module: implement the "module-sync" exports condition

This patch implements a "module-sync" exports condition
for packages to supply a sycnrhonous ES module to the
Node.js module loader, no matter it's being required
or imported. This is similar to the "module" condition
that bundlers have been using to support `require(esm)`
in Node.js, and allows dual-package authors to opt into
ESM-first only newer versions of Node.js that supports
require(esm) while avoiding the dual-package hazard.

```json
{
  "type": "module",
  "exports": {
    "node": {
      // On new version of Node.js, both require() and import get
      // the ESM version
      "module-sync": "./index.js",
      // On older version of Node.js, where "module" and
      // require(esm) are not supported, use the transpiled CJS version
      // to avoid dual-package hazard. Library authors can decide
      // to drop support for older versions of Node.js when they think
      // it's time.
      "default": "./dist/index.cjs"
    },
    // On any other environment, use the ESM version.
    "default": "./index.js"
  }
}
```

We end up implementing a condition with a different name
instead of reusing "module", because existing code in the
ecosystem using the "module" condition sometimes also expect
the module resolution for these ESM files to work in CJS
style, which is supported by bundlers, but the native
Node.js loader has intentionally made ESM resolution
different from CJS resolution (e.g. forbidding `import
'./noext'` or `import './directory'`), so it would be
semver-major to implement a `"module"` condition
without implementing the forbidden ESM resolution rules.
For now, this just implments a new condition as semver-minor
so it can be backported to older LTS.

Refs: https://webpack.js.org/guides/package-exports/#target-environment-independent-packages

doc/api/modules.md

@@ -335,9 +335,12 @@ LOAD_PACKAGE_IMPORTS(X, DIR)
 1. Find the closest package scope SCOPE to DIR.
 2. If no scope was found, return.
 3. If the SCOPE/package.json "imports" is null or undefined, return.
-4. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE),
-  ["node", "require"]) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
-5. RESOLVE_ESM_MATCH(MATCH).
+4. If `--experimental-require-module` is enabled
+  a. let CONDITIONS = ["node", "require", "module-sync"]
+  b. Else, let CONDITIONS = ["node", "require"]
+5. let MATCH = PACKAGE_IMPORTS_RESOLVE(X, pathToFileURL(SCOPE),
+  CONDITIONS) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
+6. RESOLVE_ESM_MATCH(MATCH).
 
 LOAD_PACKAGE_EXPORTS(X, DIR)
 1. Try to interpret X as a combination of NAME and SUBPATH where the name
@@ -346,9 +349,12 @@ LOAD_PACKAGE_EXPORTS(X, DIR)
    return.
 3. Parse DIR/NAME/package.json, and look for "exports" field.
 4. If "exports" is null or undefined, return.
-5. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(DIR/NAME), "." + SUBPATH,
-   `package.json` "exports", ["node", "require"]) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
-6. RESOLVE_ESM_MATCH(MATCH)
+5. If `--experimental-require-module` is enabled
+  a. let CONDITIONS = ["node", "require", "module-sync"]
+  b. Else, let CONDITIONS = ["node", "require"]
+6. let MATCH = PACKAGE_EXPORTS_RESOLVE(pathToFileURL(DIR/NAME), "." + SUBPATH,
+   `package.json` "exports", CONDITIONS) <a href="esm.md#resolver-algorithm-specification">defined in the ESM resolver</a>.
+7. RESOLVE_ESM_MATCH(MATCH)
 
 LOAD_PACKAGE_SELF(X, DIR)
 1. Find the closest package scope SCOPE to DIR.

doc/api/packages.md

@@ -665,6 +665,10 @@ specific to least specific as conditions should be defined:
   formats include CommonJS, JSON, native addons, and ES modules
   if `--experimental-require-module` is enabled. _Always mutually
   exclusive with `"import"`._
+* `"module-sync"` - matches no matter the package is loaded via `import`,
+  `import()` or `require()`. The format is expected to be ES modules that does
+  not contain top-level await in its module graph - if it does,
+  `ERR_REQUIRE_ASYNC_MODULE` will be thrown when the module is `require()`-ed.
 * `"default"` - the generic fallback that always matches. Can be a CommonJS
   or ES module file. _This condition should always come last._
 
@@ -755,7 +759,7 @@ Any number of custom conditions can be set with repeat flags.
 
 ### Community Conditions Definitions
 
-Condition strings other than the `"import"`, `"require"`, `"node"`,
+Condition strings other than the `"import"`, `"require"`, `"node"`, `"module-sync"`,
 `"node-addons"` and `"default"` conditions
 [implemented in Node.js core](#conditional-exports) are ignored by default.
 
