This week I re-encountered a problem that has been plaguing me for months, but I never took the time to try to debug it before now. The problem came with a really specific and confusing symptom: any time I installed a newer version of Ruby on my machine, a daily cronjob running a Ruby script would start failing. The specific error was always an FFI exception, with the exact error message FFI::DynamicLibrary.load_library: Could not open library.
