Preserve module identity for symlinks (#4761)

* Preserve module identity for symlinks

* Add changelog entry

* Remove unintentional whitespace change

* Revert integration test to undebuggable state

CHANGELOG.md

@@ -15,6 +15,7 @@
 * `[jest-message-util]` Always remove node internals from stacktraces ([#4695](https://github.com/facebook/jest/pull/4695))
 * `[jest-resolve]` changes method of determining builtin modules to include missing builtins ([#4740](https://github.com/facebook/jest/pull/4740))
 * `[pretty-format]` Prevent error in pretty-format for window in jsdom test env ([#4750](https://github.com/facebook/jest/pull/4750))
+* `[jest-resolve]` Preserve module identity for symlinks ([#4761](https://github.com/facebook/jest/pull/4761))
 
 ### Features
 * `[jest-environment-*]` [**BREAKING**] Add Async Test Environment APIs, dispose is now teardown ([#4506](https://github.com/facebook/jest/pull/4506))

integration_tests/resolve/__tests__/resolve.test.js

@@ -56,3 +56,9 @@ test('should resolve filename.json', () => {
   expect(testRequire('../test4')).not.toThrow();
   expect(platform.extension).toBe('json');
 });
+
+test('should preserve identity for symlinks', () => {
+  expect(require('../../../packages/jest-resolve')).toBe(
+    require('jest-resolve')
+  );
+});

packages/jest-resolve/src/default_resolver.js

@@ -94,6 +94,11 @@ function resolveSync(target: Path, options: ResolverOptions): Path {
     if (isDirectory(dir)) {
       result = resolveAsFile(name) || resolveAsDirectory(name);
     }
+    if (result) {
+      // Dereference symlinks to ensure we don't create a separate
+      // module instance depending on how it was referenced.
+      result = fs.realpathSync(result);
+    }
     return result;
   }