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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Finalization of library level tasks
Date: Wed, 18 Apr 2018 11:58:52 +0200
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Xref: reader02.eternal-september.org comp.lang.ada:51580
Date: 2018-04-18T11:58:52+02:00
List-Id: <comp.lang.ada>

On 18/04/2018 10:52, Egil H H wrote:
> Undefined?
> 
> 9.9(2), "a task is callable unless it is completed or abnormal"
> 9.3(5), "A task is said to be completed when the execution of its corresponding task_body is completed"

OK, it is defined enough to confirm that T'Callable cannot be used to 
break finalization deadlock. If not at the library level then only way 
seems to be a helper task at the same level. After second thought no 
shared variable and a controlled object is needed in the helper task. 
The required construct:

    select
       terminate; -- Terminate if asked
    else
       null;
    end select;

can be emulated using Callable on the helper task:

    task Helper is
       entry Never;
    end Helper;

    task body Helper is
    begin
       select
          accept Never;
       or terminate;
       end select;
    end Helper;

    task body Worker_Task is
    begin
       while Helper'Callable loop
          ... -- Do something
       end loop;
    end Worker_Task;

When Helper and Worker_Task have same master then Worker_Task can break 
finalization deadlock by looking at the Helper.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de