs

What the code did (before)

job-tracker-ui/src/pages/ProfilePage.tsx:

useEffect(() => {
  return () => {
    pdfCarousel.forEach((item) => item.pdfUrl && URL.revokeObjectURL(item.pdfUrl));
  };
}, [pdfCarousel]);          // <-- deps on pdfCarousel

A cleanup with [pdfCarousel] deps runs its teardown before every re-run, i.e. on every change to pdfCarousel, not just on unmount.

Why it broke

buildPdfCarousel() seeds all templates, then savePdfToCarousel() replaces each seed in place, one setPdfCarousel call at a time (ProfilePage.tsx:400-410). Trace with templates A, B, C:

1. [A₁, B₀, C₀] (A built, B/C seeds without URLs) — cleanup revoked prior [A₀,B₀,C₀] (no URLs). OK.
2. [A₁, B₁, C₀] (B built) — cleanup runs on the previous array [A₁,B₀,C₀] → revokes A₁'s URL, but A₁ is still present in the new array and still shown when the user flips the carousel to A.
3. [A₁, B₁, C₁] (C built) — cleanup revokes [A₁,B₁,C₀] → revokes B₁ too.

Result: after building an N-template deck, every preview except the last points at a revoked (broken) blob URL.

Root cause

Wrong effect dependency scope: a resource that should be released once, on unmount was tied to a value-change dependency, so React's "cleanup-before-next-run" semantics turned it into a per-change revoke. Compounded by the fact that legitimate drop paths already revoke explicitly (savePdfToCarousel replace at :402-403, resetPdfCarousel clear at :378-384), making the effect's revocation redundant and destructive.

Why it is not a leak

On unmount the effect did revoke the current array (deps capture the latest value), so URLs were freed. The bug wastes nothing and retains nothing — it releases too eagerly. It is a correctness bug (broken previews), filed here because Phase 3.5 explicitly covers "image/media resources … released".

Fix (commit eed9b1f)

Track the carousel in a ref; revoke only on unmount (empty-deps effect). Drop paths keep their explicit revokes. Verified: profile-page.test.tsx 5/5.

Why the "memory grows" symptom is not a leak here

Per the mission's Final Rule, distinguishing the four causes:

• Expected caching — MUI emotion style cache, react-scripts dev tooling, and route component state grow then plateau; not unbounded.
• Delayed GC — detached nodes from closed dialogs/pages are collected on the next major GC, not instantly; a rising sawtooth is normal.
• Browser behaviour — bfcache, image decode buffers, and devtools retention inflate numbers in a way unrelated to app code.
• Genuine leak — would require a retained root (listener, timer, global ref, live connection). None exists in this codebase (see the vector table in the main report).

Contributing (non-defect) observations

• Extraction-poll churn (ProfilePage.tsx:315-324): interval recreated every 4s while a run is active because extractionRuns is in the deps and mutates each poll. Harmless; optionally stabilise (see improvements doc).