# OS Rootfs Fingerprint and Surface Cache

## Module
Scanner

## Status
VERIFIED

## Description
Root filesystem fingerprinting to uniquely identify OS layers, paired with a surface cache that avoids re-analyzing unchanged OS layers across scans.

## Implementation Details
- **Rootfs Fingerprinting**:
  - `src/Scanner/__Libraries/StellaOps.Scanner.Analyzers.OS/Internal/OsRootfsFingerprint.cs` - `OsRootfsFingerprint` generates unique fingerprints for OS root filesystem layers based on package database state
- **Surface Cache**:
  - `src/Scanner/__Libraries/StellaOps.Scanner.Analyzers.OS/Internal/OsAnalyzerSurfaceCache.cs` - `OsAnalyzerSurfaceCache` caches OS analysis results keyed by rootfs fingerprint, avoiding re-analysis of unchanged OS layers

## E2E Test Plan
- [ ] Scan a container image and verify an OS rootfs fingerprint is generated based on the package database state
- [ ] Rescan the same image and verify the surface cache returns cached results without re-analyzing the OS layer
- [ ] Scan a different image sharing the same base OS layer and verify the cache hit reuses previously computed results
- [ ] Scan an image with a modified OS layer and verify the cache miss triggers fresh OS analysis
- [ ] Verify fingerprint stability (same layer produces identical fingerprints across scans)

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## Verification

| Check | Result |
|-------|--------|
| Tier 0 - Source files exist | PASS |
| Tier 1 - Build + code review | PASS |
| Tier 2 - Integration tests | PASS |
| Verified | 2026-02-13T18:10:00Z |