fs: add virtual file system support

Add a read-only virtual file system (VFS) that can be mounted at a
specific path prefix, enabling standard fs APIs to work transparently
with in-memory files.

Key features:
- fs.createVirtual() to create VFS instances
- Support for files, directories, and symbolic links
- Full async/sync/promise API support (readFile, stat, readdir, etc.)
- File descriptor operations (open, read, close)
- createReadStream() support
- fs.glob() integration
- CJS require() and ESM import() support via module hooks
- Virtual process.chdir() for relative path resolution
- SEA integration via sea.getVfs() and sea.hasAssets()
- Test runner mock.fs() for file system mocking

The VFS is read-only by design and uses virtual file descriptors
(10000+) to avoid conflicts with real file descriptors.

Author: mcollina

doc/api/fs.md

@@ -238,6 +238,94 @@ const raw = getRawAsset('a.jpg');
 See documentation of the [`sea.getAsset()`][], [`sea.getAssetAsBlob()`][],
 [`sea.getRawAsset()`][] and [`sea.getAssetKeys()`][] APIs for more information.
 
+### Virtual File System (VFS) for assets
+
+> Stability: 1 - Experimental
+
+Instead of using the `node:sea` API to access individual assets, you can use
+the Virtual File System (VFS) to access bundled assets through standard `fs`
+APIs. The VFS automatically populates itself with all assets defined in the
+SEA configuration and mounts them at a virtual path (default: `/sea`).
+
+To use the VFS with SEA:
+
+```cjs
+const fs = require('node:fs');
+const sea = require('node:sea');
+
+// Check if SEA assets are available
+if (sea.hasAssets()) {
+  // Initialize and mount the SEA VFS
+  const vfs = sea.getVfs();
+
+  // Now you can use standard fs APIs to read bundled assets
+  const config = JSON.parse(fs.readFileSync('/sea/config.json', 'utf8'));
+  const data = fs.readFileSync('/sea/data/file.txt');
+
+  // Directory operations work too
+  const files = fs.readdirSync('/sea/assets');
+
+  // Check if a bundled file exists
+  if (fs.existsSync('/sea/optional.json')) {
+    // ...
+  }
+}
+```
+
+The VFS supports the following `fs` operations on bundled assets:
+
+* `readFileSync()` / `readFile()` / `promises.readFile()`
+* `statSync()` / `stat()` / `promises.stat()`
+* `lstatSync()` / `lstat()` / `promises.lstat()`
+* `readdirSync()` / `readdir()` / `promises.readdir()`
+* `existsSync()`
+* `realpathSync()` / `realpath()` / `promises.realpath()`
+* `accessSync()` / `access()` / `promises.access()`
+* `openSync()` / `open()` - for reading
+* `createReadStream()`
+
+#### Loading modules from VFS in SEA
+
+The default `require()` function in a SEA only supports loading Node.js
+built-in modules. To load JavaScript modules bundled as assets, you must use
+[`module.createRequire()`][]:
+
+```cjs
+const { createRequire } = require('node:module');
+const sea = require('node:sea');
+
+// Initialize VFS
+sea.getVfs();
+
+// Create a require function that works with VFS
+const seaRequire = createRequire('/sea/');
+
+// Now you can require bundled modules
+const myModule = seaRequire('/sea/lib/mymodule.js');
+const utils = seaRequire('/sea/utils/helpers.js');
+```
+
+This is necessary because SEA uses a special embedder require that doesn't go
+through the standard module resolution hooks that VFS registers.
+
+#### Custom mount prefix
+
+By default, the VFS is mounted at `/sea`. You can specify a custom prefix
+when initializing the VFS:
+
+```cjs
+const fs = require('node:fs');
+const sea = require('node:sea');
+
+const vfs = sea.getSeaVfs({ prefix: '/app' });
+
+// Assets are now accessible under /app
+const config = fs.readFileSync('/app/config.json', 'utf8');
+```
+
+Note: `sea.getVfs()` returns a singleton. The `prefix` option is only used
+on the first call; subsequent calls return the same cached instance.
+
 ### Startup snapshot support
 
