Re: Ada Protected Object Tutorial #1

["Robert I. Eachus" <eachus@mitre.org>]

Kaz Kylheku wrote:
  
> What do you do if you need a protected procedure to call out into
> some other module which then calls back?
> 
> Using explicit locks, I just unlock, do the call, lock again
> thereby avoiding deadlock.

    Although I think that the other issues have been explained
throughly, there are a couple of special things about using protected
objects in this situation.
First note that we are discussing protected OBJECTs.  A protected object
can and will have state variables which are protected by the object. 
But, since a protected object can and will run in multiple threads, per
thread locking is no help.

    However, if you want to do allow potentially recursive calls on a
per thread basis, it is not that hard to do, and surprisingly the
overhead is trivial.  Have the calling task pass in its
identity--Ada.Task_ID.Current_Task--and save it when locking.  Of course
the easy implementation is to use a value of Null_Task_ID to mean
unlocked.

    But there is a much more interesting case.   Assume you have an
array of state variable sets, including Task_IDs.   For example, you are
interfacing to a (local) database on behalf of many client tasks.  Each
task can have a transaction in process, with the protected object
serializing interaction with the data, while the calling tasks can
interleave other (potentially blocking) actions into the transaction.

-- 

                                        Robert I. Eachus

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