Debugging background tasks inside loops and transactions

I picked up a bug fix the other day that I was able to solve fairly quickly based on a hunch, but even after I’d solved it, I didn’t totally understand why. So I went hunting (because c’mon, the why is the fun part!)

The crux of the problem was this: We were doing an operation on a list of objects, the whole loop wrapped in a database transaction (more detail on defining your own transactions and using it as a context manager here), and then calling a background task for each of them. Users were reporting that the action that was…