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module: add support for node:‑prefixed require(…) calls #37246

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Mar 19, 2021
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module: add support for node:‑prefixed require(…) calls #37246

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069b5df
module: add support for `node:`‑prefixed `require(…)` calls
ExE-Boss Feb 6, 2021
95391fe
repl: add auto‑completion for `node:`‑prefixed `require(…)` calls
ExE-Boss Feb 6, 2021
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4 changes: 4 additions & 0 deletions doc/api/esm.md
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Original file line number Diff line number Diff line change
Expand Up @@ -204,6 +204,10 @@ import _ from 'data:application/json,"world!"';
added:
- v14.13.1
- v12.20.0
changes:
- version: REPLACEME
pr-url: https://github.com/nodejs/node/pull/37246
description: Added `node:` import support to `require(...)`.
-->

`node:` URLs are supported as an alternative means to load Node.js builtin
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26 changes: 24 additions & 2 deletions doc/api/modules.md
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Expand Up @@ -280,6 +280,12 @@ irrespective of whether or not `./foo` and `./FOO` are the same file.
## Core modules

<!--type=misc-->
<!-- YAML
changes:
- version: REPLACEME
pr-url: https://github.com/nodejs/node/pull/37246
description: Added `node:` import support to `require(...)`.
-->

Node.js has several modules compiled into the binary. These modules are
described in greater detail elsewhere in this documentation.
Expand All @@ -291,6 +297,11 @@ Core modules are always preferentially loaded if their identifier is
passed to `require()`. For instance, `require('http')` will always
return the built in HTTP module, even if there is a file by that name.

Core modules can also be identified using the `node:` prefix, in which case
it bypasses the `require` cache. For instance, `require('node:http')` will
always return the built in HTTP module, even if there is `require.cache` entry
by that name.
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## Cycles

<!--type=misc-->
Expand Down Expand Up @@ -642,8 +653,19 @@ error.

Adding or replacing entries is also possible. This cache is checked before
native modules and if a name matching a native module is added to the cache,
no require call is
going to receive the native module anymore. Use with care!
only `node:`-prefixed require calls are going to receive the native module.
Use with care!

```js
const assert = require('assert');
const realFs = require('fs');

const fakeFs = {};
require.cache.fs = { exports: fakeFs };

assert.strictEqual(require('fs'), fakeFs);
assert.strictEqual(require('node:fs'), realFs);
```

#### `require.extensions`
<!-- YAML
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3 changes: 2 additions & 1 deletion lib/internal/modules/cjs/helpers.js
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Expand Up @@ -34,8 +34,9 @@ const cjsConditions = new SafeSet(['require', 'node', ...userConditions]);

function loadNativeModule(filename, request) {
const mod = NativeModule.map.get(filename);
if (mod) {
if (mod?.canBeRequiredByUsers) {
debug('load native module %s', request);
// compileForPublicLoader() throws if mod.canBeRequiredByUsers is false:
mod.compileForPublicLoader();
return mod;
}
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17 changes: 15 additions & 2 deletions lib/internal/modules/cjs/loader.js
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Expand Up @@ -110,7 +110,8 @@ let hasLoadedAnyUserCJSModule = false;
const {
ERR_INVALID_ARG_VALUE,
ERR_INVALID_MODULE_SPECIFIER,
ERR_REQUIRE_ESM
ERR_REQUIRE_ESM,
ERR_UNKNOWN_BUILTIN_MODULE,
} = require('internal/errors').codes;
const { validateString } = require('internal/validators');
const pendingDeprecation = getOptionValue('--pending-deprecation');
Expand Down Expand Up @@ -770,6 +771,17 @@ Module._load = function(request, parent, isMain) {
}

const filename = Module._resolveFilename(request, parent, isMain);
if (StringPrototypeStartsWith(filename, 'node:')) {
// Slice 'node:' prefix
const id = StringPrototypeSlice(filename, 5);

const module = loadNativeModule(id, request);
if (!module?.canBeRequiredByUsers) {
throw new ERR_UNKNOWN_BUILTIN_MODULE(filename);
}

return module.exports;
}
Comment on lines +774 to +784
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What is the motivation for short-circuiting the cache here, something that has never been done previously for native modules?

I don't see why this scheme should be any different to any other in this regard.

