vfs: answer for unowned paths under reserved root

lib/internal/vfs/setup.js

@@ -35,6 +35,7 @@ const { assertEncoding, setVfsHandlers } = require('internal/fs/utils');
 const permission = require('internal/process/permission');
 const { getOptionValue } = require('internal/options');
 const nativeModulesBinding = internalBinding('modules');
+const { UV_ENOENT } = internalBinding('uv');
 let debug = require('internal/util/debuglog').debuglog('vfs', (fn) => {
   debug = fn;
 });
@@ -115,29 +116,50 @@ function deregisterVFS(vfs) {
 }
 
 /**
- * Resolves a path string to the active VFS that owns it, or null.
- * Ownership is decidable from the path alone: all mount points live
- * under the reserved `${os.devNull}/vfs/<id>` namespace, so a single
- * prefix comparison rejects every real-file-system path and a map
- * lookup finds the owning layer. The normalized path is returned
+ * Resolves a path string to the reserved VFS root, or null for a path
+ * outside it. Ownership is decidable from the path alone: all mount
+ * points live under the reserved `${os.devNull}/vfs/<id>` namespace, so
+ * a single prefix comparison rejects every real-file-system path and a
+ * map lookup finds the owning layer. The normalized path is returned
  * alongside the layer so downstream helpers can skip renormalization.
+ *
+ * A path under the root that no active layer owns comes back with
+ * `vfs: null` rather than as `null`, because the two cases must not be
+ * treated alike by the module loader. The loader manufactures such
+ * paths itself: resolving a mount point as a directory first probes the
+ * sibling names `<mount>.js`, `<mount>.json`, ..., and a package.json
+ * walk-up passes the parents of the mount point. They cannot name
+ * anything real, but on Windows the root sits under `\\.\nul`, and
+ * `\\.\nul\<anything>` opens the NUL device, which stats as a character
+ * device and reads as empty. Handed to the native loader, such a probe
+ * "finds" a file and the walk-up above it rejects the empty device as an
+ * invalid package.json, so the loader must answer for the whole root.
  * @param {string} inputPath
- * @returns {{ vfs: object, normalized: string }|null}
+ * @returns {{ vfs: object|null, normalized: string }|null}
  */
-function findVFS(inputPath) {
+function findVFSOrRoot(inputPath) {
   const normalized = normalizeMountedPath(inputPath);
   if (!StringPrototypeStartsWith(normalized, normalizedVfsRootPrefix)) {
     return null;
   }
   const layerId = getLayerIdFromPath(normalized);
-  if (layerId === -1) return null;
-  const vfs = activeVFSLayers.get(layerId);
+  const vfs = layerId === -1 ? undefined : activeVFSLayers.get(layerId);
   if (vfs === undefined || !vfs.shouldHandleNormalized(normalized)) {
-    return null;
+    return { vfs: null, normalized };
   }
   return { vfs, normalized };
 }
 
+/**
+ * Resolves a path string to the active VFS that owns it, or null.
+ * @param {string} inputPath
+ * @returns {{ vfs: object, normalized: string }|null}
+ */
+function findVFS(inputPath) {
+  const r = findVFSOrRoot(inputPath);
+  return r === null || r.vfs === null ? null : r;
+}
+
 /**
  * Drop the cache entries under `vfs`'s mount point from the
  * JS-reachable loader caches. Real-fs entries and other-VFS entries
@@ -210,16 +232,16 @@ function findVFSForStat(filename) {
 }
 
