Fixing 'ArgumentError: Invalid Domain' After Upgrading to Ruby 2.7.7

Recently I tasked myself with upgrading one of the apps I support as a part of my day job from Ruby version 2.7.6 to 2.7.7.

After doing so, I immediately started running into this issue, only while running tests, and only when those tests were run in CI:

ArgumentError:
       invalid domain: ".example.com"

Strange. None of the other apps I upgraded had this issue, and I’m not setting anything domain-specific like this in my app code, so the first thing I thought to do was go back to 2.7.6 to make sure I didn’t jack something up. Of course it worked fine so there had to be something special about this upgrade that was causing issues.

Skipping the part where RAILS_ENV=test && CI=true were really just a red herring, the head-desk-ing that took place therein, and knowing that the version bump was primarily due to CVE-2021-33621 caused by the cgi rubygem, I had what felt like an entirely random thought¹ to check the cgi rubygem itself for this message.

Sure enough, it’s there, plain as day².

Feeling like I was on a roll, my next thought was to check any open issues for that rubygem. I can’t imagine I’m the first to run into this, right?

Definitely not (tangentially, this PR). Turns out™, in the course of updating cgi, cookie logic was changed to disallow cookies with preceding periods, because who does that, anymore?

I guess I do. 🤷🏼‍♂️

Ok, so what’s the fix? Well, on its own, Ruby 2.7.7 still holds the version of cgi that disallows cookie domains with leading periods, so you’ve got two options:

1. Stop using leading periods for your cookie domains
2. Upgrade cgi by explicitly defining it in your Gemfile with gem 'cgi', '~> 0.3.6'

On the face of it, doing the more currently-appropriate thing is probably the right move, but I didn’t have a ton of time to make those changes and ensure nothing broke, so I took the second option. I’ll probably come back to this and do it the better³ way in the future.

Or not. If Ruby 2.7.next (and equivalent 3.x.next branches) pick up cgi 0.3.6 or a later version, I may never.