Sunday 5 June 2011

Bugs: they hide where you're not looking

I bought a new Pogoplug over the weekend (only £50 new at PC World), and after being genuinely impressed by the Pogoplug software, decided it was far too easy and put PlugApps Linux on it. These 'plug' type devices are fairly amazing - cheap, very low power (measured mine at just under 4 watts, with only a single USB flash stick), but with a decent 1.2GHz ARM processor. I'm already thinking my next computer might be another 'plug.

After hacking for I while (why won't my printer work?), I decided to check whether my pylibftdi package worked on it. To my shock, a quick 'pacman -S libftdi; pip install pylibftdi', installed fine, and I could open a device connection to a FTDI device! But then things got worse. Trying to run examples/ failed with an exception in BitBangDevice, and I quickly realised that the API changes I'd done in 0.8 to make device access more 'file-like' had broken things in BitBangDriver. I was slightly sad that I'd released something where the examples didn't even work, and part of the whole reason the package might be useful to people (the abstraction over bit-bang device operation) was broken.

pylibftdi is fairly simple, and basically consists of Driver, Device, and BitBangDevice classes. Most of the interesting is in the Device class - so this is where I started when I finally got round to adding some tests for the 0.8 release. Having achieved reasonable coverage (though shamefully less than the 100% Uncle Bob demands), I considered my initial testing 'done'. I knew there was more to add later, and had (and still have) full intentions to 'get around to it'.

What I failed to anticipate has the unintended side-effect of writing tests. In the same way teachers might teach how to pass an exam rather than the depth and breadth of a subject, once tests exist, the purpose can simply be to pass them. Old manual acceptance tests get ignored as the 'old way' of doing things. Ideally of course this isn't a problem, because full test cases exist for every feature and code-path in the system, but that was very far from the case here. So somehow, my standard acceptance test (do the example programs still work) got omitted, in preference for 'there are tests now, so it must be better! And the tests pass!'

So beware - a little testing can be a dangerous thing. The bugs hide where you're not looking for them. This is great motivation for achieving full test coverage, for automating acceptance testing (as well as unit / component level testing) so far as possible, and for being humble when it comes to writing tests. My motivations for writing them in the first place were two-fold: the feeling it was 'what I should do', and the idea that at some future point when I added or refactored things later I could be confident I hadn't broken things. I had no thought that the software was already broken; that I needed tests.

Anyway, pylibftdi 0.8.1 is now out, with trivial but important fixes and lots more tests.