feat(ci): warm Next.js builds via Turbopack persistent cache on a sticky disk (#5869)

* feat(ci): warm Next.js builds via Turbopack persistent cache on a sticky disk

- enable experimental.turbopackFileSystemCacheForBuild behind
  NEXT_TURBOPACK_BUILD_CACHE so only the CI check build opts in; production
  image builds stay on the default cold path until the feature stabilizes
- mount ./apps/sim/.next/cache as a Blacksmith sticky disk (cache-mount)
  instead of actions/cache: the turbopack cache is ~5 GB, which a sticky disk
  mounts in ~1s while an actions/cache round-trip would eat the win
- measured locally: 105s cold compile vs 22s warm (4.8x)

* chore(ci): drop the superseded actions/cache comment and restore trailing newline

.github/workflows/test-build.yml

@@ -236,21 +236,17 @@ jobs:
           key: ${{ github.repository }}-turbo-cache-build-${{ github.event_name }}${{ github.event.pull_request.head.repo.fork && '-fork' || '' }}
           path: ./.turbo
 
-      # Keyed per commit so EVERY run saves its refreshed cache — a key without
-      # a unique suffix is written once and then frozen (GitHub caches are
-      # immutable per key; a primary-key hit skips the save), which left builds
-      # compiling against a cache stale since the last lockfile change. The
-      # restore-keys prefix match picks the most recently saved cache for this
-      # lockfile, falling back across lockfile changes.
-      - name: Restore Next.js build cache
-        uses: actions/cache@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5
+      # Turbopack's persistent build cache (NEXT_TURBOPACK_BUILD_CACHE below)
+      # writes ~5 GB into .next/cache — a sticky disk mounts it in ~1s where an
+      # actions/cache round-trip would eat the warm-build win. Same event/fork
+      # namespacing as the other mounts; the GitHub fallback inside cache-mount
+      # still uses actions/cache with a run_id-suffixed key.
+      - name: Mount Next.js build cache
+        uses: ./.github/actions/cache-mount
         with:
+          provider: ${{ vars.CI_PROVIDER }}
+          key: ${{ github.repository }}-nextjs-cache-${{ github.event_name }}${{ github.event.pull_request.head.repo.fork && '-fork' || '' }}
           path: ./apps/sim/.next/cache
-          key: ${{ runner.os }}-nextjs-${{ hashFiles('bun.lock') }}-${{ github.sha }}-${{ github.run_id }}-${{ github.run_attempt }}
-          restore-keys: |
-            ${{ runner.os }}-nextjs-${{ hashFiles('bun.lock') }}-${{ github.sha }}-
-            ${{ runner.os }}-nextjs-${{ hashFiles('bun.lock') }}-
-            ${{ runner.os }}-nextjs-
 
       - name: Install dependencies
         run: bun install --frozen-lockfile
@@ -266,4 +262,7 @@ jobs:
           AWS_REGION: 'us-west-2'
           ENCRYPTION_KEY: '7cf672e460e430c1fba707575c2b0e2ad5a99dddf9b7b7e3b5646e630861db1c' # dummy key for CI only
           TURBO_CACHE_DIR: .turbo
-        run: bunx turbo run build --filter=sim
+          # Opt into Turbopack's persistent build cache (beta) for this CI
+          # check build only — measured 105s cold vs 22s warm locally.
+          NEXT_TURBOPACK_BUILD_CACHE: '1'
+        run: bunx turbo run build --filter=sim

apps/sim/next.config.ts

@@ -144,6 +144,12 @@ const nextConfig: NextConfig = {
   experimental: {
     optimizeCss: !isDev,
     turbopackFileSystemCacheForDev: false,
+    /**
+     * Turbopack's persistent build cache (beta) — opt-in via env so only the
+     * CI check build uses it; production image builds stay on the default
+     * cold-build path until the feature stabilizes.
+     */
+    turbopackFileSystemCacheForBuild: process.env.NEXT_TURBOPACK_BUILD_CACHE === '1',
     preloadEntriesOnStart: false,
     optimizePackageImports: [
       'lodash',