Stephanie has another debugging mystery to share. Earlier this year, Joël mentioned that he was experimenting with a bookmark manager to keep track of helpful and interesting articles. He's happy to report that it's working very well for him!
Together, they discuss tactics to ensure the easiest route also upholds app health and aids fellow developers. They explore streamlining test fixes over mere re-runs and how to motivate desired actions across teams and individuals.
Raindrop.io (https://raindrop.io/)
Railway Oriented Programming (https://fsharpforfunandprofit.com/rop/)
Transcript:
JOËL: Hello and welcome to another episode of The Bike Shed, a weekly podcast from your friends at thoughtbot about developing great software. I'm Joël Quenneville.
STEPHANIE: And I'm Stephanie Minn. And together, we're here to share a bit of what we've learned along the way.
JOËL: So, Stephanie, what's new in your world?
STEPHANIE: So, I have another debugging mystery to share with the group. I was working on a bug fix and was trying to figure out what went wrong with some plain text that we're rendering from a controller with ERB. And I was looking at the ERB file, and I was like, great, like, I see the method in question that I need to go, you know, figure out why it's not returning what we think it's supposed to return. I went down to go check out that method. I read through it ran the tests.
Things were looking all fine and dandy, but I did know that the bug was specific to a particular, I guess, type of the class that the method was being called on, where this type was configured via a column in the database. You know, if it was set to true or not, then that signaled that this was a special thing. You know, I made sure that the test case for that specific type of object was returning what we thought it was supposed to. But strangely, the output in the plain text was different from what our method was returning.
And I was really confused for a while because I thought, surely, it must be the method that is the problem here [laughs]. But it turns out, in our controller, we were actually doing a side effect on that particular type of object if it were the case. So, after it was set to an instance variable, we called another method that essentially overwrote all of its associations and really changed the way that you would interact with that method, right? And that was the source of the bug is that we were expecting the associations to return what we, you know, thought it would, but this side effect was very subvertly changing that behavior.
JOËL: Would it be fair to call this a classic mutation bug?
STEPHANIE: Yeah. I didn't know there was a way to describe that, but that sounds exactly right. It was a classic mutation bug. And I think the assumption that I was making was that the controller code was, hopefully, just pretty straightforward, right? I was thinking, oh, it's just rendering plain text, so not [laughs] much stuff could really be happening in there, and that it must be the method in question that was causing the issues.
But, you know, once I had to revisit that assumption and took a look at the controller code, I was like, oh, that is clearly incorrect. And from there, I was able to spot some, you know, suspicious-looking lines that led me to that line that did the mutation and, ultimately, the answer to our mystery.
JOËL: Was the mutation happening directly in the controller? Or was this a situation where you're passing this object to a method somewhere, and that method, you know, in another object or some other file is doing the mutation on the record that you passed in?
STEPHANIE: Yeah, that's a great question. I think that could have been a very likely situation. But, in this particular case, it was a little more obvious just in the controller code, which was nice, right? Because then I didn't have to go digging into all of these other functions that may or may not be the ones that is doing the mutation. But the thing that was re