After some new code was checked in at work we encountered this issue in our CI as part of the build step to run RubyCritic over our code. I’d never seen it before, and the source of the error was in buffer.rb of the parser gem library:

'source=': invalid byte sequence in US-ASCII (EncodingError)

I did some digging and I found that this is where RubyCritic parses each file into an abstract syntax tree for analysis. It seemed like there was a character in the file that could not be parsed correctly, and eventually I found a StackOverflow post that pointed to a tool called iconv that can be used to convert between different character encodings, and that if a conversion is unsuccessful it will throw an error and return code 1. Now this was all well and good but the error I was getting from buffer.rb did not tell me the currently erroring file – the best I could do was modify my local gem source to give me a list of the files that passed through the RubyCritic library for analysis.

Then, now that I had a list of files, I could run each file through iconv to check which one had invalid ASCII characters. Of course I am a programmer and thus lazy so I wasn’t going to sit there and run it manually on every damn file, so I just made a ruby script to run it on each file in my list (of which there were hundreds):

def run 
  SOURCE_FILES.each do |file|
    file_path = SOURCE_DIR # source dir is the full path of the root directory
    puts file_path
    puts `iconv -f us-ascii #{file_path} > /dev/null; echo $`

I ran the script and it found the file easily by finding the one that returned 1. Then, all I did to fix the issue was delete the code that had been changed in the previous commit, re-typed it manually, then saved the file. I ran my script again and the issue was solved!