"Stack level too deep?", I asked myself, perplexed. All I did was add a gem and a few lines of initialization code.
The gem that I added was a client for a configuration service. For fun, let's call this configuration service "caas" for Configuration as a Service. Values can be added to it via migrations, which work similarly to ActiveRecord migrations. Any pending migrations are run at deploy.
I wrote some test migrations and ran them successfully in my local development environment. Deployment to QA, however, failed. Let's replicate that failure locally by setting
qa and running the migration Rake task.
OK. Let's run with
Not much help. I already knew that there was a problem running the migrations. At this point, my teammates and I spent quite some time thinking about the failure and reading code. Then it hit me: even though there's no backtrace, there must be an exception associated with the "stack level too deep" message. Some searching turned up an answer:
I temporarily changed the client gem to rescue
SystemStackError, updated the host application
Gemfile to refer to a local copy of the gem, and ran the migrations again.
Perfect. Now we know exactly where the problem is.
If you encounter a problem and have nothing but an error message to go on, determine whether it comes from an exception that you can rescue, even if only temporarily. Speculation and intuition are useful, but a backtrace will tell you exactly where you need to look.