1 files changed, 117 insertions, 0 deletions
diff --git a/config/helpers/incremental_webpack_compiler/compiler.js b/config/helpers/incremental_webpack_compiler/compiler.js
new file mode 100644
index 00000000000..480d7fa3263
--- /dev/null
+++ b/config/helpers/incremental_webpack_compiler/compiler.js
@@ -0,0 +1,117 @@
+/* eslint-disable max-classes-per-file */
+
+const path = require('path');
+const { History, HistoryWithTTL } = require('./history');
+const log = require('./log');
+
+const onRequestEntryPoint = (app, callback) => {
+  app.use((req, res, next) => {
+    const fileName = path.basename(req.url);
+
+    /**
+     * We are only interested in files that have a name like `pages.foo.bar.chunk.js`
+     * because those are the ones corresponding to our entry points.
+     *
+     * This filters out hot update files that are for example named "pages.foo.bar.[hash].hot-update.js"
+     */
+    if (fileName.startsWith('pages.') && fileName.endsWith('.chunk.js')) {
+      const entryPoint = fileName.replace(/\.chunk\.js$/, '');
+      callback(entryPoint);
+    }
+
+    next();
+  });
+};
+
+/**
+ * The NoopCompiler does nothing, following the null object pattern.
+ */
+class NoopCompiler {
+  constructor() {
+    this.enabled = false;
+  }
+
+  // eslint-disable-next-line class-methods-use-this
+  filterEntryPoints(entryPoints) {
+    return entryPoints;
+  }
+
+  // eslint-disable-next-line class-methods-use-this
+  logStatus() {}
+
+  // eslint-disable-next-line class-methods-use-this
+  setupMiddleware() {}
+}
+
+/**
+ * The HistoryOnlyCompiler only records which entry points have been requested.
+ * This is so that if the user disables incremental compilation, history is
+ * still recorded. If they later enable incremental compilation, that history
+ * can be used.
+ */
+class HistoryOnlyCompiler extends NoopCompiler {
+  constructor(historyFilePath) {
+    super();
+    this.history = new History(historyFilePath);
+  }
+
+  setupMiddleware(app) {
+    onRequestEntryPoint(app, (entryPoint) => {
+      this.history.onRequestEntryPoint(entryPoint);
+    });
+  }
+}
+
+// If we force a recompile immediately, the page reload doesn't seem to work.
+// Five seconds seem to work fine and the user can read the message
+const TIMEOUT = 5000;
+
+/**
+ * The IncrementalWebpackCompiler tracks which entry points have been
+ * requested, and only compiles entry points visited within the last `ttl`
+ * days.
+ */
+class IncrementalWebpackCompiler {
+  constructor(historyFilePath, ttl) {
+    this.enabled = true;
+    this.history = new HistoryWithTTL(historyFilePath, ttl);
+  }
+
+  filterEntryPoints(entrypoints) {
+    return Object.fromEntries(
+      Object.entries(entrypoints).map(([entryPoint, paths]) => {
+        if (this.history.isRecentlyVisited(entryPoint)) {
+          return [entryPoint, paths];
+        }
+        return [entryPoint, ['./webpack_non_compiled_placeholder.js']];
+      }),
+    );
+  }
+
+  logStatus(totalCount) {
+    log(`Currently compiling route entrypoints: ${this.history.size} of ${totalCount}`);
+  }
+
+  setupMiddleware(app, server) {
+    onRequestEntryPoint(app, (entryPoint) => {
+      const wasVisitedRecently = this.history.onRequestEntryPoint(entryPoint);
+      if (!wasVisitedRecently) {
+        log(`Have not visited ${entryPoint} recently. Adding to compilation.`);
+
+        setTimeout(() => {
+          server.middleware.invalidate(() => {
+            if (server.sockets) {
+              server.sockWrite(server.sockets, 'content-changed');
+            }
+          });
+        }, TIMEOUT);
+      }
+    });
+  }
+}
+
+module.exports = {
+  NoopCompiler,
+  HistoryOnlyCompiler,
+  IncrementalWebpackCompiler,
+};