Jest: --detectLeaks reporting leaks on a very minimal test environment

[kand]

Do you want to request a _feature_ or report a _bug_?

_bug_

What is the current behavior?

Jest --detectLeaks is reporting leaks even on very minimal test suites.

I've set up an example repo here: https://github.com/kand/jest-22-mem-leak

There are two test files:

describe('memory leak suite 1', () => {

  it('happens', () => {
    console.log('suite 1 > test 1 > ran test');
  });
});

describe('memory leak suite 2', () => {

  it('happens', () => {
    console.log('suite 2 > test 1 > ran test');
  });
});

As well as some added console.log statements in node_modules/jest-leak-detector/build/index.js:

// line 51
    console.log('[IN JEST (line 52)] attaching GC listener to', value.constructor.name);
    weak(value, () => {
      console.log('[IN JEST (line 54)] garbage collected value');
      this._isReferenceBeingHeld = false
    });
    this._isReferenceBeingHeld = true;

    // Ensure value is not leaked by the closure created by the "weak" callback.
    value = null;
  }

  isLeaking() {
    this._runGarbageCollector();

    console.log('[IN JEST (line 66)] checking leak');
    return this._isReferenceBeingHeld;
  }
// line 69

Which produces the following output when tests are run:

yarn test v1.0.2
$ jest --detectLeaks --runInBand
 FAIL  ./file-1.test.js
  ● Test suite failed to run

    EXPERIMENTAL FEATURE!
    Your test suite is leaking memory. Please ensure all references are cleaned.

    There is a number of things that can leak memory:
      - Async operations that have not finished (e.g. fs.readFile).
      - Timers not properly mocked (e.g. setInterval, setTimeout).
      - Keeping references to the global scope.

      at node_modules/jest-cli/build/test_scheduler.js:115:22

[IN JEST (line 52)] attaching GC listener to JSDOMEnvironment
[IN JEST (line 66)] checking leak
[IN JEST (line 52)] attaching GC listener to JSDOMEnvironment
 FAIL  ./file-2.test.js
  ● Test suite failed to run

    EXPERIMENTAL FEATURE!
    Your test suite is leaking memory. Please ensure all references are cleaned.

    There is a number of things that can leak memory:
      - Async operations that have not finished (e.g. fs.readFile).
      - Timers not properly mocked (e.g. setInterval, setTimeout).
      - Keeping references to the global scope.

      at node_modules/jest-cli/build/test_scheduler.js:115:22

Test Suites: 2 failed, 2 total
Tests:       0 total
Snapshots:   0 total
Time:        0.734s
Ran all test suites.
[IN JEST (line 54)] garbage collected value
[IN JEST (line 66)] checking leak
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

It doesn't appear that either of these tests should be producing a memory leak. Memory leaks are also reported when --runInBand is not used. It looks like the leak detector is checking for memory leaks before the JSDOMEnvironment object is cleaned up, which causes jest to report a memory leak, though I'm sure this is not the root cause.

I believe these issues are related:

https://github.com/facebook/jest/issues/1893
https://github.com/tmpvar/jsdom/issues/1682
https://github.com/tmpvar/jsdom/issues/1682

But 2 are closed and the open one hasn't had activity in a while.

What is the expected behavior?

No memory leaks are reported.

Please provide your exact Jest configuration and mention your Jest, node,
yarn/npm version and operating system.

[email protected]
[email protected]
[email protected]
[email protected]
macOS Sierra (version 10.12.6)

[rickhanlonii]

Note that this fails for both jsdom and node environments and for all Jest 22 releases

Fix is in the above PR

[dylanwulf]

This is happening to me in jest v25

[davimello28]

Also with me in Jest v25. Mine was even smaller:

describe('MemoryLeakTest', function () {
  it('should do nothing', function () {
    expect(true).toBeTruthy()
  })
})

./node_modules/jest/bin/jest.js --detectLeaks

[dylanwulf]

I submitted a new issue for this problem in v25: #9507