fix: lazy loading to improve startup time slows down large programs

tools/@aws-cdk/lazify/lib/index.ts

@@ -160,6 +160,7 @@ export function transformFileContents(filename: string, contents: string, progre
 
         const file = require.resolve(requiredModule, { paths: [path.dirname(filename)] });
         // FIXME: Should probably do this in a subprocess
+        // FIXME: Maybe we should use the cjs-lexer
         const module = require(file);
         const entries = Object.keys(module);
 
@@ -218,6 +219,9 @@ class ExpressionGenerator {
   /**
    * Create an lazy getter for a particular value at the module level
    *
+   * The name, and the lexer
+   * -----------------------
+   *
    * Since Node statically analyzes CommonJS modules to determine its exports
    * (using the `cjs-module-lexer` module), we need to trick it into recognizing
    * these exports as legitimate.
@@ -240,7 +244,7 @@ class ExpressionGenerator {
    *
    * ```
    * exports.myExport = void 0;
-   * Object.defineProperty(exports', 'm' + 'yExport', { ... });
+   * Object.defineProperty(exports, 'm' + 'yExport', { ... });
    * ```
    *
    * Then the code passes the lexer: it detects `myExport` as an export, and it
@@ -258,11 +262,33 @@ class ExpressionGenerator {
    * ```
    * let _noFold;
    * exports.myExport = void 0;
-   * Object.defineProperty(exports', _noFold = 'myExport', { ... });
+   * Object.defineProperty(exports, _noFold = 'myExport', { ... });
    * ```
    *
    * This takes advantage of the fact that the return value of an `<x> = <y>` expression
    * returns `<y>`, but has a side effect so cannot be safely optimized away.
+   *
+   * The returned value
+   * ------------------
+   *
+   * If we only generate a getter:
+   *
+   * ```
+   * Object.defineProperty(exports, _noFold = 'myExport', { get: () => require('./file').myExport });
+   * ```
+   *
+   * If the same member is requested more than once, the same getter will be
+   * executed multiple times.  What we'll do instead is reify the lazy value on
+   * the `exports` object, so that the getter is only executed on the first access,
+   * and subsequent accesses and read the value directly.
+   *
+   * ```
+   * Object.defineProperty(exports, _noFold = 'myExport', { get: () => {
+   *   const value = require('./file').myExport;
+   *   Object.defineProperty(exports, _noFold = 'myExport', { value });
+   *   return value;
+   * });
+   * ```
    */
   public moduleGetter(
     exportName: string,
@@ -273,11 +299,7 @@ class ExpressionGenerator {
 
     const ret = [];
     if (!this.emittedNoFold) {
-      ret.push(
-        factory.createVariableStatement([],
-          factory.createVariableDeclarationList([
-            factory.createVariableDeclaration('_noFold'),
-          ])));
+      ret.push(this.createVariables(factory.createVariableDeclaration('_noFold')));
 
       this.emittedNoFold = true;
     }
@@ -291,7 +313,31 @@ class ExpressionGenerator {
         ts.SyntaxKind.EqualsToken,
         factory.createVoidZero())),
       // Object.defineProperty(exports, _noFold = "<name>", { get: () => ... });
-      factory.createExpressionStatement(factory.createCallExpression(
+      this.createDefinePropertyStatement(exportName, [
+        factory.createPropertyAssignment('get',
+          factory.createArrowFunction(undefined, undefined, [], undefined, undefined,
+            factory.createBlock([
+              this.createVariables(factory.createVariableDeclaration('value', undefined, undefined,
+                moduleFormatter(
+                  factory.createCallExpression(factory.createIdentifier('require'), undefined, [factory.createStringLiteral(moduleName)])))),
+              this.createDefinePropertyStatement(exportName, [factory.createShorthandPropertyAssignment(factory.createIdentifier('value'))]),
+              factory.createReturnStatement(factory.createIdentifier('value')),
+            ]),
+          ),
+        ),
+      ]),
+    );
+    return ret;
+  }
+
+  private createVariables(...vars: ts.VariableDeclaration[]) {
+    return this.factory.createVariableStatement([], this.factory.createVariableDeclarationList(vars));
+  }
+
+  private createDefinePropertyStatement(exportName: string, members: ts.ObjectLiteralElementLike[]) {
+    const factory = this.factory;
+
+    return factory.createExpressionStatement(factory.createCallExpression(
         factory.createPropertyAccessExpression(factory.createIdentifier('Object'), factory.createIdentifier('defineProperty')),
         undefined,
         [
@@ -300,15 +346,10 @@ class ExpressionGenerator {
           factory.createObjectLiteralExpression([
             factory.createPropertyAssignment('enumerable', factory.createTrue()),
             factory.createPropertyAssignment('configurable', factory.createTrue()),
-            factory.createPropertyAssignment('get',
-              factory.createArrowFunction(undefined, undefined, [], undefined, undefined,
-                moduleFormatter(
-                  factory.createCallExpression(factory.createIdentifier('require'), undefined, [factory.createStringLiteral(moduleName)])))),
+            ...members,
           ]),
         ]
-      )
-    ));
-    return ret;
+      ));
   }
 
   /**

tools/@aws-cdk/lazify/test/no-double-getter.test.ts

@@ -0,0 +1,49 @@
+import * as fs from 'fs-extra';
+import * as path from 'path';
+import { transformFileContents } from '../lib';
+import { parse } from 'cjs-module-lexer';
+
+// Write a .js file in this directory that will be imported by tests below
+beforeEach(async () => {
+  await fs.writeFile(path.join(__dirname, 'some-module.js'), [
+    'Object.defineProperty(module.exports, "foo", {',
+    // Necessary otherwise the way we find exported symbols (by actually including the file and iterating keys)
+    // won't find this symbol.
+    '  enumerable: true,',
+    '  get: () => {',
+    '    console.log("evaluating getter");',
+    '    return 42;',
+    '  }',
+    '})',
+  ].join('\n'), { encoding: 'utf-8' });
+});
+
+test('replace re-export with getter', () => {
+  const fakeFile = path.join(__dirname, 'index.ts');
+  const transformed = transformFileContents(fakeFile, [
+    '__exportStar(require("./some-module"), exports);'
+  ].join('\n'));
+
+  const mod = evalModule(transformed);
+
+  const logMock = jest.spyOn(console, 'log');
+  expect(mod.foo).toEqual(42);
+  expect(mod.foo).toEqual(42);
+
+  expect(logMock).toHaveBeenCalledTimes(1);
+});
+
+/**
+ * Fake NodeJS evaluation of a module
+ */
+function evalModule(x: string) {
+  const code = [
+    '(function() {',
+    'const exports = {};',
+    'const module = { exports };',
+    x,
+    'return exports;',
+    '})()',
+  ].join('\n');
+  return eval(code);
+}