The new checkpoint test uncovered a hang that can occur in the logging code. The job is on tinderbox. The only interesting stack is thread 1:
Although the local slot variable had been optimized out, I managed to figure out which slot. Here is the slot's contents. Note that it is in a closed state and that the slot has an error set on it.
I am also not sure how/why this closed slot is still the active slot because there are many free slots available.