Move ESM loaders off-thread

doc/api/esm.md

@@ -7,6 +7,9 @@
 <!-- YAML
 added: v8.5.0
 changes:
+ - version: REPLACEME
+ pr-url: https://github.com/nodejs/node/pull/44710
+ description: Loader hooks are executed off the main thread.
  - version:
  - v18.6.0
  - v16.17.0
@@ -325,6 +328,9 @@ added:
  - v13.9.0
  - v12.16.2
 changes:
+ - version: REPLACEME
+ pr-url: https://github.com/nodejs/node/pull/44710
+ description: This API now returns a string synchronously instead of a Promise.
  - version:
  - v16.2.0
  - v14.18.0
@@ -340,29 +346,26 @@ command flag enabled.
 * `specifier`{string} The module specifier to resolve relative to `parent`.
 * `parent`{string|URL} The absolute parent module URL to resolve from. If none
  is specified, the value of `import.meta.url` is used as the default.
-* Returns:{Promise}
+* Returns:{string}
 
 Provides a module-relative resolution function scoped to each module, returning
-the URL string.
+the URL string. In alignment with browser behavior, this now returns
+synchronously.
 
-<!-- eslint-skip -->
+> **Caveat** This can result in synchronous file-system operations, which
+> can impact performance similarly to `require.resolve`.
 
 ```js
-const dependencyAsset = await import.meta.resolve('component-lib/asset.css');
+const dependencyAsset = import.meta.resolve('component-lib/asset.css');
 ```
 
 `import.meta.resolve` also accepts a second argument which is the parent module
-from which to resolve from:
-
-<!-- eslint-skip -->
+from which to resolve:
 
 ```js
-await import.meta.resolve('./dep', import.meta.url);
+import.meta.resolve('./dep', import.meta.url);
 ```
 
-This function is asynchronous because the ES module resolver in Node.js is
-allowed to be asynchronous.
-
 ## Interoperability with CommonJS
 
 ### `import` statements
@@ -730,6 +733,11 @@ A hook that returns without calling `next<hookName>()` _and_ without returning
 `shortCircuit: true` also triggers an exception. These errors are to help
 prevent unintentional breaks in the chain.
 
+Hooks are run in a separate thread, isolated from the main. That means it is a
+different [realm](https://tc39.es/ecma262/#realm). The hooks thread may be
+terminated by the main thread at any time, so do not depend on asynchronous
+operations to (like `console.log`) complete.
+
 #### `resolve(specifier, context, nextResolve)`
 
 <!-- YAML
@@ -762,7 +770,7 @@ changes:
  Node.js default `resolve` hook after the last user-supplied `resolve` hook
  * `specifier`{string}
  * `context`{Object}
-* Returns:{Object}
+* Returns:{Object|Promise}
  * `format`{string|null|undefined} A hint to the load hook (it might be
  ignored)
  `'builtin' | 'commonjs' | 'json' | 'module' | 'wasm'`
@@ -772,6 +780,9 @@ changes:
  terminate the chain of `resolve` hooks. **Default:** `false`
  * `url`{string} The absolute URL to which this input resolves
 
