[Experimental] Move Babel transpile into a cached thread pool (#9331)

* Add cached off-thread babel transpile

* Update webpack-config.ts

Co-authored-by: Tim Neutkens <tim@timneutkens.nl>

packages/next/build/webpack-config.ts

@@ -642,7 +642,30 @@ export default async function getBaseWebpackConfig(
             }
             return /node_modules/.test(path)
           },
-          use: defaultLoaders.babel,
+          use: config.experimental.babelMultiThread
+            ? [
+                // Move Babel transpilation into a thread pool (2 workers, unlimited batch size).
+                // Applying a cache to the off-thread work avoids paying transfer costs for unchanged modules.
+                {
+                  loader: 'cache-loader',
+                  options: {
+                    cacheContext: dir,
+                    cacheDirectory: path.join(dir, '.next', 'cache', 'webpack'),
+                    cacheIdentifier: `webpack${isServer ? '-server' : ''}${
+                      config.experimental.modern ? '-hasmodern' : ''
+                    }`,
+                  },
+                },
+                {
+                  loader: 'thread-loader',
+                  options: {
+                    workers: 2,
+                    workerParallelJobs: Infinity,
+                  },
+                },
+                defaultLoaders.babel,
+              ]
+            : defaultLoaders.babel,
         },
       ].filter(Boolean),
     },

packages/next/package.json

@@ -81,6 +81,7 @@
     "babel-plugin-syntax-jsx": "6.18.0",
     "babel-plugin-transform-define": "2.0.0",
     "babel-plugin-transform-react-remove-prop-types": "0.4.24",
+    "cache-loader": "4.1.0",
     "chalk": "2.4.2",
     "ci-info": "2.0.0",
     "compression": "1.7.4",
@@ -128,6 +129,7 @@
     "style-loader": "1.0.0",
     "styled-jsx": "3.2.4",
     "terser": "4.4.2",
+    "thread-loader": "2.1.3",
     "unfetch": "4.1.0",
     "url": "0.11.0",
     "url-polyfill": "1.1.7",