On Timezones, Testing and Deals with the Devil

Timezones make my head hurt. Not the concept, that is easy, but writing code that works correctly in different timezones. Throw daylight savings correction into the mix and you’ll often find me in a corner rocking gently. The problem of course is that I frequently have to deal with data that isn’t just the standard unix timestamp (or various orders of magnitude away from). A number of times a load of information has been thrown away, never to be recovered which makes life interesting. You end up having to decide which is the best path to take when all of them are wrong in various ways.

So why bring this up? Well America has just jumped to daylight savings time. And wouldn’t you know it, a few of Mozilla’s automated tests failed. Which is awesome of course. Always good to know something is up. The tricky thing with test failures is figuring out why they are failing. The automated tinderbox are remote machines you generally don’t have access to to debug on. This can be pretty unhelpful if the test doesn’t provide enough logged information. I’ve been pretty guilty of this myself, while a test that spots problems is fantastic, what is even better is one that provides enough information in the log to figure out why it failed. Think of it this way. If you write a test that (months, years) later fails then you’ll likely be asked why. What information do you need to find out? A few suggestions I’ve learned the hard way:

  • Always log the found and expected values when comparing (do_check_eq and similar does it for you but there are cases where you don’t use that).
  • If you have a loop in the test be sure to log what iteration the test failed in. A message that “false wasn’t true” isn’t a lot of use when that check is performed a hundred times.
  • If it is a long test why not just log a few checkpoints along the way, if the test crashes somewhere at least you’ll have a rough idea of where.

I will probably come back with more suggestions by the end of the week. I needed approval for a late fix I needed to land on Saturday and shaver was the only driver around at the time. He is often getting on at people (quite rightly) for not writing tests for the code they submit, so I made a deal with him that in return for the approval that I’d spend 2 days this week writing tests. I expect the majority of them to be testing for the UI side of the add-ons manager which is essentially untested right now and right now I have no clue how to go about testing. Should be fun!

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