 /**
- * Finds the VFS owning `filename` and reads it.
+ * Reads `filename` from the VFS that owns it, reporting a missing file
+ * or a directory the way the native loader's read does.
+ * @param {object} vfs The VFS owning filename
  * @param {string} filename The absolute path to read
  * @param {string|object} options Read options
- * @returns {{ vfs: object, content: Buffer|string }|null}
+ * @returns {Buffer|string}
  */
-function findVFSForRead(filename, options) {
-  const r = findVFS(filename);
-  if (r === null) return null;
+function readVFS(vfs, filename, options) {
   try {
-    return { vfs: r.vfs, content: r.vfs.readFileSync(filename, options) };
+    return vfs.readFileSync(filename, options);
   } catch (e) {
     const code = e?.code;
     if (code === 'ENOENT' || code === 'EISDIR') {
@@ -794,30 +816,40 @@ function installModuleLoaderOverrides() {
   // wrapLoaderMethod then falls through to the native binding.
   setLoaderOverrides({
     internalModuleStat(filename) {
-      const result = findVFSForStat(filename);
-      return result !== null ? result.result : undefined;
+      const r = findVFSOrRoot(filename);
+      if (r === null) return undefined;
+      return r.vfs === null ? UV_ENOENT : vfsStat(r.vfs, filename);
     },
     readFileSync(filename, options) {
       const pathStr = typeof filename === 'string' ? filename :
         (filename instanceof URL ? fileURLToPath(filename) : String(filename));
-      const result = findVFSForRead(pathStr, options);
-      return result !== null ? result.content : undefined;
+      const r = findVFSOrRoot(pathStr);
+      if (r === null) return undefined;
+      if (r.vfs === null) throw createENOENT('open', pathStr);
+      return readVFS(r.vfs, pathStr, options);
     },
     realpathSync(filename) {
-      return findVFSWith(filename, 'realpath', (vfs, n) => vfs.realpathSync(n));
+      const r = findVFSOrRoot(filename);
+      if (r === null) return undefined;
+      if (r.vfs === null || !r.vfs.existsSync(filename)) {
+        throw createENOENT('realpath', filename);
+      }
+      return r.vfs.realpathSync(filename);
     },
     getResolutionRoot(pathStr) {
-      const r = findVFS(pathStr);
+      const r = findVFSOrRoot(pathStr);
       if (r === null) return undefined;
-      const mountPoint = r.vfs.mountPoint;
       // The boundary is compared as a plain string prefix by the
-      // callers, so only report it when the input carries the mount
-      // point verbatim.
-      return StringPrototypeStartsWith(pathStr, mountPoint) ?
-        mountPoint : undefined;
+      // callers, so only report it when the input carries it verbatim.
+      // An unowned path stops at the reserved root itself, so no
+      // node_modules lookup walks out into the real file system.
+      const boundary = r.vfs === null ?
+        getNormalizedVfsRoot() : r.vfs.mountPoint;
+      return StringPrototypeStartsWith(pathStr, boundary) ?
+        boundary : undefined;
     },
     legacyMainResolve(pkgPath, main, base) {
-      if (findVFS(pkgPath) === null) return undefined;
+      if (findVFSOrRoot(pkgPath) === null) return undefined;
 
       for (let i = 0; i < legacyMainResolveExtensions.length; i++) {
         const byMain = i <= kResolvedByMainIndexNode;
@@ -835,14 +867,14 @@ function installModuleLoaderOverrides() {
       throw new ERR_MODULE_NOT_FOUND(initial, base, undefined);
     },
     getFormatOfExtensionlessFile(filePath) {
-      let result;
+      const r = findVFSOrRoot(filePath);
+      if (r === null) return undefined;
+      let content;
       try {
-        result = findVFSForRead(filePath, null);
+        content = r.vfs === null ? null : readVFS(r.vfs, filePath, null);
       } catch {
         return internalConstants.EXTENSIONLESS_FORMAT_JAVASCRIPT;
       }
-      if (result === null) return undefined;
-      const content = result.content;
       // Wasm magic bytes: 0x00 0x61 0x73 0x6d
       if (content && content.length >= 4 &&
           content[0] === 0x00 && content[1] === 0x61 &&
@@ -852,8 +884,9 @@ function installModuleLoaderOverrides() {
       return internalConstants.EXTENSIONLESS_FORMAT_JAVASCRIPT;
     },
     readPackageJSON(jsonPath, isESM, base, specifier) {
-      const r = findVFS(jsonPath);
+      const r = findVFSOrRoot(jsonPath);
       if (r === null) return undefined;
+      if (r.vfs === null) return kLoaderOverrideNoResult;
       const { vfs } = r;
       if (vfsStat(vfs, jsonPath) !== 0) return kLoaderOverrideNoResult;
       let content;
@@ -868,8 +901,9 @@ function installModuleLoaderOverrides() {
         content, jsonPath, isESM, base, specifier);
     },
     getNearestParentPackageJSON(checkPath) {
-      const r = findVFS(checkPath);
+      const r = findVFSOrRoot(checkPath);
       if (r === null) return undefined;
+      if (r.vfs === null) return kLoaderOverrideNoResult;
       const found = findVFSPackageJSON(r.vfs, checkPath, r.normalized);
       return found.tuple ?? kLoaderOverrideNoResult;
     },
@@ -884,8 +918,11 @@ function installModuleLoaderOverrides() {
       } else {
         filePath = resolved;
       }
-      const r = findVFS(filePath);
+      const r = findVFSOrRoot(filePath);
       if (r === null) return undefined;
+      // The "not found" marker is the package.json beside the queried
+      // path, which is what the native binding reports for it.
+      if (r.vfs === null) return join(dirname(filePath), 'package.json');
       const found = findVFSPackageJSON(r.vfs, filePath, r.normalized);
       if (found.tuple !== undefined) return found.tuple;
       return found.sentinel;
@@ -901,8 +938,9 @@ function installModuleLoaderOverrides() {
       } else {
         filePath = url;
       }
-      const r = findVFS(filePath);
+      const r = findVFSOrRoot(filePath);
       if (r === null) return undefined;
+      if (r.vfs === null) return kLoaderOverrideNoResult;
       const found = findVFSPackageJSON(r.vfs, filePath, r.normalized);
       if (found.tuple !== undefined) {
         // Tuple shape: [name, main, type, imports, exports, filePath].