@@ -886,6 +890,17 @@ $ node other.js
 
 ## Dual CommonJS/ES module packages
 
+<!-- This section should not be in the API documentation:
+
+1. It teaches opinionated practices that some consider dangerous, see
+   https://github.com/nodejs/node/issues/52174
+2. It will soon be obsolete when we unflag --experimental-require-module.
+3. It's difficult to understand a multi-file structure via long texts and snippets in
+   a markdown document.
+
+TODO(?): Move this section to its own repository with example folders.
+-->
+
 Prior to the introduction of support for ES modules in Node.js, it was a common
 pattern for package authors to include both CommonJS and ES module JavaScript
 sources in their package, with `package.json` [`"main"`][] specifying the
@@ -898,7 +913,7 @@ ignores) the top-level `"module"` field.
 Node.js can now run ES module entry points, and a package can contain both
 CommonJS and ES module entry points (either via separate specifiers such as
 `'pkg'` and `'pkg/es-module'`, or both at the same specifier via [Conditional
-exports][]). Unlike in the scenario where `"module"` is only used by bundlers,
+exports][]). Unlike in the scenario where top-level `"module"` field is only used by bundlers,
 or ES module files are transpiled into CommonJS on the fly before evaluation by
 Node.js, the files referenced by the ES module entry point are evaluated as ES
 modules.

lib/internal/modules/esm/get_format.js

@@ -91,7 +91,7 @@ function underNodeModules(url) {
 let typelessPackageJsonFilesWarnedAbout;
 function warnTypelessPackageJsonFile(pjsonPath, url) {
   typelessPackageJsonFilesWarnedAbout ??= new SafeSet();
-  if (!typelessPackageJsonFilesWarnedAbout.has(pjsonPath)) {
+  if (!underNodeModules(url) && !typelessPackageJsonFilesWarnedAbout.has(pjsonPath)) {
     const warning = `Module type of ${url} is not specified and it doesn't parse as CommonJS.\n` +
       'Reparsing as ES module because module syntax was detected. This incurs a performance overhead.\n' +
       `To eliminate this warning, add "type": "module" to ${pjsonPath}.`;

lib/internal/modules/esm/utils.js

@@ -83,6 +83,9 @@ function initializeDefaultConditions() {
     ...userConditions,
   ]);
   defaultConditionsSet = new SafeSet(defaultConditions);
+  if (getOptionValue('--experimental-require-module')) {
+    defaultConditionsSet.add('module-sync');
+  }
 }
 
 /**

lib/internal/modules/helpers.js

@@ -81,6 +81,9 @@ function initializeCjsConditions() {
     ...addonConditions,
     ...userConditions,
   ]);
+  if (getOptionValue('--experimental-require-module')) {
+    cjsConditions.add('module-sync');
+  }
 }
 
 /**

test/es-module/test-import-module-conditional-exports-module.mjs

@@ -0,0 +1,29 @@
+// Flags: --experimental-require-module
+
+import '../common/index.mjs';
+import assert from 'node:assert';
+import * as staticImport from '../fixtures/es-modules/module-condition/import.mjs';
+import { import as _import } from '../fixtures/es-modules/module-condition/dynamic_import.js';
+
+async function dynamicImport(id) {
+  const result = await _import(id);
+  return result.resolved;
+}
+
+assert.deepStrictEqual({ ...staticImport }, {
+  import_module_require: 'import',
+  module_and_import: 'module',
+  module_and_require: 'module',
+  module_import_require: 'module',
+  module_only: 'module',
+  module_require_import: 'module',
+  require_module_import: 'module',
+});
+
+assert.strictEqual(await dynamicImport('import-module-require'), 'import');
+assert.strictEqual(await dynamicImport('module-and-import'), 'module');
+assert.strictEqual(await dynamicImport('module-and-require'), 'module');
+assert.strictEqual(await dynamicImport('module-import-require'), 'module');
+assert.strictEqual(await dynamicImport('module-only'), 'module');
+assert.strictEqual(await dynamicImport('module-require-import'), 'module');
+assert.strictEqual(await dynamicImport('require-module-import'), 'module');

test/es-module/test-require-module-conditional-exports-module.js

@@ -0,0 +1,15 @@
+// Flags: --experimental-require-module
+'use strict';
+
+require('../common');
+const assert = require('assert');
+
+const loader = require('../fixtures/es-modules/module-condition/require.cjs');
+
+assert.strictEqual(loader.require('import-module-require').resolved, 'module');
+assert.strictEqual(loader.require('module-and-import').resolved, 'module');
+assert.strictEqual(loader.require('module-and-require').resolved, 'module');
+assert.strictEqual(loader.require('module-import-require').resolved, 'module');
+assert.strictEqual(loader.require('module-only').resolved, 'module');
+assert.strictEqual(loader.require('module-require-import').resolved, 'module');
+assert.strictEqual(loader.require('require-module-import').resolved, 'require');

test/fixtures/es-modules/module-condition/dynamic_import.js

@@ -0,0 +1,5 @@
+function load(id) {
+  return import(id);
+}
+
+export { load as import };

test/fixtures/es-modules/module-condition/import.mjs

@@ -0,0 +1,7 @@
+export { resolved as import_module_require } from 'import-module-require';
+export { resolved as module_and_import } from 'module-and-import';
+export { resolved as module_and_require } from 'module-and-require';
+export { resolved as module_import_require } from 'module-import-require';
+export { resolved as module_only } from 'module-only';
+export { resolved as module_require_import } from 'module-require-import';
+export { resolved as require_module_import } from 'require-module-import';

test/fixtures/es-modules/module-condition/require.cjs

@@ -0,0 +1 @@
+exports.require = require;