Re: task synchronization and activation

[Jean-Pierre Rosen <rosen@adalog.fr>]

Robert A Duff a �crit :
[...]
> I can believe that that was the reasoning of the Ada 83 designers.
> But I still don't quite agree with it.
> 
> For one thing, if you want to handle exceptions in the decl part,
> just change this:
> 
>     task body T is
>         ... -- possible exception here?
>     begin
>         ...
>     end T;
> 
> to this:
> 
>     task body T is
>     begin
>         declare
>             ... -- possible exception here?
>         begin
>             ...
>         end;
>     exception
>         ...
>     end T;
> 
> and now the task can handle it (because it's no longer in the task's
> declarative part).  So it's a case of "Doctor, it hurts when I...."
> "So don't do that."  ;-)

It is not the same thing *at all*. Here, you are putting the 
responsibility on the task's side, but the issue is to be safe from the 
caller's point of view.

Let's look at it differently. A program unit needs some subtasks to do 
its job. If the subtasks cannot be started for any reason (including 
storage_error), then the program unit cannot work, and it'd better be 
noticed of that; most likely, the program unit will raise an exception 
or find a fall-back strategy. Since asynchronous exceptions are horrible 
(Java did that mistake, and had to back-up), this must happen 
synchronously, and the logical point is just before executing any statement.

Now, you can argue that the subtasks may fail at any later time; that's 
true, and Tasking_Error may be raised for any interaction. However, 
failing activation is basically not having the resources to start the 
servers, and is a much more severe condition.


-- 
---------------------------------------------------------
            J-P. Rosen (rosen@adalog.fr)
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