>What is the external interface of the kernel? What is its surface area? A kernel is so central and massive the only way to test its complete contract with the

>What is the external interface of the kernel? What is its surface area? A kernel is so central and massive the only way to test its complete contract with the user (space) is... just to run stuff and see if it breaks. Having never interacted with the kernel, I have to assume it isn't just one massive file right? It's broken into separate files and components? And if you ever want to modify or refactor one of those components its nice to have the confidence that your code changes are safe without having to rebuild the whole thing and run your integration/behavior/e2e tests. Especially if you aren't the one who originally wrote the component you're modifying and don't know what the intended behavior for edge cases was. Obviously if you're mocking everything then the tests might be pointless(there can still value in preserving in the test what you assumed the behavior was), but I feel like in some ways people have over-corrected on TDD and claiming unit tests are pointless.