perf(metadata): cache scanMetadataFiles to skip the redundant 2nd per-build app-tree walk

[hyf0]

No dependencies — reviewable and mergeable on its own. Touches only server/metadata-routes.ts + index.ts.

What

Memoizes scanMetadataFiles(appDir) per appDir (mirroring app-router's cachedGraph), and invalidates it alongside the route graph when app files change in dev.

Why

scanMetadataFiles walks the entire app tree with recursive fs.readdirSync to find file-based metadata routes (favicon, icon, opengraph-image, sitemap, robots, manifest, …). It is called from load(RESOLVED_RSC_ENTRY) (index.ts), which fires once per build pass that roots at the RSC entry — measured at exactly 2× per vinext build on the 5000-route fixture — and it had no cache, so the whole tree was walked twice. The sibling call right next to it, appRouter(...), is already memoized via cachedGraph; this just gives the metadata scan the same treatment.

Measured

Direct timing of scanMetadataFiles on the 5000-route fixture (5,037 dirs):

call	before	after
1st (miss)	~100 ms	~100 ms
2nd (per build)	~100 ms	~0 ms

So it removes one full app-tree walk per build: ~100 ms at 5,000 routes, scaling ~linearly with directory count (~200 ms at 10,000). Modest, but it's provably-redundant work and the change is tiny.

Correctness

• The cache is invalidated in invalidateAppRoutingModules() — the same path the route-graph cache uses. Metadata-file add/remove already routes there via shouldInvalidateAppRouteFile → isMetadataRouteFile, so a new favicon/sitemap/opengraph-image in dev clears the cache and re-scans (no staleness).
• Metadata files don't change within a build, so the per-build cache is always correct there.
• Callers treat the result read-only (no push/sort/splice), so returning the cached array is safe — same as cachedGraph.
• Metadata suites pass — metadata-routes, file-based-metadata, app-router-metadata-routes, metadata-route-build-data, metadata-route-response, streaming-metadata: 174 tests. vp lint clean.

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commit: cea2b29

[github-actions]

Performance benchmarks

Compared 50c18c8 against base 91c9c18 using alternating same-runner rounds. Next.js was unchanged and skipped.

0 improved · 0 regressed · 6 within ±1.5%

Scenario	Framework	Baseline	Current	Change
Client bundle size (gzip)	vinext	122.0 KB	122.0 KB	⚫ +0.0%
Client entry size (gzip)	vinext	117.0 KB	117.0 KB	⚫ +0.0%
Dev server cold start	vinext	2.55 s	2.53 s	⚫ -0.7%
Production build time	vinext	3.06 s	3.04 s	⚫ -0.7%
RSC entry closure size (gzip)	vinext	94.3 KB	94.3 KB	⚫ -0.0%
Server bundle size (gzip)	vinext	160.0 KB	160.0 KB	⚫ -0.0%

View detailed results and traces

_{🟢 improvement · 🔴 regression · ⚫ change below 1.5% · paired base/head}