Challenge: The code review bug that gives me nightmares

I was writing a fairly low level piece of code recently, this is deep in the guts of how RavenDB handles query. I’m adding a cache for some processing inside of RavenDB and the performance benefits are amazing (three orders of magnitude better). As you can imagine, this is the sort of things that I would really like to get into the main codebase. So I did all the usual taxes, created a pull request and moved to other things. Part of our process is to review all pull requests by another pair of eyes. And while there was some minor comments about the code, there was one comment asking about a particular line that had be break out in cold sweat.

I created a simpler reproduction for discussion purposes, here is the code:

Take a look, and let’s me know if you see the bug and its consequences.