module: enable existing machinery for deferred import of static modules

This commit enables deferred import for statically imported modules,
by using the corresponding functionality in V8. It adds two tests:
 - smoke test exercising the basic syntax
 - test for deferred import of module with its own imports

More test coverage should be added for mixing ESM and CJS modules,
as well as TypeScript modules.

Refs: https://github.com/tc39/proposal-defer-import-eval
Signed-off-by: Maya Lekova <maya@igalia.com>

Author: MayaLekova

eslint.config.mjs

@@ -19,6 +19,7 @@ const { globalIgnores } = await importEslintTool('eslint/config');
 const { default: js } = await importEslintTool('@eslint/js');
 const { default: babelEslintParser } = await importEslintTool('@babel/eslint-parser');
 const babelPluginSyntaxImportSource = resolveEslintTool('@babel/plugin-syntax-import-source');
+const babelPluginImportDefer = resolveEslintTool('@babel/plugin-syntax-import-defer');
 const { default: jsdoc } = await importEslintTool('eslint-plugin-jsdoc');
 const { default: regexpPlugin } = await importEslintTool('eslint-plugin-regexp');
 const { default: markdown } = await importEslintTool('@eslint/markdown');
@@ -105,6 +106,7 @@ export default [
         babelOptions: {
           plugins: [
             babelPluginSyntaxImportSource,
+            babelPluginImportDefer,
           ],
         },
         requireConfigFile: false,

lib/internal/modules/esm/module_job.js

@@ -28,6 +28,7 @@ const {
   kErrored,
   kEvaluated,
   kEvaluating,
+  kDeferPhase,
   kEvaluationPhase,
   kInstantiated,
   kUninstantiated,
@@ -164,7 +165,7 @@ class ModuleJobBase {
         debug(`ModuleJobBase.syncLink() ${this.url} -> ${request.specifier}`, job);
         assert(!isPromise(job));
         assert(job.module instanceof ModuleWrap);
-        if (request.phase === kEvaluationPhase) {
+        if (this.shouldRunModule(request.phase)) {
           ArrayPrototypePush(evaluationDepJobs, job);
         }
         modules[idx] = job.module;
@@ -199,6 +200,13 @@ class ModuleJobBase {
       }
     }
   }
+
+  shouldLinkModule(phase) {
+    return phase >= kDeferPhase;
+  }
+  shouldRunModule(phase) {
+    return phase === kEvaluationPhase;
+  }
 }
 
 /* A ModuleJob tracks the loading of a single Module, and the ModuleJobs of
@@ -227,7 +235,7 @@ class ModuleJob extends ModuleJobBase {
       this.modulePromise = PromiseResolve(moduleOrModulePromise);
     }
 
-    if (this.phase === kEvaluationPhase) {
+    if (this.shouldLinkModule(this.phase)) {
       // Promise for the list of all dependencyJobs.
       this.linked = this.link(requestType);
       // This promise is awaited later anyway, so silence
@@ -279,7 +287,7 @@ class ModuleJob extends ModuleJobBase {
       const dependencyJobPromise = this.loader.getOrCreateModuleJob(this.url, request, requestType);
       const modulePromise = PromisePrototypeThen(dependencyJobPromise, (job) => {
         debug(`ModuleJob.asyncLink() ${this.url} -> ${request.specifier}`, job);
-        if (request.phase === kEvaluationPhase) {
+        if (this.shouldRunModule(request.phase)) {
           ArrayPrototypePush(evaluationDepJobs, job);
         }
         return job.modulePromise;
@@ -380,7 +388,7 @@ class ModuleJob extends ModuleJobBase {
   }
 
   runSync(parent) {
-    assert(this.phase === kEvaluationPhase);
+    assert(this.shouldRunModule(this.phase));
     assert(this.module instanceof ModuleWrap);
     let status = this.module.getStatus();
 
