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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Tail recursion upon task destruction
Newsgroups: comp.lang.ada
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Organization: cbb software GmbH
Date: Tue, 17 Nov 2009 11:17:38 +0100
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Date: 2009-11-17T11:17:38+01:00
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Consider a task encapsulated in an object in either way:

   type Device is
      Driver : Driver_Task (Device'Access);

or

   type Device is
      Driver : not null access Driver_Task := Driver_Task (Device'Access);

Let the object is allocated dynamically and we wanted to destroy it from
the task. It seems that there is no way to do this:

   task Driver_Task (Object : not null access Device) is
       procedure Free is
          new Ada.Unchecked_Deallocation (Device, Device_Ptr)
       Self : Device_Ptr;
   begin
      
       ...

      accept Shut_Down;
      Self := Object.all'Unchecked_Access; -- Or whatever way

      Free (Self);  -- This will deadlock
   end Driver_Task;

The core problem is that a task cannot destroy itself, because that would
block for task termination, which never to happen.

What I do to solve this is an extra "collector task" to await for a
rendezvous with Driver_Tasks, accepting a pointer to the Device and then
after leaving the rendezvous, freeing it. That looks tedious.

Don't we need some kind of "tail recursion" for this destruction pattern?

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