Re: Asynchronous channels in Ada

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 21/02/2016 23:57, Hadrien Grasland wrote:

> Now, something goes wrong on the producer side, for any reason, which
> prevents this task from correctly producing its next data packet.

That cannot happen. You should reconsider your design if you really 
think it could. When the communication channel is up the producer must 
function.

I do not consider the case of a bug in the producer code. But anything 
else is just a valid state. That includes all handled exceptions on the 
producer side.

And any valid producer state must have a corresponding valid state on 
the subscriber's side. This is just basics of any communication protocol 
design.

> Transmit a specially crafted data packet representing an error (i.e. an exception).

This is not an "error" it is a valid state.

There is no problem to marshal an exception to the subscriber side. You 
make the data object like this:

type Data (Abnormal : Boolean := False) is record
    case Abnormal is
       when True =>
          Reason : Ada.Exceptions.Exception_ID;
       when False =>
          ... -- Normal data
    end case;
end record;

Remove default value for Abnormal if your FIFO can handle indefinite 
objects. However, assuming that abnormal data objects are not frequently 
to transmit, making them indefinite might be not necessary.

Regarding bugs, the safest way to handle a bug is to kill the 
application as early as possible. It makes debugging easier.

> I wonder if I could find a way to build such a communication channel
> and reuse existing FIFO implementations without incurring twice the
> synchronization overhead. Most likely I should look into unsynchronized
> FIFO implementations then.

I think any implementation would go.

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