Non-termination of test execution in Jest when tested function mutates its arguments

🐛 Bug Report

In some situations test execution doesn’t appear to terminate. It also does not run into a timeout. I suspect this can happen when the tested function mutates its arguments. I have only observed this with Jest, so I’m not sure if this is rather a Jest issue.

To Reproduce

Consider the following example (full setup available in this repository):

const fc = require('fast-check')

// the function to be tested
function nasty(array) {
    array.push(1) // nasty argument mutation
    return array
}

test('demo test', () => {
    fc.assert(fc.property(
        fc.array(fc.integer()),
        array_before => {
            const array_after = nasty(array_before)

            // some random assertion that should eventually fail
            expect(array_after.length).toBeLessThenOrEqual(3)
        }
    ))
})

If I run this with Jest, it looks like this. Note, if I pass a copy of array_before into nasty, the test run terminates immediately (with a failure as expected). So I really think the argument mutation is the offender here.

https://user-images.githubusercontent.com/26570572/192302053-fb7586fb-a8e1-4c1a-b44a-d5b9d41a90c1.mov

Your environment

Packages / Softwares	Version(s)
fast-check	3.1.4
node	v16.0.5
jest	29.0.3

[dubzzz]

Good news: I have some ideas to make it work with, I hope, no runtime footprint but probably a huge memory footprint as I’ll have to keep one copy of the array in memory to make the code work. I’ll work on it soon and assess impacts to check whether acceptable or not. The risk of side effects being huge, it does not seem that an issue to try to limit that kind of issues to the end user a little bit more if feasible without any excessive impact.

[dubzzz]

Let’s keep it opened for now so that I can have a reminder that there is possibly a quick improvement to do for the shrinker of arrays.