Mirah Office Hours: Mirah doesn’t work on JRuby 1.6.4

Unlike last week where I totally rocked it, this week I didn’t feel like I got as much forward motion. Too much time spent trying to diagnose #144 and not enough on other things.

What I set out to do
Boogs to smash

  • class name manging of dashes #138
  • Java source generation boogs eg #120
  • class loading issues on JRuby 1.6.4 #144
  • Features to rock

  • argv/ARGV
  • make loading extensions client code friendly
  • Experimenting

  • Shatner revisited
  • Dubious revisited
  • REPL for awesome
  • I like to be ambitious, what can I say.

    Of that I got the class naming bug and one Java source bug fixed.

    The rest of my time was spent on trying to figure out what was going on with the class loading problem. To be honest, I should have probably given up on it after I couldn’t figure out what was wrong and moved on to other bugs I could fix. Given more time, either I or somebody else would have come up with a solution. But, I love hard puzzles, and this was a good one.

    The first thing I did was try to narrow down the possible places the bug was originating from. Unfortunately the stacktrace was pretty unhelpful in this regard because it didn’t tell me what happened between the outer call and the place where the class lookup failed.

    I tried poking around and putting in debug statements. Eventually I decided that the bug was somewhere in the interpreter in something that had changed between 1.6.3 and 1.6.4.

    Backup, what was the bug?
    The bug was that when you attempted to load a macro, a ClassNotFoundException would be raised about the MacroClassName, with an extra ‘$’ on the end.


    java.lang.ClassNotFoundException: Square$Extension1$

    So, I looked through JRuby’s source with an eye for the string “$”. I was figuring that where there was a $ appended to a classname, there might be some code related to the extra lookup that was failing.

    rgrep ‘\”$\”‘ src

    Mostly I found false positives, there are a quite a few places that deal with “$”s. But not too many that append “$”‘s to strings. I found a couple things that stood out, and one in particular was especially interesting because it added a “$” to the end of a class name and then attempted to load it. And, the method it was in was changed between 1.6.3 & 1.6.4. handleScalaSingletons

    To be honest, that wasn’t really the point I knew that it was the problem. I did a whole bunch more debugging, including trying out git’s bisect feature (which is pretty awesome, by the way).

    handleScalaSingleton previously caught Exception, but in 1.6.4 it was changed to catch the specific exceptions that can be raised from loading a class. The problem was that the exception being raised was wrapped.

    JRuby internally wraps exceptions raised in Ruby in a RaisedException class, which is fine in itself. It unwraps it before sending back to client Java methods, when it makes sense. The problem it that it is passed around in the interpreter as a RaisedException, not as the type of native exception that was raise from Ruby. When a class loader implemented in Ruby raised a ClassNotFoundException, it would get wrapped and raised in the interpreter as a RaisedException, so it wouldn’t get caught by a catch for a ClassNotFoundException.

    There’s now a pull request for fixing it that @abscondment wrote.

    Comments are closed.