jest-worker farm memory leak

[IIIEII]

Parent process continuously grow memory usage while executing multiple child worker processes that return large chunks of data.

I prepared sample repository to reproduce issue: https://github.com/IIIEII/jest-worker-farm-leak
It has worker that returns large array with strings.
Parent process executes it three times with pause of 10s between.

Steps:

1. Run sample script in inspect mode node --inspect --expose-gc src/parent.js
2. Connect with chrome inspector and record allocation timeline

3. Here you can see that data returned from worker are stored in memory

It seems that problem lies here: https://github.com/facebook/jest/blob/7a64ede2163eba4ecc725f448cd92102cd8c14aa/packages/jest-worker/src/Farm.ts#L136-L138

Possible solution:

1. Make change to node_modules/jest-worker/build/Farm.js file:

Index: node_modules/jest-worker/build/Farm.js
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/node_modules/jest-worker/build/Farm.js b/node_modules/jest-worker/build/Farm.js
--- a/node_modules/jest-worker/build/Farm.js	(date 1625231320301)
+++ b/node_modules/jest-worker/build/Farm.js	(date 1625231320301)
@@ -140,10 +140,11 @@
     // and other properties of the task object, such as task.request can be
     // garbage collected.
 
-    const taskOnEnd = task.onEnd;
+    let taskOnEnd = task.onEnd;
 
     const onEnd = (error, result) => {
       taskOnEnd(error, result);
+      taskOnEnd = null;
 
       this._unlock(workerId);
 

2. Run sample script in inspect mode node --inspect --expose-gc src/parent.js
3. Connect with chrome inspector and record allocation timeline

4. Here you can see that data are in GC roots

[sibelius]

@IIIEII can you send a pull request to fix this?

can you provide a patch-package (https://github.com/ds300/patch-package) so more people can try your solution?

[SimenB]

Very much happy to take a PR changing this like suggested

[SimenB]

https://github.com/facebook/jest/releases/tag/v28.0.0-alpha.6

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