Implement resolver.unstable_enableSymlinks

packages/metro-file-map/src/HasteFS.js

@@ -203,4 +203,8 @@ export default class HasteFS implements MutableFileSystem {
     }
     return this.#files.get(this._normalizePath(filePath));
   }
+
+  getRealPath(filePath: Path): Path {
+    throw new Error('HasteFS.getRealPath() is not implemented.');
+  }
 }

packages/metro-file-map/src/flow-types.js

@@ -163,6 +163,7 @@ export interface FileSystem {
   getAllFiles(): Array<Path>;
   getDependencies(file: Path): ?Array<string>;
   getModuleName(file: Path): ?string;
+  getRealPath(file: Path): ?string;
   getSerializableSnapshot(): FileData;
   getSha1(file: Path): ?string;
 

packages/metro-file-map/src/lib/TreeFS.js

@@ -171,6 +171,19 @@ export default class TreeFS implements MutableFileSystem {
     return files;
   }
 
+  getRealPath(filePath: Path): ?string {
+    const normalPath = this._normalizePath(filePath);
+    const metadata = this.#files.get(normalPath);
+    if (metadata && metadata[H.SYMLINK] === 0) {
+      return fastPath.resolve(this.#rootDir, normalPath);
+    }
+    const result = this._lookupByNormalPath(normalPath, {follow: true});
+    if (!result || result.node instanceof Map) {
+      return null;
+    }
+    return fastPath.resolve(this.#rootDir, result.normalPath);
+  }
+
   addOrModify(filePath: Path, metadata: FileMetaData): void {
     const normalPath = this._normalizePath(filePath);
     this.bulkAddOrModify(new Map([[normalPath, metadata]]));

packages/metro-file-map/src/lib/__tests__/TreeFS-test.js

@@ -86,6 +86,28 @@ describe.each([['win32'], ['posix']])('TreeFS on %s', platform => {
     });
   });
 
+  describe('getRealPath', () => {
+    test.each([
+      [p('/project/foo/link-to-another.js'), p('/project/foo/another.js')],
+      [p('/project/foo/link-to-bar.js'), p('/project/bar.js')],
+      [p('link-to-foo/link-to-another.js'), p('/project/foo/another.js')],
+      [p('/project/root/outside/external.js'), p('/outside/external.js')],
+      [p('/outside/../project/bar.js'), p('/project/bar.js')],
+    ])('%s -> %s', (givenPath, expectedRealPath) =>
+      expect(tfs.getRealPath(givenPath)).toEqual(expectedRealPath),
+    );
+
+    test.each([
+      [p('/project/foo')],
+      [p('/project/bar.js/bad-parent')],
+      [p('/project/root/outside')],
+      [p('/project/link-to-nowhere')],
+      [p('/project/not/exists')],
+    ])('returns null for directories or broken paths: %s', givenPath =>
+      expect(tfs.getRealPath(givenPath)).toEqual(null),
+    );
+  });
+
   describe('matchFilesWithContext', () => {
     test('non-recursive, skipping deep paths', () => {
       expect(

packages/metro-resolver/package.json

@@ -15,6 +15,9 @@
     "absolute-path": "^0.0.0",
     "invariant": "^2.2.4"
   },
+  "devDependencies": {
+    "invariant": "^2.2.4"
+  },
   "license": "MIT",
   "engines": {
     "node": ">=14.17.0"

packages/metro-resolver/src/__tests__/package-exports-test.js

@@ -26,6 +26,7 @@ describe('with package exports resolution disabled', () => {
       }),
       originModulePath: '/root/src/main.js',
       unstable_enablePackageExports: false,
+      unstable_getRealPath: null,
     };
 