 The `useSnapshot` field can be used to enable startup snapshot support. In this
@@ -553,6 +641,7 @@ to help us document them.
 [Mach-O]: https://en.wikipedia.org/wiki/Mach-O
 [PE]: https://en.wikipedia.org/wiki/Portable_Executable
 [Windows SDK]: https://developer.microsoft.com/en-us/windows/downloads/windows-sdk/
+[`module.createRequire()`]: module.md#modulecreaterequirefilename
 [`process.execPath`]: process.md#processexecpath
 [`require()`]: modules.md#requireid
 [`require.main`]: modules.md#accessing-the-main-module

doc/api/single-executable-applications.md

@@ -2334,6 +2334,94 @@ test('mocks a counting function', (t) => {
 });
 ```
 
+### `mock.fs([options])`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1.0 - Early development
+
+* `options` {Object} Optional configuration options for the mock file system.
+  The following properties are supported:
+  * `prefix` {string} The mount point prefix for the virtual file system.
+    **Default:** `'/mock'`.
+  * `files` {Object} An optional object where keys are file paths (relative to
+    the VFS root) and values are the file contents. Contents can be strings,
+    Buffers, or functions that return strings/Buffers.
+* Returns: {MockFSContext} An object that can be used to manage the mock file
+  system.
+
+This function creates a mock file system using the Virtual File System (VFS).
+The mock file system is automatically cleaned up when the test completes.
+
+## Class: `MockFSContext`
+
+The `MockFSContext` object is returned by `mock.fs()` and provides the
+following methods and properties:
+
+* `vfs` {VirtualFileSystem} The underlying VFS instance.
+* `prefix` {string} The mount prefix.
+* `addFile(path, content)` Adds a file to the mock file system.
+* `addDirectory(path[, populate])` Adds a directory to the mock file system.
+* `existsSync(path)` Checks if a path exists (path is relative to prefix).
+* `restore()` Manually restores the file system to its original state.
+
+The following example demonstrates how to create a mock file system for testing:
+
+```js
+const { test } = require('node:test');
+const assert = require('node:assert');
+const fs = require('node:fs');
+
+test('reads configuration from mock file', (t) => {
+  const mockFs = t.mock.fs({
+    prefix: '/app',
+    files: {
+      '/config.json': JSON.stringify({ debug: true }),
+      '/data/users.txt': 'user1\nuser2\nuser3',
+    },
+  });
+
+  // Files are accessible via standard fs APIs
+  const config = JSON.parse(fs.readFileSync('/app/config.json', 'utf8'));
+  assert.strictEqual(config.debug, true);
+
+  // Check file existence
+  assert.strictEqual(fs.existsSync('/app/config.json'), true);
+  assert.strictEqual(fs.existsSync('/app/missing.txt'), false);
+
+  // Use mockFs.existsSync for paths relative to prefix
+  assert.strictEqual(mockFs.existsSync('/config.json'), true);
+});
+
+test('supports dynamic file content', (t) => {
+  let counter = 0;
+  const mockFs = t.mock.fs({ prefix: '/dynamic' });
+
+  mockFs.addFile('/counter.txt', () => {
+    counter++;
+    return String(counter);
+  });
+
+  // Each read calls the function
+  assert.strictEqual(fs.readFileSync('/dynamic/counter.txt', 'utf8'), '1');
+  assert.strictEqual(fs.readFileSync('/dynamic/counter.txt', 'utf8'), '2');
+});
+
+test('supports require from mock files', (t) => {
+  t.mock.fs({
+    prefix: '/modules',
+    files: {
+      '/math.js': 'module.exports = { add: (a, b) => a + b };',
+    },
+  });
+
+  const math = require('/modules/math.js');
+  assert.strictEqual(math.add(2, 3), 5);
+});
+```
+
 ### `mock.getter(object, methodName[, implementation][, options])`
 
 <!-- YAML

doc/api/test.md

@@ -3206,6 +3206,19 @@ function globSync(pattern, options) {
   return new Glob(pattern, options).globSync();
 }
 
+const lazyVfs = getLazy(() => require('internal/vfs/virtual_fs').VirtualFileSystem);
+
+/**
+ * Creates a new virtual file system instance.
+ * @param {object} [options] Configuration options
