Race condition in the MutationTestExecutor

[nicojs]

The MutationTestExecutor has a race condition here:

stryker-js/packages/core/src/process/4-mutation-test-executor.ts

Lines 146 to 163 in aa92c52

	const group$ = this.checkerPool
	.schedule(input$.pipe(bufferTime(CHECK_BUFFER_MS)), (checker, mutants) => checker.group(checkerName, mutants))
	.pipe(mergeMap((mutantGroups) => mutantGroups));
	const checkTask$ = this.checkerPool
	.schedule(group$, (checker, group) => checker.check(checkerName, group))
	.pipe(
	mergeMap((mutantGroupResults) => mutantGroupResults),
	shareReplay(),
	map(([mutantRunPlan, checkResult]) =>
	checkResult.status === CheckStatus.Passed
	? mutantRunPlan
	: {
	plan: PlanKind.EarlyResult as const,
	mutant: this.mutationTestReportHelper.reportCheckFailed(mutantRunPlan.mutant, checkResult),
	}
	)
	);
	return checkTask$;

As you can see, .schedule() is called twice in quick succession. This isn't supported by the Pool class. Only one schedule() can be called at the same time. A call to schedule() creates some internal bookkeeping variables that are local to the function itself, any subsequent calls don't share that state and instead cause new bookkeeping variables to be created. This results in parallel calls to the checker (calls to check while a call to group is active).

[nicojs]

I've created the branch feat/pool-schedule which contains a unit test that forces the bug.

I have no clear solution for this. I'm thinking that what we want to achieve (a pool of resources shared by consumers) is not possible with standard RXJS operators. Maybe we should simply program it ourselves using plain old arrays or something 😢

[nicojs]

I've also opened a stackoverflow question about it. You never know 🤞
https://stackoverflow.com/questions/71689376/how-to-use-rxjs-to-share-a-pool-of-resources-between-multiple-consumers