How do you test (currently) unreachable code?

Question

I have a function that needs to do something like "try to move to node X in a directed graph", and my current implementation uses an indefinite loop like this

while(true) {    
    
    trySomething();    

    if (checkAreWeThereYet()) {
        break;
    }    

    if (inferInfiniteLoopIsHappening()) {
        throw;
    }
}

But with the current implementation I haven't found a way (yet) to actually trigger an infinite loop and test that last part (which I added in precisely because there was a bug in a previous iteration of the code that led to an infinite loop) to make sure it's working as intended.

I've been thinking of mocking some of the implementation details to fake "making a mistake" that would then lead to an infinite loop, but that strikes me as maybe not a best practice (?)

The only online resources I've been able to find so far around this are for the "simpler" but analogous "switch statement's default case is currently unreachable, but might be hit if a bug is introduced in upstream code" scenario, but I'm wondering if I've just been googling the wrong terms (?)

Answer 1

It is a classic case for TDD.

I would recommend deleting the current design and drive a new design with automated checks, rather than trying to write tests to adhere to your design.

For more details on TDD, I would recommend this presentation on TDD by Bob Martin.

I've been thinking of mocking some of the implementation details to fake "making a mistake" that would then lead to an infinite loop, but that strikes me as maybe not a best practice

Your new design (after some refactoring) probably will be different from your current implementation. E.g., you have three responsibilities in this function: The looping, the trySomething, and the dealing with the new state in the graph. These probably will end up in different objects, which then you can inject as you wish on your main object.

while(!analyzer.areWeDone()) {    
    actor.trySomething();    
}

As demonstrated in the presentation, the final design will be created from the simplest cases (where analyzer.areWeDone is true) to the most completed ones, where all branches are covered.