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On this topic, should we throw or defer to cache on unknown builtins?


const cachedModule = Module._cache[filename];
if (cachedModule !== undefined) {
Expand Down Expand Up @@ -841,7 +853,8 @@ Module._load = function(request, parent, isMain) {
};

Module._resolveFilename = function(request, parent, isMain, options) {
if (NativeModule.canBeRequiredByUsers(request)) {
if (StringPrototypeStartsWith(request, 'node:') ||
NativeModule.canBeRequiredByUsers(request)) {
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This bifurcation will have implications for mocking, but I can appreciate the benefit too.

I'd still prefer a more convergent path here eg, to return node:fs for both node:fs and fs inputs, and then to still populate a cache['fs'] entry in the node:fs case as the same object for backwards compatibility. Such a change would hopefully be mostly compatible but would probably need to be a separate major nontheless. Worth thinking about at least.

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Existing mocking tools (such as Jest) provide their own require(…) implementation, since they need to be able to bypass the mocked module using jest.requireActual(…).

return request;
}

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2 changes: 1 addition & 1 deletion lib/internal/modules/esm/translators.js
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Expand Up @@ -282,7 +282,7 @@ translators.set('builtin', async function builtinStrategy(url) {
debug(`Translating BuiltinModule ${url}`);
// Slice 'node:' scheme
const id = StringPrototypeSlice(url, 5);
const module = loadNativeModule(id, url, true);
const module = loadNativeModule(id, url);
if (!StringPrototypeStartsWith(url, 'node:') || !module) {
throw new ERR_UNKNOWN_BUILTIN_MODULE(url);
}
Expand Down
4 changes: 2 additions & 2 deletions lib/repl.js
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Expand Up @@ -1113,7 +1113,7 @@ REPLServer.prototype.setPrompt = function setPrompt(prompt) {
};

const importRE = /\bimport\s*\(\s*['"`](([\w@./:-]+\/)?(?:[\w@./:-]*))(?![^'"`])$/;
const requireRE = /\brequire\s*\(\s*['"`](([\w@./-]+\/)?(?:[\w@./-]*))(?![^'"`])$/;
const requireRE = /\brequire\s*\(\s*['"`](([\w@./:-]+\/)?(?:[\w@./:-]*))(?![^'"`])$/;
const fsAutoCompleteRE = /fs(?:\.promises)?\.\s*[a-z][a-zA-Z]+\(\s*["'](.*)/;
const simpleExpressionRE =
/(?:[a-zA-Z_$](?:\w|\$)*\??\.)*[a-zA-Z_$](?:\w|\$)*\??\.?$/;
Expand Down Expand Up @@ -1275,7 +1275,7 @@ function complete(line, callback) {
}

if (!subdir) {
ArrayPrototypePush(completionGroups, _builtinLibs);
ArrayPrototypePush(completionGroups, _builtinLibs, nodeSchemeBuiltinLibs);
}
} else if (RegExpPrototypeTest(importRE, line) &&
this.allowBlockingCompletions) {
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2 changes: 2 additions & 0 deletions test/es-module/test-esm-dynamic-import.js
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Expand Up @@ -51,6 +51,8 @@ function expectFsNamespace(result) {

expectModuleError(import('node:unknown'),
'ERR_UNKNOWN_BUILTIN_MODULE');
expectModuleError(import('node:internal/test/binding'),
'ERR_UNKNOWN_BUILTIN_MODULE');
expectModuleError(import('./not-an-existing-module.mjs'),
'ERR_MODULE_NOT_FOUND');
expectModuleError(import('http://example.com/foo.js'),
Expand Down
18 changes: 13 additions & 5 deletions test/parallel/test-repl-tab-complete.js
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Expand Up @@ -31,6 +31,10 @@ const assert = require('assert');
const path = require('path');
const fixtures = require('../common/fixtures');
const { builtinModules } = require('module');
const publicModules = builtinModules.filter(
(lib) => !lib.startsWith('_') && !lib.includes('/'),
);

const hasInspector = process.features.inspector;

if (!common.isMainThread)
Expand Down Expand Up @@ -239,9 +243,9 @@ putIn.run(['.clear']);

