d-borisov / sinon-gc

Showcase to undestand nuances on how sinonjs affects V8 GC

sinon-gc

Showcase to understand nuances on how sinonjs affects V8 GC

description

I found that sinon affects garbage collection mechanism in NodeJS. Objects that have interacted with sinon's mock objects are not removed by GC.

My local setup is:

$ uname -a
Linux dborisov-laptop 4.15.0-163-generic #171-Ubuntu SMP Fri Nov 5 11:55:11 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

$  cat /etc/os-release
NAME="Ubuntu"
VERSION="18.04.3 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.3 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

$ node -v
v16.13.0

$ npm -v
8.1.0

steps to reproduce

Just run test npm script

$ npm run-script test

> @dborisov/[email protected] test
> node --inspect-brk --expose-gc --trace-gc index.js

Debugger listening on ws://127.0.0.1:9229/98df6527-f3c7-4109-96eb-590490d43dc3
For help, see: https://nodejs.org/en/docs/inspector         <-- LABEL_1
Debugger attached.
array with objects
gc() A       <-- LABEL_2
array cleared
gc() B       <-- LABEL_3
Waiting for the debugger to disconnect...

When you get a string labeled as LABEL_1 from stdout please connect to the application with debugger. I use Google Chrome DevTools (chrome://inspect).

When you get a string labeled as LABEL_2 make a heapdump. I've uploaded mine - see heapdump_1.

When you get a string labeled as LABEL_3 make one more heapdump. I've uploaded mine - see heapdump_2.

Now search for object of SomeClass in dumps.

First heapdump

Second heapdump

One can see that behavior for the same objects differs depending on sinon.

1. Object that was identified as this.id = 'withReal' exists in the first dump but not in the second. This is correct, there is no active link to the object because an array that had a link to the object was replaced with empty one.

2. Object that was identified as this.id = 'withMock' exists in both dumps which seems incorrect. This object has passed the same algorithm as the first object and I expect that mock-object should be swept out by GC as well.