Everything Let Me Down (Or, How I Messed Up a Simple Task)
28 February 2020

I wrote some code for a site about music. At the top of each page, it has a mouseover which tells you what equipment was used to play the records discussed below. It does this by way of a hash which details all the gear that’s ever been in the hi-fi, and when it came and went. For each post it filters the hash according to the date the post was written, and returns the correct system. It then further filters depending on whether the reviewer listened through headphones or speakers.

It didn’t work.

Visiting the site showed the source and amplification, but not the transducers. No speakers, no headphones. I fired up a local instance.

It worked fine.

Until it didn’t.

If you started off viewing a “listening through speakers” page, you got the speakers, until you viewed a “listening through headphones” page, at which point the speakers disappeared and were not replaced by headphones. And vice-versa.

I had unit tests to check each transducer type, and they were passing.

Looking at the code, I found the transducer filtering was done with Ruby’s #delete_if method. Now, delete_if should probably be called delete_if!, because it modifies the object on which it operates. So when I took out the headphones or speakers, they were gone from the hi-fi hash for ever. I replaced delete_if with reject, and everything was as it should be.

It’s a stupid, elementary mistake, but I think it raises a couple of interesting points.

1. The New Toy

Why did I use delete_if? When I looked at the code, I barely recognized it, and a quick grep -r through my work directory shows I’ve never used it anywhere else. I remember, around the time I wrote this code, prepping for an interview, and reading through the reference part of the pickaxe book. There were a whole bunch of Enumerable methods I didn’t know, and I probably used it because I’d just learnt about it, without understanding exactly what it did.

We do this a lot. We get a new toy, be it as complex as Kubernetes or as simple as delete_if, and we throw it at the first vaguely related problem we see. And we’re probably wrong, and we’ll probably regret it.

2. Trusting Your Tests

When came to fix the bug, I immediately looked at my tests. The code had a build pipeline, and there were unit tests for class which creates the hi-fi list, and an acceptance test which looked at the rendered HTML. They passed, and they passed because every test set up a nice fresh environment in which to run.

The headphones test ran on a brand-new hi-fi object. The speakers test ran on a brand new HiFi object, so the effects of permanently deleting the other type of transducer were never seen. The acceptance test ran on a randomly generated page, not on a page which had been through the change in state. This is (broadly) the way we’re told to write tests: ordering is bad, and idempotency is king. If I’d had lousy order-dependent, resource-recycling tests, they’d have caught this problem before the first release. (Though, of course, they’d have caused lots of other problems.)

A code review would have caught this. Someone would have said “why are you using #delete_if, and I’d have said “… I… don’t… know…”, and changed it to reject. That person might also have pointed out exactly what delete_if does, and I’d have learnt the easy way instead of the hard.

3. Changing Stuff

I wrote this code a couple of years ago, before I started using Clojure and Rust, and had the penny-drop moment about immutability. Now, even when I write Ruby, I try to think functionally, and do whatever’s reasonable to avoid mutating anything.

If you can’t change existing objects, you can trust those beautifully distinct, idempotent tests. You also don’t have to worry about locking and synchronization and all that awful stuff.

4. Those Damn Expressive Languages

Some people would blame Ruby for giving me the scope to make a mistake like this. ‘Why does Enumerable need so much “syntactic sugar”? You wouldn’t make this mistake in Go.’

True, but I’d much rather, whether writing or reading code, have all those Enumerable methods compacting and explaining than only a for loop, and everything in longhand.

I’d rather have the rope, even if I did accidentally wrap it round my neck this time.

Lesson Learnt

Though I’d like to say “I wouldn’t make this mistake now”, I wouldn’t have made it then if I’d bothered to look up exactly how delete_if works. That’s the best lesson: on any scale, dev or ops, don’t use something without fully understanding it. Don’t use something “because it does this” if you don’t know enough to grasp that “it also does that”.