+ * @param {boolean} [options.fallthrough] Whether to fall through to real fs on miss
+ * @param {boolean} [options.moduleHooks] Whether to enable require/import hooks
+ * @returns {VirtualFileSystem}
+ */
+function createVirtual(options) {
+  const VirtualFileSystem = lazyVfs();
+  return new VirtualFileSystem(options);
+}
 
 module.exports = fs = {
   appendFile,
@@ -3223,6 +3236,7 @@ module.exports = fs = {
   cp,
   cpSync,
   createReadStream,
+  createVirtual,
   createWriteStream,
   exists,
   existsSync,

lib/fs.js

@@ -50,6 +50,10 @@ const {
 const { MockTimers } = require('internal/test_runner/mock/mock_timers');
 const { Module } = require('internal/modules/cjs/loader');
 const { _load, _nodeModulePaths, _resolveFilename, isBuiltin } = Module;
+const { join } = require('path');
+
+// Lazy-load VirtualFileSystem to avoid loading VFS code if fs mocking is not used
+let VirtualFileSystem;
 function kDefaultFunction() {}
 const enableModuleMocking = getOptionValue('--experimental-test-module-mocks');
 const kSupportedFormats = [
@@ -402,6 +406,72 @@ class MockPropertyContext {
 
 const { restore: restoreProperty } = MockPropertyContext.prototype;
 
+/**
+ * Context for mocking the file system using VFS.
+ */
+class MockFSContext {
+  #vfs;
+  #prefix;
+
+  constructor(vfs, prefix) {
+    this.#vfs = vfs;
+    this.#prefix = prefix;
+  }
+
+  /**
+   * Gets the underlying VirtualFileSystem instance.
+   * @returns {VirtualFileSystem}
+   */
+  get vfs() {
+    return this.#vfs;
+  }
+
+  /**
+   * Gets the mount prefix for the mock file system.
+   * @returns {string}
+   */
+  get prefix() {
+    return this.#prefix;
+  }
+
+  /**
+   * Adds a file to the mock file system.
+   * @param {string} path - The path of the file.
+   * @param {string|Buffer|Function} content - The file content.
+   */
+  addFile(path, content) {
+    this.#vfs.addFile(path, content);
+  }
+
+  /**
+   * Adds a directory to the mock file system.
+   * @param {string} path - The path of the directory.
+   * @param {Function} [populate] - Optional callback to populate the directory.
+   */
+  addDirectory(path, populate) {
+    this.#vfs.addDirectory(path, populate);
+  }
+
+  /**
+   * Checks if a path exists in the mock file system.
+   * @param {string} path - The path to check (relative to prefix).
+   * @returns {boolean}
+   */
+  existsSync(path) {
+    const fullPath = join(this.#prefix, path);
+    return this.#vfs.existsSync(fullPath);
+  }
+
+  /**
+   * Restores the file system to its original state.
+   */
+  restore() {
+    this.#vfs.unmount();
+  }
+}
+
+const { restore: restoreFS } = MockFSContext.prototype;
+
 class MockTracker {
   #mocks = [];
   #timers;
@@ -725,6 +795,45 @@ class MockTracker {
     });
   }
 
+  /**
+   * Creates a mock file system using VFS.
+   * @param {object} [options] - Options for the mock file system.
+   * @param {string} [options.prefix] - The mount prefix for the VFS.
+   * @param {object} [options.files] - Initial files to add (path: content pairs).
+   * @returns {MockFSContext} The mock file system context.
+   */
+  fs(options = kEmptyObject) {
+    validateObject(options, 'options');
+    const { prefix = '/mock', files } = options;
+    if (files !== undefined) {
+      validateObject(files, 'options.files');
+    }
+
+    VirtualFileSystem ??= require('internal/vfs/virtual_fs').VirtualFileSystem;
+    const vfs = new VirtualFileSystem({ __proto__: null, moduleHooks: true });
+
+    // Add initial files if provided
+    if (files) {
+      const paths = ObjectKeys(files);
+      for (let i = 0; i < paths.length; i++) {
+        const path = paths[i];
+        vfs.addFile(path, files[path]);
+      }
+    }
+
+    // Mount the VFS at the specified prefix
+    vfs.mount(prefix);
+
+    const ctx = new MockFSContext(vfs, prefix);
+    ArrayPrototypePush(this.#mocks, {
+      __proto__: null,
+      ctx,
+      restore: restoreFS,
+    });
+
+    return ctx;
+  }
+
   /**
    * Resets the mock tracker, restoring all mocks and clearing timers.
    */