Semaphores are hard to use

[cstanfill]

Semaphores are a little tricky to use, since independent parts of the program can watch different semaphores. If some piece of code that only cares about semaphore A gets woken up for semaphore B, it has no idea what to do about this - and can't just eat the wakeup.
Right now this can be worked around by unwatching the unexpected semaphore, and then once it's done with its semaphore logic, rewatching it and setting it for the next wait operation to pick up.