With modern fake timers, an infinite loop contaminates all following tests

[quasicomputational]

🐛 Bug Report

Test cases that fail due to excessive timer recursion can cause failures in following tests, as the infinitely-recursing timer is not cleared between tests.

Jest version: 26.2.2

To Reproduce

jest.useFakeTimers("modern");

it("loops", () => {
    const f = () => {
        setImmediate(f);
    };
    f();
    jest.runAllTimers();
});

it("should work", () => {
    jest.runAllTimers();
});

Expected behavior

Despite the first test failing, the second ought to be green.

Observed behaviour

The second test fails due to timer recursion.

Adding clearMocks: true and resetMocks: true doesn't help matters.

[mohamedsgap]

@SimenB is this issue relative! can i consider it as "good first issue" ? try to work on it!

[mrazauskas]

The first test simply throws: 'Aborting after running 100000 timers, assuming an infinite loop!' But before throwing it still calls setImmediate(f). If I get it right, calling jest.runAllTimers() in the second test is trying to execute the scheduled call back and is hitting the same infinite loop.

All gets solved if fake timers are reset in the second test:

jest.useRealTimers().useFakeTimers("modern").runAllTimers();

Alternatively, one could use afterEach hook to clear all pending timers:

afterEach(() => {
  jest.clearAllTimers();
});

Or to reinstall fakes:

afterEach(() => {
   jest.useRealTimers();
 });

Hm.. Perhaps before throwing it is possible to clean up a timer causing infinite loop in Sinon's Fake Timers? But what if there is another timer which will cause infinite loop soon after the error? That’s complicated. Looks like it is better to clean up manually between tests.

[SimenB]

I don't think we should automatically clean up timers between tests, that's up to the author

[github-actions]

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