Observation: The current stress test failed once

[rayluo]

I happen to observe one occurrence of garbled cache file, happened on macOS, under 80 concurrent processes with 0 waiting interval.

This would probably not affect real world usage, but I'm keeping this record here, so that we could revisit this at a later time. It can possibly be improved by rework the locking mechanism (#63).

CC: @SomkaPe