Hi. OP here.

Thanks for the comments about atomicity vs. thread-safety. Absolutely on point. The article started out demonstrating what happened with concurrent Array mutation, but then I put in that += operation and didn't address it. Sorry for not making the distinction. Atomicity is absolutely a different issue than a thread-safe collection. I'm publishing something new tomorrow that addresses this point.

To bring things back to code, the point I was originally trying to make is that this code is not thread-safe.

  array = []
  threads = []

  10.times do
    threads << Thread.new do
      100.times { array.push(rand) }
    end
  end

  threads.each(&:join)
  # 10 threads each inserted 100 values, result should be 1000
  puts array.size

Specifically, too many Ruby programmers won't think twice about this operation not being thread-safe:

  array.push(item)

But there's no such guarantee. This is demonstrated nicely when this code example is run on an implementation with no global lock, try it on JRuby.