properly dispose Mocha instance in watch mode; closes #4495

lib/cli/watch-run.js

@@ -41,6 +41,9 @@ exports.watchParallelRun = (
       // I don't know why we're cloning the root suite.
       const rootSuite = mocha.suite.clone();
 
+      // ensure we aren't leaking event listeners
+      mocha.dispose();
+
       // this `require` is needed because the require cache has been cleared.  the dynamic
       // exports set via the below call to `mocha.ui()` won't work properly if a
       // test depends on this module (see `required-tokens.spec.js`).
@@ -100,6 +103,9 @@ exports.watchRun = (mocha, {watchFiles, watchIgnore}, fileCollectParams) => {
       // I don't know why we're cloning the root suite.
       const rootSuite = mocha.suite.clone();
 
+      // ensure we aren't leaking event listeners
+      mocha.dispose();
+
       // this `require` is needed because the require cache has been cleared.  the dynamic
       // exports set via the below call to `mocha.ui()` won't work properly if a
       // test depends on this module (see `required-tokens.spec.js`).

test/integration/helpers.js

@@ -415,8 +415,12 @@ async function runMochaWatchAsync(args, opts, change) {
   if (typeof opts === 'string') {
     opts = {cwd: opts};
   }
-  opts = {sleepMs: 2000, ...opts, fork: process.platform === 'win32'};
-  opts.stdio = ['pipe', 'pipe', 'inherit'];
+  opts = {
+    sleepMs: 2000,
+    stdio: ['pipe', 'pipe', 'inherit'],
+    ...opts,
+    fork: process.platform === 'win32'
+  };
   const [mochaProcess, resultPromise] = invokeMochaAsync(
     [...args, '--watch'],
     opts
@@ -539,24 +543,25 @@ function sleep(time) {
 module.exports = {
   DEFAULT_FIXTURE,
   SPLIT_DOT_REPORTER_REGEXP,
+  copyFixture,
 
   createTempDir,
+  escapeRegExp,
+  getSummary,
   invokeMocha,
   invokeMochaAsync,
   invokeNode,
-  getSummary,
+  replaceFileContents,
   resolveFixturePath,
-  toJSONResult,
-  escapeRegExp,
   runMocha,
-  runMochaJSON,
   runMochaAsync,
+  runMochaJSON,
   runMochaJSONAsync,
   runMochaWatchAsync,
   runMochaWatchJSONAsync,
-  copyFixture,
-  touchFile,
-  replaceFileContents
+  sleep,
+  toJSONResult,
+  touchFile
 };
 
 /**

test/integration/options/watch.spec.js

@@ -5,6 +5,8 @@ const path = require('path');
 const {
   copyFixture,
   runMochaWatchJSONAsync,
+  sleep,
+  runMochaWatchAsync,
   touchFile,
   replaceFileContents,
   createTempDir,
@@ -343,5 +345,33 @@ describe('--watch', function() {
       it('mochaHooks.afterAll runs as expected', setupHookTest('afterAll'));
       it('mochaHooks.afterEach runs as expected', setupHookTest('afterEach'));
     });
+
+    it('should not leak event listeners', function() {
+      this.timeout(20000);
+      const testFile = path.join(tempDir, 'test.js');
+      copyFixture(DEFAULT_FIXTURE, testFile);
+
+      return expect(
+        runMochaWatchAsync(
+          [testFile],
+          {cwd: tempDir, stdio: 'pipe'},
+          async () => {
+            // we want to cause _n + 1_ reruns, which should cause the warning
+            // to occur if the listeners aren't properly destroyed
+            const iterations = new Array(process.getMaxListeners() + 1);
+            // eslint-disable-next-line no-unused-vars
+            for await (const _ of iterations) {
+              touchFile(testFile);
+              await sleep(1000);
+            }
+          }
+        ),
+        'when fulfilled',
+        'to satisfy',
+        {
+          output: expect.it('not to match', /MaxListenersExceededWarning/)
+        }
+      );
+    });
   });
 });