test: test cases for circular symlinked deps and peer deps

Adds a test case in the known-test cases to catch nodejs#5950
Adds a test to verify that circular symlinked deps do not recurse
forever.

test/known_issues/test-symlinked-peer-modules.js

@@ -0,0 +1,60 @@
+/* eslint no-irregular-whitespace: 0 */
+'use strict';
+// Refs: https://github.com/nodejs/node/pull/5950
+
+// This test illustrates the problem that symlinked modules are unable
+// to find their peer dependencies. This was fixed in #5950 but that is
+// reverted because that particular way of fixing it causes too much
+// breakage (breakage that was not caught by either CI or CITGM on multiple
+// runs.
+
+const common = require('../common');
+const fs = require('fs');
+const path = require('path');
+const assert = require('assert');
+
+common.refreshTmpDir();
+
+const tmpDir = common.tmpDir;
+
+// Creates the following structure
+// {tmpDir}
+// ├── app
+// │   ├── index.js
+// │   └── node_modules
+// │       ├── moduleA -> {tmpDir}/moduleA
+// │       └── moduleB
+// │           ├── index.js
+// │           └── package.json
+// └── moduleA
+//     ├── index.js
+//     └── package.json
+
+const moduleA = path.join(tmpDir, 'moduleA');
+const app = path.join(tmpDir, 'app');
+const moduleB = path.join(app, 'node_modules', 'moduleB');
+const moduleA_link = path.join(app, 'node_modules', 'moduleA');
+
+fs.mkdirSync(moduleA);
+fs.writeFileSync(path.join(moduleA, 'package.json'),
+                 JSON.stringify({name: 'moduleA', main: 'index.js'}), 'utf8');
+fs.writeFileSync(path.join(moduleA, 'index.js'),
+                 'module.exports = require(\'moduleB\');');
+
+fs.mkdirSync(app);
+fs.writeFileSync(path.join(app, 'index.js'),
+                 '\'use strict\'; require(\'moduleA\');');
+fs.mkdirSync(path.join(app, 'node_modules'));
+
+fs.mkdirSync(moduleB);
+fs.writeFileSync(path.join(moduleB, 'package.json'),
+                 JSON.stringify({name: 'moduleB', main: 'index.js'}), 'utf8');
+fs.writeFileSync(path.join(moduleB, 'index.js'),
+                 'module.exports = 1;');
+
+fs.symlinkSync(moduleA, moduleA_link);
+
+// This should not throw, but it does
+assert.doesNotThrow(() => {
+  console.log(require(path.join(app, 'index')));
+});

test/sequential/test-module-circular-symlinks.js

@@ -0,0 +1,49 @@
+/* eslint no-irregular-whitespace: 0 */
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const path = require('path');
+const fs = require('fs');
+
+// {tmpDir}
+// ├── index.js
+// └── node_modules
+//     ├── moduleA
+//     │   ├── index.js
+//     │   └── node_modules
+//     │       └── moduleB -> {tmpDir}/node_modules/moduleB
+//     └── moduleB
+//         ├── index.js
+//         └── node_modules
+//             └── moduleA -> {tmpDir}/node_modules/moduleA
+
+common.refreshTmpDir();
+const tmpDir = common.tmpDir;
+
+const node_modules = path.join(tmpDir, 'node_modules');
+const moduleA = path.join(node_modules, 'moduleA');
+const moduleB = path.join(node_modules, 'moduleB');
+const moduleA_link = path.join(moduleB, 'node_modules', 'moduleA');
+const moduleB_link = path.join(moduleA, 'node_modules', 'moduleB');
+
+fs.writeFileSync(path.join(tmpDir, 'index.js'),
+                 'module.exports = require(\'moduleA\');', 'utf8');
+fs.mkdirSync(node_modules);
+fs.mkdirSync(moduleA);
+fs.mkdirSync(moduleB);
+fs.writeFileSync(path.join(moduleA, 'index.js'),
+                 'module.exports = {b: require(\'moduleB\')};', 'utf8');
+fs.writeFileSync(path.join(moduleB, 'index.js'),
+                 'module.exports = {a: require(\'moduleA\')};', 'utf8');
+fs.mkdirSync(path.join(moduleA, 'node_modules'));
+fs.mkdirSync(path.join(moduleB, 'node_modules'));
+fs.symlinkSync(moduleA, moduleA_link);
+fs.symlinkSync(moduleB, moduleB_link);
+
+// Ensure that the symlinks are not followed forever...
+const obj = require(path.join(tmpDir, 'index'));
+assert.ok(obj);
+assert.ok(obj.b);
+assert.ok(obj.b.a);
+assert.ok(!obj.b.a.b);