[sdab]

Ah, yep I see the race. Here is a fix: https://gist.github.com/sdab/d7ba036b2b7f4b5626cd

Thanks for the puzzle, its hard to find good concurrency problems.

Edit: Looked through your solution. You are right, we thought of similar things. I started out by wanting a multi condition variable, but didnt want to implement it :). I ended up getting something similar in a roundabout way.