Re: Elegant 'abort' of sleeping task

[Lutz Donnerhacke <lutz@iks-jena.de>]

* Simon Apperley wrote:
> I'm looking at the design of a piece of server code which has to handle
> calls that also pass a timeout value. The target system is aerospace
> related, and dynamically creating tasks 'on the fly' just is not an
> option.

If you are not hiding your own process details, you can pass the timeout
value down to the lowest level routines. Running on an approbriate OS, all
syscalls has a separate timeout parameter, therefore you are done. ;-)

> I want to be able to set up a single task to handle the timeout from
> the head of a delta-queue of timeouts, but have found a problem. If I
> have the timeout implemented as a task stuck in a 'delay' call, and a
> more immediate timeout comes in, I want to wake up the sleeping task,
> re-calculate the delta-queue and then sleep on the new, shorter,
> delay. So far the only way I can see to do this is to use abort, and
> set up the task again, which seems a bit of a brute force approach.

Do not use delay: Use the syscall to sleep a specified amount of time or
shorter if an event occur. In Posix enviroments, select(2) or poll(2) comes
to mind. But if possible look for calls like epoll(2).

> Has anyone got any suggestions on how I can interrupt the sleep call,
> without using a polling approach that would just consume CPU time at
> the expense of the other code in the system.

Depends on your system. Most can do this easily.

> I could use direct calls to the underlying RTOS, but I'd rather keep
> as much as possible within the Ada language.

Write a wrapper "procedure Pause_Max (seconds : in out Delay_T; events : out
Event_Array; last_event : out Natural)" or "function Pause_Max (seconds :
Delay_T) return event_or_remaining_time_t".

> I did wonder about delay until TIME, and having another task change
> TIME, but that seems rather un-safe to me as it starts making
> assumptions about the underlying run-time implementation.

This will not work, because it will not stop in the case of events.