     expect(Resolver.resolve(context, 'test-pkg', null)).toEqual({

packages/metro-resolver/src/__tests__/symlinks-test.js

@@ -0,0 +1,48 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+'use strict';
+
+import type {ResolutionContext} from '../index';
+
+const FailedToResolvePathError = require('../errors/FailedToResolvePathError');
+const Resolver = require('../index');
+import {createResolutionContext} from './utils';
+
+const fileMap = {
+  '/root/project/foo.js': '',
+  '/root/project/baz/index.js': '',
+  '/root/project/baz.js': {realPath: null},
+  '/root/project/link-to-foo.js': {realPath: '/root/project/foo.js'},
+};
+
+const CONTEXT: ResolutionContext = {
+  ...createResolutionContext(fileMap, {enableSymlinks: true}),
+  originModulePath: '/root/project/foo.js',
+};
+
+it('resolves to a real path when the chosen candidate is a symlink', () => {
+  expect(Resolver.resolve(CONTEXT, './link-to-foo', null)).toEqual({
+    type: 'sourceFile',
+    filePath: '/root/project/foo.js',
+  });
+});
+
+it('does not resolve to a broken symlink', () => {
+  // ./baz.js is a broken link, baz/index.js is real
+  expect(() => Resolver.resolve(CONTEXT, './baz.js', null)).toThrow(
+    FailedToResolvePathError,
+  );
+  expect(Resolver.resolve(CONTEXT, './baz', null)).toEqual({
+    type: 'sourceFile',
+    filePath: '/root/project/baz/index.js',
+  });
+});

packages/metro-resolver/src/__tests__/utils.js

@@ -10,12 +10,15 @@
  */
 
 import type {ResolutionContext} from '../index';
+import invariant from 'invariant';
 
 /**
  * Data structure approximating a file tree. Should be populated with complete
  * paths mapping to file contents.
  */
-type MockFileMap = {[path: string]: string};
+type MockFileMap = $ReadOnly<{
+  [path: string]: ?(string | $ReadOnly<{realPath: ?string}>),
+}>;
 
 /**
  * Create a new partial `ResolutionContext` object given a mock file structure.
@@ -24,15 +27,21 @@ type MockFileMap = {[path: string]: string};
  */
 export function createResolutionContext(
   fileMap: MockFileMap,
+  {enableSymlinks}: $ReadOnly<{enableSymlinks?: boolean}> = {},
 ): $Diff<ResolutionContext, {originModulePath: string}> {
   return {
     allowHaste: true,
     customResolverOptions: {},
     disableHierarchicalLookup: false,
-    doesFileExist: (filePath: string) => filePath in fileMap,
     extraNodeModules: null,
-    getPackage: (packageJsonPath: string) =>
-      JSON.parse(fileMap[packageJsonPath]),
+    getPackage: (packageJsonPath: string) => {
+      invariant(
+        typeof fileMap[packageJsonPath] === 'string',
+        '%s is not a regular file',
+        packageJsonPath,
+      );
+      return JSON.parse(fileMap[packageJsonPath]);
+    },
     getPackageForModule: () => null,
     isAssetFile: () => false,
     mainFields: ['browser', 'main'],
@@ -46,5 +55,23 @@ export function createResolutionContext(
     unstable_conditionNames: [],
     unstable_conditionsByPlatform: {},
     unstable_enablePackageExports: false,
+    ...(enableSymlinks === true
+      ? {
+          doesFileExist: (filePath: string) =>
+            // Should return false unless realpath(filePath) exists. We mock shallow
+            // dereferencing.
+            fileMap[filePath] != null &&
+            (typeof fileMap[filePath] === 'string' ||
+              typeof fileMap[filePath].realPath === 'string'),
+          unstable_getRealPath: filePath =>
+            typeof fileMap[filePath] === 'string'
+              ? filePath
+              : fileMap[filePath]?.realPath,
+        }
+      : {
+          doesFileExist: (filePath: string) =>
+            typeof fileMap[filePath] === 'string',
+          unstable_getRealPath: null,
+        }),
   };
 }

packages/metro-resolver/src/index.js

@@ -20,6 +20,7 @@ export type {
   FileAndDirCandidates,
   FileCandidates,
   FileResolution,
+  GetRealPath,
   IsAssetFile,
   ResolutionContext,
   Resolution,

packages/metro-resolver/src/resolve.js

@@ -460,7 +460,12 @@ function resolveSourceFileForExt(
   if (redirectedPath === false) {
     return {type: 'empty'};
   }
-  if (context.doesFileExist(redirectedPath)) {
+  if (context.unstable_getRealPath) {
+    const maybeRealPath = context.unstable_getRealPath(redirectedPath);
+    if (maybeRealPath != null) {
+      return maybeRealPath;
+    }
+  } else if (context.doesFileExist(redirectedPath)) {
     return redirectedPath;
   }
   context.candidateExts.push(extension);

packages/metro-resolver/src/types.js

@@ -64,6 +64,7 @@ export type PackageInfo = $ReadOnly<{
  * Check existence of a single file.
  */
 export type DoesFileExist = (filePath: string) => boolean;
+export type GetRealPath = (path: string) => ?string;
 export type IsAssetFile = (fileName: string) => boolean;
 
