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From: Natasha Kerensikova <lithiumcat@instinctive.eu>
Newsgroups: comp.lang.ada
Subject: Re: Termination of tasks waiting on a protected queue
Date: Sun, 18 May 2014 19:54:16 +0000 (UTC)
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Date: 2014-05-18T19:54:16+00:00
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Hello,

On 2014-05-18, Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> wrote:
> I have done something similar for pools of worker tasks. I used an
> instance of PragmARC.Queue_Unbounded_Blocking to hold the job requests
> rather than rolling my own for each task pool. The solution to
> termination was to use a timed entry call to the queue, with another
> protected object with a function to indicate if the pool tasks should
> end.

That's a solution I considered, along with the idea of outputing a
special value in the blocking protected entry to indicate that tasks
should end (choosing between both solutions depending on how unnatural
adding the shutdown order to the queue is, and how badly I want the
termination order to be immediate).

But I haven't managed to design a robust way of triggering the
indication that tasks should end.

This is a library-level queue and a library level task providing a
service to the whole application. So there is no object whose
finalization can be used as a trigger for the task shutdown order.
The only solution I see is to provide an explicit Shudown procedure that
sends the task shutdown signal, but it is fragile: what if the client
forgets to call it? or if an exception make control flow jump around it?
and what if the client calls it too early, and more jobs are added to
the queue?

> Obviously not Ravenscar compliant.

Well Ravenscar was only involved in my coming up with such a solution.
As soon as I'm thinking about task termination, I'm leaving Ravenscar
realm anyway.



Thanks for your help,
Natasha