> had never been reported before, despite being largely the same for 9 years

I'll admit to encountering weird "er, why is that randomly nil" errors quite regularly with JRuby when load testing webservers, which, er, I've never reported. It's never been obvious where the fault was, really. And it always seemed unlikely to be your fault, tsk ;)

> It's better to avoid using closure state if you know it's going to be called across threads

Yeah. In this case it was a hash of name -> lambda pairs which boiled down to variations on:

    ->(obj) do
      case obj.bla
        when /foo(.*)/ then "FOO_#{$1.upcase}"
        when /bla(.*)/ then $1
      end
    end

Called in lots of tight loops from a pool of worker threads. I refactored it into something neater, but it still should have worked fine :)

It would be worth proposing to ruby-core that captured closures are thread-local, but that would break a lot of code that actually depends on the sharing. Programming is hard :-(

Huh, if the local various was only scoped to the closure block, and not above it, I would never expect it to be shared. I would think "avoid using closure state" means exactly that, use only local variables scoped no higher than the closure block itself. (It's true this can sometimes be difficult to ensure in ruby; block local variables can help).

Do I understand things right, or am I wrong here?

I guess the question is where the regexp special vars are scoped to though, I see how that's not entirely clear.

You're largely right. The problem is that $~ (and related vars) and $_ are scoped to the nearest method body. If they were scoped to the closure itself, there'd be no problem.