testMe.complete('require(\'', common.mustCall(function(error, data) {
assert.strictEqual(error, null);
builtinModules.forEach((lib) => {
publicModules.forEach((lib) => {
assert(
data[0].includes(lib) || lib.startsWith('_') || lib.includes('/'),
data[0].includes(lib) && data[0].includes(`node:${lib}`),
`${lib} not found`
);
});
Expand All @@ -258,11 +262,15 @@ testMe.complete("require\t( 'n", common.mustCall(function(error, data) {
assert.strictEqual(error, null);
assert.strictEqual(data.length, 2);
assert.strictEqual(data[1], 'n');
// require(...) completions include `node:`-prefixed modules:
publicModules.forEach((lib, index) =>
assert.strictEqual(data[0][index], `node:${lib}`));
assert.strictEqual(data[0][publicModules.length], '');
// There is only one Node.js module that starts with n:
assert.strictEqual(data[0][0], 'net');
assert.strictEqual(data[0][1], '');
assert.strictEqual(data[0][publicModules.length + 1], 'net');
assert.strictEqual(data[0][publicModules.length + 2], '');
// It's possible to pick up non-core modules too
data[0].slice(2).forEach((completion) => {
data[0].slice(publicModules.length + 3).forEach((completion) => {
assert.match(completion, /^n/);
});
}));
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42 changes: 42 additions & 0 deletions test/parallel/test-require-node-prefix.js
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@@ -0,0 +1,42 @@
'use strict';

require('../common');
const assert = require('assert');
const fs = require('fs');

const errUnknownBuiltinModuleRE = /^No such built-in module: /u;

// For direct use of require expressions inside of CJS modules,
// all kinds of specifiers should work without issue.
{
assert.strictEqual(require('fs'), fs);
assert.strictEqual(require('node:fs'), fs);

assert.throws(
() => require('node:unknown'),
{
code: 'ERR_UNKNOWN_BUILTIN_MODULE',
message: errUnknownBuiltinModuleRE,
},
);

assert.throws(
() => require('node:internal/test/binding'),
{
code: 'ERR_UNKNOWN_BUILTIN_MODULE',
message: errUnknownBuiltinModuleRE,
},
);
}

// `node:`-prefixed `require(...)` calls bypass the require cache:
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this differs from other builtins, I love it, but want to be sure it is intentional.

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this actually seems like an issue - it means someone who replaces a builtin intentionally can’t replace one of these (altho presumably mutating a builtin would be visible in both). That could cause an issue where someone wants to use a package to instrument or lock down fs (and thus, policies are not an ergonomic option, unless I’m misunderstanding how policies work), but accidentally leaves a gaping security hole when an attacker requires node:fs.

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policies should intercept before any of the resolution helpers even get called

node/lib/internal/modules/cjs/helpers.js

Line 52 in 36cc0ee

if (redirects) {
, if any module redirection is in place (including complete shutdown) then it would never call out to module.require and get here without some kind of opt-in to the behavior (like dependencies:true).

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At the very least this needs documentation and probably a strong indication people using policies + instrumenting fs now need to update the policy.

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My point was that policies aren’t currently required to ensure this for CJS - a package (not the app itself) can do it. Forcing policies to be the mechanism means packages are no longer capable of abstracting this for users.

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Totally understood. Given that policies are the superior mechanism for app developers to lock things down, what's the benefit of adding special, inconsistent, cache-bypassing behavior for require with a node: prefix?

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@ljharb i have no strong opinion on if we should diverge, but cache first behavior of CJS isn't shared by ESM and isn't robust against things like core destructuring the original impls. My like is just that it isn't a confusing situation like with fs/.

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but cache first behavior of CJS isn't shared by ESM and isn't robust against things like core destructuring the original impls.

This is demonstrated by:

const fs = require('fs');

const fakeModule = {};
require.cache.fs = { exports: fakeModule };

assert.strictEqual((await import('fs')).default, fs);

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indeed, but that doesn’t mean it’s a good idea to make CJS inconsistent with itself.

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I don't think CJS is able to be claimed as consistent with itself since in general the natives don't normally populate the cache. I think it was just ad-hoc written together and the cache behavior is evolutionary not intentional or well understood (even by me).

{
const fakeModule = {};

require.cache.fs = { exports: fakeModule };

assert.strictEqual(require('fs'), fakeModule);
assert.strictEqual(require('node:fs'), fs);

delete require.cache.fs;
}