Fix module registry memory leak.

[scotthovestadt]

Summary

This leak is caused by this line:
https://github.com/facebook/jest/blob/01044dabf2253287a5a84e6a87b8505b559e9257/packages/jest-runtime/src/index.ts#L693

The moduleRegistry is preserved in the closure and never GC'd, along with all modules within the registry, making it a pretty nasty memory leak. I'm fairly certain it's also a bug fix: when you reset the registry, this reference to the registry continued referencing the old version. That doesn't make sense after a reset.

This code is now a map and easy to clear but until very recently it was a normal Object which explains why it was 'cleared' in this way.

Memory Impact

command: yarn jest --logHeapUsage --w=1 packages/

--expose-gc enabled to trigger gc manually after each test for accurate readings on the leak

without circus:
295 MB -> 262 MB (11% savings)

with circus:
848 MB -> 762 MB (10% savings)

Test plan

• All tests pass.
• Reason for leak identified.
• Memory impact quantified.
• Hoping CI for Circus doesn't OOM now. :)

[scotthovestadt]

🎉🎉🎉

Circus CI no longer fails with an OOM error, which means this has definitely made an impact.

Just need a sanity check on the behavior change and if it makes sense. In the case where we want to preserve the old behavior (I still think it's a bug), it can be done in a way that doesn't create a large memory leak.

@cpojer take a look if you have a moment, maybe you can confirm it's a bug for me?

[codecov-io]

Codecov Report

Merging #8282 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #8282   +/-   ##
=======================================
  Coverage   62.13%   62.13%           
=======================================
  Files         266      266           
  Lines       10664    10664           
  Branches     2590     2590           
=======================================
  Hits         6626     6626           
  Misses       3448     3448           
  Partials      590      590

Impacted Files	Coverage Δ
packages/jest-runtime/src/index.ts	69.39% <100%> (ø)	⬆️

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[SimenB]

This is so awesome!

Changelog? 😀

[SimenB]

I'm fairly certain it's also a bug fix: when you reset the registry, this reference to the registry continued referencing the old version. That doesn't make sense after a reset.

I agree. Hopefully nobody (unwittingly) relied on getting old versions of modules after a reset

[thymikee]

nice!

[jeysal]

Great work identifying this ❤️

[jeysal]

Also cc @dubzzz fast-check is not to blame after all ;)

[cpojer]

This is great! Memory leaks are the worst.

[peterdemartini]

@scotthovestadt can we get a release with this PR? Our tests are broken in CI due to memory leaks and it would be nice to be able to test this.

[scotthovestadt]

@peterdemartini Yes, soon! There will be a release within a week.

Please report back and let me know if this has solved it for you after the release.

[johannessjoberg]

Any updates concerning the release @scotthovestadt ? 🙏

[ulrichb]

Please release 🙏

My Jest integration tests eat 10 GB ... :/

[ulrichb]

Oh. This has been released with 24.8 and I have 24.8. Oh my dear. 10 GB !

[SimenB]

This was released as part of jest 24.8.0

[github-actions]

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