@@ -427,7 +435,7 @@ class ModuleJob extends ModuleJobBase {
 
   async run(isEntryPoint = false) {
     debug('ModuleJob.run()', this.module);
-    assert(this.phase === kEvaluationPhase);
+    assert(this.shouldRunModule(this.phase));
     await this.#instantiate();
     if (isEntryPoint) {
       globalThis[entry_point_module_private_symbol] = this.module;
@@ -475,7 +483,7 @@ class ModuleJobSync extends ModuleJobBase {
     assert(this.module instanceof ModuleWrap);
     this.linked = undefined;
     this.type = importAttributes.type;
-    if (phase === kEvaluationPhase) {
+    if (this.shouldLinkModule(phase)) {
       this.linked = this.link(requestType);
     }
   }
@@ -494,7 +502,7 @@ class ModuleJobSync extends ModuleJobBase {
   }
 
   async run() {
-    assert(this.phase === kEvaluationPhase);
+    assert(this.shouldRunModule(this.phase));
     // This path is hit by a require'd module that is imported again.
     const status = this.module.getStatus();
     debug('ModuleJobSync.run()', status, this.module);
@@ -523,7 +531,7 @@ class ModuleJobSync extends ModuleJobBase {
 
   runSync(parent) {
     debug('ModuleJobSync.runSync()', this.module);
-    assert(this.phase === kEvaluationPhase);
+    assert(this.shouldRunModule(this.phase));
     // TODO(joyeecheung): add the error decoration logic from the async instantiate.
     this.module.instantiate();
     // If --experimental-print-required-tla is true, proceeds to evaluation even

src/module_wrap.cc

@@ -555,6 +555,8 @@ ModulePhase to_phase_constant(ModuleImportPhase phase) {
   switch (phase) {
     case ModuleImportPhase::kEvaluation:
       return kEvaluationPhase;
+    case ModuleImportPhase::kDefer:
+      return kDeferPhase;
     case ModuleImportPhase::kSource:
       return kSourcePhase;
     default:
@@ -1682,6 +1684,7 @@ void ModuleWrap::CreatePerContextProperties(Local<Object> target,
   V(Module::Status, kErrored);
 
   V(ModulePhase, kEvaluationPhase);
+  V(ModulePhase, kDeferPhase);
   V(ModulePhase, kSourcePhase);
 #undef V
 }

src/module_wrap.h

@@ -35,7 +35,8 @@ enum HostDefinedOptions : int {
 
 enum ModulePhase : int {
   kSourcePhase = 1,
-  kEvaluationPhase = 2,
+  kDeferPhase = 2,
+  kEvaluationPhase = 3,
 };
 
 /**

test/es-module/test-defer-import-eval.mjs

@@ -0,0 +1,24 @@
+// Flags: --js-defer-import-eval
+
+// Tests that defer import actually evaluates the imported module
+// only when properties that it exports are accessed.
+
+import '../common/index.mjs';
+import * as assert from 'assert';
+
+globalThis.eval_list = [];
+
+import defer * as deferred from '../fixtures/es-modules/module-deferred-eval.mjs';
+
+assert.strictEqual(globalThis.eval_list.length, 0);
+
+// Attempts to define a property on the deferred module. This should
+// trigger its execution, similar to accessing the `foo` property.
+assert.throws(() => Object.defineProperty(deferred, 'newProp', { value: 15 }), TypeError);
+assert.strictEqual(deferred.foo, 42);
+
+// Check that the module has been evaluated at this point.
+assert.partialDeepStrictEqual(['defer-1'], globalThis.eval_list);
+
+// Clean-up
+delete globalThis.eval_list;

test/es-module/test-defer-import-with-module-tree.mjs

@@ -0,0 +1,5 @@
+// Flags: --js-defer-import-eval
+
+import '../common/index.mjs';
+
+import defer * as deferred from '../fixtures/es-modules/module-with-module-tree.mjs'

test/fixtures/es-modules/module-deferred-eval.mjs

@@ -0,0 +1,8 @@
+if (!globalThis.eval_list) {
+  globalThis.eval_list = [];
+}
+globalThis.eval_list.push('defer-1');
+
+export const foo = 42;
+
+console.log('executed');

test/fixtures/es-modules/module-with-module-tree.mjs

@@ -0,0 +1,3 @@
+import './module-deferred-eval.mjs';
+
+export const bar = 64;

tools/eslint/package-lock.json

@@ -5,6 +5,7 @@
   "dependencies": {
     "@babel/core": "^8.0.0-rc.6",
     "@babel/eslint-parser": "^8.0.0-rc.6",
+    "@babel/plugin-syntax-import-defer": "^8.0.0-rc.6",
     "@babel/plugin-syntax-import-source": "^8.0.0-rc.6",
     "@eslint/js": "^10.0.1",
     "@eslint/markdown": "^8.0.2",