 /**
@@ -135,6 +136,7 @@ export type ResolutionContext = $ReadOnly<{
     [platform: string]: $ReadOnlyArray<string>,
   }>,
   unstable_enablePackageExports: boolean,
+  unstable_getRealPath?: ?GetRealPath,
 }>;
 
 export type CustomResolutionContext = $ReadOnly<{

packages/metro/src/node-haste/DependencyGraph.js

@@ -199,12 +199,23 @@ class DependencyGraph extends EventEmitter {
       projectRoot: this._config.projectRoot,
       resolveAsset: (dirPath: string, assetName: string, extension: string) => {
         const basePath = dirPath + path.sep + assetName;
-        const assets = [
+        let assets = [
           basePath + extension,
           ...this._config.resolver.assetResolutions.map(
             resolution => basePath + '@' + resolution + 'x' + extension,
           ),
-        ].filter(candidate => this._fileSystem.exists(candidate));
+        ];
+
+        if (this._config.resolver.unstable_enableSymlinks) {
+          assets = assets
+            .map(candidate => this._fileSystem.getRealPath(candidate))
+            .filter(Boolean);
+        } else {
+          assets = assets.filter(candidate =>
+            this._fileSystem.exists(candidate),
+          );
+        }
+
         return assets.length ? assets : null;
       },
       resolveRequest: this._config.resolver.resolveRequest,
@@ -214,6 +225,9 @@ class DependencyGraph extends EventEmitter {
         this._config.resolver.unstable_conditionsByPlatform,
       unstable_enablePackageExports:
         this._config.resolver.unstable_enablePackageExports,
+      unstable_getRealPath: this._config.resolver.unstable_enableSymlinks
+        ? path => this._fileSystem.getRealPath(path)
+        : null,
     });
   }
 
@@ -236,14 +250,19 @@ class DependencyGraph extends EventEmitter {
     const containerName =
       splitIndex !== -1 ? filename.slice(0, splitIndex + 4) : filename;
 
-    // TODO Calling realpath allows us to get a hash for a given path even when
+    // Prior to unstable_enableSymlinks:
+    // Calling realpath allows us to get a hash for a given path even when
     // it's a symlink to a file, which prevents Metro from crashing in such a
     // case. However, it doesn't allow Metro to track changes to the target file
     // of the symlink. We should fix this by implementing a symlink map into
     // Metro (or maybe by implementing those "extra transformation sources" we've
     // been talking about for stuff like CSS or WASM).
+    //
+    // This is unnecessary with a symlink-aware fileSystem implementation.
+    const resolvedPath = this._config.resolver.unstable_enableSymlinks
+      ? containerName
+      : fs.realpathSync(containerName);
 
-    const resolvedPath = fs.realpathSync(containerName);
     const sha1 = this._fileSystem.getSha1(resolvedPath);
 
     if (!sha1) {

packages/metro/src/node-haste/DependencyGraph/ModuleResolution.js

@@ -15,6 +15,7 @@ import type {
   CustomResolver,
   DoesFileExist,
   FileCandidates,
+  GetRealPath,
   IsAssetFile,
   Resolution,
   ResolveAsset,
@@ -74,6 +75,7 @@ type Options<TPackage> = $ReadOnly<{
     [platform: string]: $ReadOnlyArray<string>,
   }>,
   unstable_enablePackageExports: boolean,
+  unstable_getRealPath: ?GetRealPath,
 }>;
 
 class ModuleResolver<TPackage: Packageish> {
@@ -136,6 +138,7 @@ class ModuleResolver<TPackage: Packageish> {
       unstable_conditionNames,
       unstable_conditionsByPlatform,
       unstable_enablePackageExports,
+      unstable_getRealPath,
     } = this._options;
 
     try {
@@ -155,6 +158,7 @@ class ModuleResolver<TPackage: Packageish> {
           unstable_conditionNames,
           unstable_conditionsByPlatform,
           unstable_enablePackageExports,
+          unstable_getRealPath,
           customResolverOptions: resolverOptions.customResolverOptions ?? {},
           originModulePath: fromModule.path,
           resolveHasteModule: (name: string) =>