Re: Basic question about select

["Jeffrey R. Carter" <spam.jrcarter.not@spam.acm.org>]

Dmitry A. Kazakov wrote:
> 
>    protected Event is
>       procedure Signal;
>       entry Wait;
>    private
>       Signaled : Boolean := False;
>    end Event;
> 
>    protected body Event is
>       procedure Signal is
>       begin
>          Signaled := True;
>       end Signal;
>       entry Wait when Signaled is
>       begin
>          Signaled := False;
>       end;
>    end Event;
>    
>    task PID is
>       entry Call;
>    end PID;
> 
>    task body PID is
>    begin
>       accept Call do Event.Wait; end;
>    end PID;
> 
> begin
>   select
>      PID.Call;  -- Blocked in the rendezvous
>   then abort
>      delay 2.0;
>      Event.Signal; -- Releases the rendezvous after 2s
>   end select;
> 
> If abort to happen after *completion* of the rendezvous then the above
> shall not deadlock. (I checked this under GNAT/Windows, it deadlocks
> there.)

Here the call to PID.Call is not queued, but is accepted immediately, and does 
not encounter an explicit requeue-with-abort. I guess that queuing on Event.Wait 
does not count as equivalent to requeue-with-abort (unless this is a compiler 
error; any language lawyers care to comment?). So the abortable part is never 
executed, leading to deadlock.

It will be interesting to add "delay 1.0;" to the task before the accept, and 
see if that changes the behavior. Then the call to PID.Call will queue, and the 
abortable part should execute.

-- 
Jeff Carter
"From this day on, the official language of San Marcos will be Swedish."
Bananas
28