Not waiting on a notifier hangs stepTiming in simulation

[virtuald]

This hangs:

from wpilib.simulation import stepTiming
import hal
hal.initializeNotifier()
stepTiming(0.2)

@ThadHouse thinks it shouldn't hang the program. @calcmogul in #3874 (comment) seems to think it should hang it. Either it should be fixed or documented.

[ThadHouse]

High level robot code has startSingle. Which means a stopped notifier is 100% perfectly valid user code. There should be no requirement that all notifiers are running to not hang sim.

[ThadHouse]

Actually, I take my above comment back. High level notifiers still always call WaitForNotifierAlarm, because they always have their own thread.

So this is purely a HAL level issue.

[virtuald]

I think that the implicit contract makes sense because missing a notifier event is undesirable. If the code leveraging the notifier performs a long-running task, the time should not increment until that task eventually calls HAL_WaitForNotifierAlarm, ensuring synchronization. Incrementing time before calling HAL_WaitForNotifierAlarm would result in time becoming out of sync.

Additionally, this implies that it would be a bug to perform a completely blocking operation without ultimately waiting on the notifier.

I think we should document the behavior more throughly and move on.

[virtuald]

If notifiersWaiterCond.wait_for(ulock, std::chrono::duration<double>(1)); in hal::WaitNotifiers times out and doesn't exit the loop, I think it should print a warning telling the user that something is using a notifier incorrectly (and add this discussion there, or a pointer to it) .