fix: closes mochajs#4552

Breaks circular references in before objects are serialized to prevent
cryptic error in parallel mode.

lib/nodejs/serializer.js

@@ -6,7 +6,7 @@
 
 'use strict';
 
-const {type} = require('../utils');
+const {type, breakCircularDeps} = require('../utils');
 const {createInvalidArgumentTypeError} = require('../errors');
 // this is not named `mocha:parallel:serializer` because it's noisy and it's
 // helpful to be able to write `DEBUG=mocha:parallel*` and get everything else.
@@ -188,14 +188,9 @@ class SerializableEvent {
    * @param {Array<object|string>} pairs - List of parent/key tuples to process; modified in-place. This JSDoc type is an approximation
    * @param {object} parent - Some parent object
    * @param {string} key - Key to inspect
-   * @param {WeakSet<Object>} seenObjects - For avoiding circular references
    */
-  static _serialize(pairs, parent, key, seenObjects) {
+  static _serialize(pairs, parent, key) {
     let value = parent[key];
-    if (seenObjects.has(value)) {
-      parent[key] = Object.create(null);
-      return;
-    }
     let _type = type(value);
     if (_type === 'error') {
       // we need to reference the stack prop b/c it's lazily-loaded.
@@ -263,13 +258,14 @@ class SerializableEvent {
       error: this.originalError
     });
 
+    // mutates the object
+    breakCircularDeps(result);
+
     const pairs = Object.keys(result).map(key => [result, key]);
-    const seenObjects = new WeakSet();
 
     let pair;
     while ((pair = pairs.shift())) {
-      SerializableEvent._serialize(pairs, ...pair, seenObjects);
-      seenObjects.add(pair[0]);
+      SerializableEvent._serialize(pairs, ...pair);
     }
 
     this.data = result.data;

lib/utils.js

@@ -647,3 +647,36 @@ exports.assignNewMochaID = obj => {
  */
 exports.getMochaID = obj =>
   obj && typeof obj === 'object' ? obj[MOCHA_ID_PROP_NAME] : undefined;
+
+/**
+ * Replaces any detected circular dependency with the string '[Circular]'
+ * Mutates original object
+ * @param inputObj {*}
+ * @returns {*}
+ */
+exports.breakCircularDeps = inputObj => {
+  const seen = new Set();
+
+  function _breakCircularDeps(obj) {
+    if (obj && typeof obj !== 'object') {
+      return obj;
+    }
+
+    if (seen.has(obj)) {
+      return '[Circular]';
+    }
+
+    seen.add(obj);
+    for (const k in obj) {
+      if (Object.prototype.hasOwnProperty.call(obj, k)) {
+        obj[k] = _breakCircularDeps(obj[k], k);
+      }
+    }
+
+    // deleting means only a seen object that is its own child will be detected
+    seen.delete(obj);
+    return obj;
+  }
+
+  return _breakCircularDeps(inputObj);
+};

test/integration/fixtures/parallel/circular-error.mjs

@@ -0,0 +1,10 @@
+import {describe,it} from "../../../../index.js";
+
+describe('test1', () => {
+  it('test', () => {
+    const error = new Error('Foo');
+    error.foo = { props: [] };
+    error.foo.props.push(error.foo);
+    throw error;
+  });
+});

test/integration/parallel.spec.js

@@ -30,4 +30,17 @@ describe('parallel run', () => {
     assert.strictEqual(result.stats.failures, 0);
     assert.strictEqual(result.stats.passes, 3);
   });
+
+  it('should correctly handle circular references in an exception', async () => {
+    const result = await runMochaJSONAsync('parallel/circular-error.mjs', [
+      '--parallel',
+      '--jobs',
+      '2',
+      require.resolve('./fixtures/parallel/testworkerid1.mjs')
+    ]);
+    assert.strictEqual(result.stats.failures, 1);
+    assert.strictEqual(result.stats.passes, 1);
+    assert.strictEqual(result.failures[0].err.message, 'Foo');
+    assert.strictEqual(result.failures[0].err.foo.props[0], '[Circular]');
+  });
 });