+> **Caveat** Despite support for returning promises and async functions, calls
+> to `resolve` may block the main thread which can impact performance.
+
 The `resolve` hook chain is responsible for telling Node.js where to find and
 how to cache a given `import` statement or expression. It can optionally return
 its format (such as `'module'`) as a hint to the `load` hook. If a format is
@@ -797,7 +808,7 @@ Node.js module specifier resolution behavior_ when calling `defaultResolve`, the
 `context.conditions` array originally passed into the `resolve` hook.
 
 ```js
-export async function resolve(specifier, context, nextResolve){
+export function resolve(specifier, context, nextResolve){
  const{parentURL = null } = context;
 
  if (Math.random() > 0.5){// Some condition.
@@ -1100,7 +1111,7 @@ const baseURL = pathToFileURL(`${cwd()}/`).href;
 // CoffeeScript files end in .coffee, .litcoffee, or .coffee.md.
 const extensionsRegex = /\.coffee$|\.litcoffee$|\.coffee\.md$/;
 
-export async function resolve(specifier, context, nextResolve){
+export function resolve(specifier, context, nextResolve){
  if (extensionsRegex.test(specifier)){
  const{parentURL = baseURL } = context;
 

lib/internal/main/worker_thread.js

@@ -10,6 +10,7 @@ const{
  ObjectDefineProperty,
  PromisePrototypeThen,
  RegExpPrototypeExec,
+ SafeWeakMap,
  globalThis:{
  Atomics,
  SharedArrayBuffer,
@@ -89,23 +90,25 @@ port.on('message', (message) =>{
  const{
  argv,
  cwdCounter,
- filename,
  doEval,
- workerData,
  environmentData,
- publicPort,
+ filename,
+ hasStdin,
  manifestSrc,
  manifestURL,
- hasStdin,
+ publicPort,
+ workerData,
  } = message;
 
- if (argv !== undefined){
- ArrayPrototypePushApply(process.argv, argv);
- }
+ if (doEval !== 'internal'){
+ if (argv !== undefined){
+ ArrayPrototypePushApply(process.argv, argv);
+ }
 
- const publicWorker = require('worker_threads');
- publicWorker.parentPort = publicPort;
- publicWorker.workerData = workerData;
+ const publicWorker = require('worker_threads');
+ publicWorker.parentPort = publicPort;
+ publicWorker.workerData = workerData;
+ }
 
  require('internal/worker').assignEnvironmentData(environmentData);
 
@@ -138,31 +141,47 @@ port.on('message', (message) =>{
  debug(`[${threadId}] starts worker script ${filename} ` +
  `(eval = ${doEval}) at cwd = ${process.cwd()}`);
  port.postMessage({type: UP_AND_RUNNING });
- if (doEval === 'classic'){
- const{evalScript } = require('internal/process/execution');
- const name = '[worker eval]'
- // This is necessary for CJS module compilation.
- // TODO: pass this with something really internal.
- ObjectDefineProperty(process, '_eval',{
- __proto__: null,
- configurable: true,
- enumerable: true,
- value: filename,
- });
- ArrayPrototypeSplice(process.argv, 1, 0, name);
- evalScript(name, filename);
- } else if (doEval === 'module'){
- const{evalModule } = require('internal/process/execution');
- PromisePrototypeThen(evalModule(filename), undefined, (e) =>{
- workerOnGlobalUncaughtException(e, true);
- });
- } else{
- // script filename
- // runMain here might be monkey-patched by users in --require.
- // XXX: the monkey-patchability here should probably be deprecated.
- ArrayPrototypeSplice(process.argv, 1, 0, filename);
- const CJSLoader = require('internal/modules/cjs/loader');
- CJSLoader.Module.runMain(filename);
+ switch (doEval){
+ case 'internal':{
+ // Create this WeakMap in js-land because V8 has no C++ API for WeakMap.
+ internalBinding('module_wrap').callbackMap = new SafeWeakMap();
+ require(filename)(workerData, publicPort);
+ break;
+ }
+
+ case 'classic':{
+ const{evalScript } = require('internal/process/execution');
+ const name = '[worker eval]'
+ // This is necessary for CJS module compilation.
+ // TODO: pass this with something really internal.
+ ObjectDefineProperty(process, '_eval',{
+ __proto__: null,
+ configurable: true,
+ enumerable: true,
+ value: filename,
+ });
+ ArrayPrototypeSplice(process.argv, 1, 0, name);
+ evalScript(name, filename);
+ break;
+ }
+
+ case 'module':{
+ const{evalModule } = require('internal/process/execution');
+ PromisePrototypeThen(evalModule(filename), undefined, (e) =>{
+ workerOnGlobalUncaughtException(e, true);
+ });
+ break;
+ }
+
+ default:{
+ // script filename
+ // runMain here might be monkey-patched by users in --require.
+ // XXX: the monkey-patchability here should probably be deprecated.
+ ArrayPrototypeSplice(process.argv, 1, 0, filename);
+ const CJSLoader = require('internal/modules/cjs/loader');
+ CJSLoader.Module.runMain(filename);
+ break;
+ }
  }
  } else if (message.type === STDIO_PAYLOAD){
  const{stream, chunks } = message;