test/parallel/test-vfs-reserved-root-unowned.js

@@ -0,0 +1,59 @@
+// Flags: --experimental-vfs --expose-internals
+'use strict';
+
+// The module loader manufactures paths under the reserved VFS root that no
+// layer owns: resolving a mount point as a directory first probes the sibling
+// names `<mount>.js`, `<mount>.json`, ..., and a package.json walk-up passes
+// the parents of the mount point. Such paths cannot name anything real, but
+// they must still be answered by the VFS instead of being handed to the native
+// loader: on Windows the reserved root sits under `\\.\nul`, and
+// `\\.\nul\<anything>` opens the NUL device, which stats as a character device
+// and reads as empty. The native loader would then "find" a file at
+// `<mount>.js` and reject the empty package.json above it as invalid JSON.
+
+require('../common');
+const assert = require('assert');
+const path = require('path');
+const { pathToFileURL } = require('url');
+const vfs = require('node:vfs');
+const { loaderMethods } = require('internal/modules/helpers');
+const { getNormalizedVfsRoot } = require('internal/vfs/router');
+
+const layer = vfs.create();
+layer.writeFileSync('/index.js', 'module.exports = "ran";');
+const mountPoint = layer.mount();
+
+const root = getNormalizedVfsRoot();
+const unowned = [
+  `${mountPoint}.js`,
+  `${mountPoint}.json`,
+  `${mountPoint}.node`,
+  path.join(root, 'package.json'),
+  path.join(root, 'nope', 'index.js'),
+];
+
+for (const p of unowned) {
+  assert.ok(loaderMethods.internalModuleStat(p) < 0, p);
+  assert.throws(() => loaderMethods.readFileSync(p), { code: 'ENOENT' }, p);
+  assert.throws(() => loaderMethods.realpathSync(p), { code: 'ENOENT' }, p);
+  assert.strictEqual(loaderMethods.getNearestParentPackageJSON(p), undefined, p);
+  assert.strictEqual(loaderMethods.readPackageJSON(p, false), undefined, p);
+  assert.strictEqual(loaderMethods.getPackageType(pathToFileURL(p).href), undefined, p);
+  // The "not found" marker is the last candidate examined, like the native
+  // binding returns.
+  assert.strictEqual(
+    loaderMethods.getPackageScopeConfig(pathToFileURL(p).href),
+    path.join(path.dirname(p), 'package.json'), p);
+  // Upward walks (node_modules lookups) stop at the reserved root rather than
+  // continuing into the real file system.
+  assert.strictEqual(loaderMethods.getResolutionRoot(p), root, p);
+}
+
+// Paths outside the reserved root are still left to the native loader.
+assert.strictEqual(loaderMethods.getResolutionRoot(__filename), undefined);
+
+// The mount point itself resolves as a directory to its index through the
+// layer, which is the sequence that produced the sibling probes above.
+assert.strictEqual(require(mountPoint), 'ran');
+
+layer.unmount();