[next] Preload common chunks (#11126)

This PR adds requiring the modules that _we know_ will be required in
the Next.js Lambda in the module scope in a attempt to make a better use
of resources.

---------

Co-authored-by: JJ Kasper <jj@jjsweb.site>

.changeset/lazy-trees-bake.md

@@ -0,0 +1,5 @@
+---
+'@vercel/next': patch
+---
+
+Load common chunks on module initialization

packages/next/src/server-build.ts

@@ -70,6 +70,30 @@ const NEXT_DATA_MIDDLEWARE_RESOLVING_VERSION = 'v12.1.7-canary.33';
 const EMPTY_ALLOW_QUERY_FOR_PRERENDERED_VERSION = 'v12.2.0';
 const CORRECTED_MANIFESTS_VERSION = 'v12.2.0';
 
+// Ideally this should be in a Next.js manifest so we can change it in
+// the future but this also allows us to improve existing versions
+const PRELOAD_CHUNKS = {
+  APP_ROUTER_PAGES: [
+    '.next/server/webpack-runtime.js',
+    'next/dist/client/components/action-async-storage.external.js',
+    'next/dist/client/components/request-async-storage.external.js',
+    'next/dist/client/components/static-generation-async-storage.external.js',
+    'next/dist/compiled/next-server/app-page.runtime.prod.js',
+  ],
+  APP_ROUTER_HANDLER: [
+    '.next/server/webpack-runtime.js',
+    'next/dist/compiled/next-server/app-route.runtime.prod.js',
+  ],
+  PAGES_ROUTER_PAGES: [
+    '.next/server/webpack-runtime.js',
+    'next/dist/compiled/next-server/pages.runtime.prod.js',
+  ],
+  PAGES_ROUTER_API: [
+    '.next/server/webpack-api-runtime.js',
+    'next/dist/compiled/next-server/pages-api.runtime.prod.js',
+  ],
+};
+
 // related PR: https://github.com/vercel/next.js/pull/52997
 // and https://github.com/vercel/next.js/pull/56318
 const BUNDLED_SERVER_NEXT_VERSION = 'v13.5.4';
@@ -1068,9 +1092,58 @@ export async function serverBuild({
         }
       }
 
+      let launcherData = group.isAppRouter ? appLauncher : launcher;
+      let preloadChunks: string[] = [];
+
+      if (process.env.VERCEL_NEXT_PRELOAD_COMMON === '1') {
+        const nextPackageDir = path.dirname(
+          resolveFrom(projectDir, 'next/package.json')
+        );
+
+        if (group.isPages) {
+          preloadChunks = PRELOAD_CHUNKS.PAGES_ROUTER_PAGES;
+        } else if (group.isApiLambda) {
+          preloadChunks = PRELOAD_CHUNKS.PAGES_ROUTER_API;
+        } else if (group.isAppRouter && !group.isAppRouteHandler) {
+          preloadChunks = PRELOAD_CHUNKS.APP_ROUTER_PAGES;
+        } else if (group.isAppRouteHandler) {
+          preloadChunks = PRELOAD_CHUNKS.APP_ROUTER_HANDLER;
+        }
+        const normalizedPreloadChunks: string[] = [];
+
+        for (const preloadChunk of preloadChunks) {
+          const absoluteChunk = preloadChunk.startsWith('.next')
+            ? path.join(projectDir, preloadChunk)
+            : path.join(nextPackageDir, '..', preloadChunk);
+
+          // ensure the chunks are actually in this layer
+          if (
+            group.pseudoLayer[
+              path.join('.', path.relative(baseDir, absoluteChunk))
+            ]
+          ) {
+            normalizedPreloadChunks.push(
+              // relative files need to be prefixed with ./ for require
+              preloadChunk.startsWith('.next')
+                ? `./${preloadChunk}`
+                : preloadChunk
+            );
+          }
+        }
+
+        if (normalizedPreloadChunks.length > 0) {
+          launcherData = launcherData.replace(
+            '// @preserve next-server-preload-target',
+            normalizedPreloadChunks
+              .map(name => `require('${name}');`)
+              .join('\n')
+          );
+        }
+      }
+
       const launcherFiles: { [name: string]: FileFsRef | FileBlob } = {
         [path.join(path.relative(baseDir, projectDir), '___next_launcher.cjs')]:
-          new FileBlob({ data: group.isAppRouter ? appLauncher : launcher }),
+          new FileBlob({ data: launcherData }),
       };
       const operationType = getOperationType({ group, prerenderManifest });
 

packages/next/src/server-launcher.ts

@@ -23,6 +23,8 @@ if (process.env.NODE_ENV !== 'production' && region !== 'dev1') {
 // eslint-disable-next-line
 const NextServer = require('__NEXT_SERVER_PATH__').default;
 
+// @preserve next-server-preload-target
+
 // __NEXT_CONFIG__ value is injected
 declare const __NEXT_CONFIG__: any;
 const conf = __NEXT_CONFIG__;