Tag Archives: integration testing

When you can’t (and shouldn’t) unit test

I’m a unit test aficionado, and, as such, have attempted to unit test what really shouldn’t be. It’s common to get excited by a new hammer and then seeing nails everywhere, and unit testing can get out of hand (cough! mocks! cough!).

I still believe that the best tests are free from side-effects, deterministic and fast. What’s important to me isn’t whether or not this fits someone’s definition of what a unit test is, but that these attributes enable the absence of slow and/or flaky tests. There is however another class of tests that are the bane of my existence: brittle tests. These are the ones that break when you change the production code despite your app/library still working as intended. Sometimes, insisting on unit tests means they break for no reason.

Let’s say we’re writing a new build system. Let’s also say that said build system works like CMake does and spits out build files for other build systems such as ninja or make. Our unit test fan comes along and writes a test like this:

assert make_output == "all: foo\nfoo: foo.c\n\tgcc -o foo foo.c"

I believe this to be a bad test, and the reason why is that it’s checking the implementation instead of the behaviour of the production code. Consider what happens when the implementation is changed without affecting behaviour:

all: foo\nfoo: foo.c\n\tgcc -o $@ $<

The behaviour is the same as before: any time `foo.c` is changed, `foo` will get recompiled. The implementation not only isn’t the same, it’s arguably better now, and yet the assertion in the test would fail. I think we can all agree that the ROI for this test is negative if this is all it takes to break it.

The orthodox unit test approach to situations like these is to mock the service in question, except most people don’t get the memo that you should only mock code you own. We don’t control GNU make, so we shouldn’t be doing that. It’s impossible to copy make exactly in a mock/stub/etc. and it’s foolish to even try. We (mostly) don’t care about the string that our code outputs, we care that make interprets that string with the correct semantics.

My conclusion is that I shouldn’t even try to write unit tests for code like this. Integration tests exist for a reason.

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Unit Testing? Do As I Say, Don’t Do As I Do

I’m a firm believer in unit testing. I’ve done more tech talks on the subject than I’d care to count, and always tell audiences the same thing: prefer unit tests, here’s a picture of the testing pyramid, keep unit tests pure (no side-effects), avoid end-to-end tests (they’re flaky, people will stop paying attention to red builds since all builds will be red). I tell them about adapters, ports and hexagonal architecture. But when it comes to using libclang to parse and translate C and C++ headers, I end up punting and writing a lot of integration tests instead. Hmm.

I know why people write tests with side-effects, and why they end up writing integration and end-to-end ones instead of the nice pure unit test happy place I advocate. It’s easier. There’s less thinking involved. A lot less. However, taking the easy path has always come back to bite me. Those kinds of tests take longer. They higher up the test pyramid you go, the flakier they get. TCP ports stay open longer than a tester would like, for instance. The network goes down. All sorts of things.

I understand why I wrote integration tests instead of unit tests when interfacing with libclang too. Like it is for everyone else, it was just easier. I failed to come up with a plan to unit test what I was doing. It didn’t help that I’d never used libclang and had no idea what the API looked like or what it allowed me to do. It also doesn’t help that libclang doesn’t have an option to take a string to the code to parse and instead takes a file name, but I can work around that.

Because of this, the dpp codebase currently suffers from that lack of separation of concerns. Code that translates C/C++ to D is now intimately tied to libclang and its quirks. If I ever try to use something other than libclang, I won’t be able to. All of the bad things I caution everybody else about? I guaranteed they happened in one of my newest projects.

Before the code collapses under its own complexity, I’ve decided to do what I should’ve done all along and am rewriting dpp so it uses layers to get away from the libclang mess. I’m still figuring it all out, but the main idea is to have a transformation layer between libclang and my code that takes its data types and converts them to a new set of AST types that are my own. From then on it should be trivial to unit test the translation of those AST types that represent C or C++ code into D. Funnily enough, the fact that I wrote so many integration tests will keep me honest since all of those old tests will still have to pass. I’m not sure how I feel about that.

I might do another blog post covering how I ended up porting a codebase with pretty much only integration tests to the unit variety. It might be of interest to anyone maintaining a legacy codebase (i.e. all of us).

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main is just another function

Last week I talked about code that isn’t unit-testable, at least not by my definition of what a unit test is. In keeping with that, this blog post will talk about testing code that has side-effects.

Recently I’d come to accept a defeatist attitude where I couldn’t think of any other way to test that passing certain command-line options to a console binary had a certain effect. I mean, the whole point is to test that running the app differently will have different consequences. As a result I ended up only ever doing end-to-end testing. And… that’s simply not where I want to be.

Then it dawned on me: main is just another function. Granted, it has a special status that makes it so you can’t call it directly from a test, but nearly all my main functions lately have looked like this:

int main(string[] args) {
    try {
        doStuff(args);
        return 0;
    } catch(Exception ex) {
        stderr.writeln(ex.msg);
        return 1;
    }
}

It should be easy enough to translate this to the equivalent C++ in your head. With main so conveniently delegating to a function that does real work, I can now easily write integration tests. After all, is there really any difference between:

doStuff(["myapp", "--option", "arg1", "arg2"]);
// assert stuff happened

And (in, say, a shell script):

./myapp --option arg1 arg2
# assert stuff happened

I’d say no. This way I have one end-to-end test for sanity’s sake, and everything else being tested from the same binary by calling the “real” main function directly.

If your main doesn’t look like the one above, and you happen to be writing C or C++, there’s another technique: use the preprocessor to rename main to something else and call it from your integration/component test. And then, as they say, Bob’s your uncle.

Happy testing!

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