Safe, Dynamic Task Creation in Ada 09 Mar 2013
ada programming

A few years ago, [Ada]( became my hobby/tinker programming language of choice, for a number of reasons, concurrency being one of them. In this post I’d like to walk you through an example of dynamic task creation in Ada, which uses Ada.Task_Termination handlers, a new feature in Ada 2005.

(If you’re familiar with Ada, you can skip this next section)

Note: You can find all this code, and more in my ada-playground repository on GitHub

Similar to C, Ada supports stack allocated variables as well as heap allocated variabels, it also defaults to stack allocation. For example:

    procedure Main is
        -- A stack allocated `Integer` object
        Enough_Memory : constant Integer := 655360;
    end Main;

If you wanted to allocate that Integer onto the heap, then you would use the new keyword:

    procedure Main is
        -- A heap allocated `Integer` pointer
        Enough_Memory : access Integer := new Integer'(655360);
    end Main;

I won’t dive too much into the minutia of what is going on here, if you’re not familiar with Ada already you can learn more about access types on the Ada Programming Wikibook. Basically we’re heap allocating a new Integer and using an access type (aka: typed pointer) to keep track of it. Keen readers will notice we didn’t do anything with that Integer access type, and we’re technically leaking the memory. To solve this we use the generic unit Ada.Unchecked_Deallocation, which gives you a facility for properly freeing memory (more details here).

Tasking Trickiness

Concurrency is part of the language in Ada, and is handled through tasking. A basic example of might be:

    with Ada.Text_IO;
    procedure Main is
        task Counter;
        task body Counter is
            for Count in 1 .. 10 loop
                Ada.Text_IO.Put_Line (Count'Img);
            end loop;
        end Counter;
    end Main;

The way tasks in Ada work means that the Counter task will be created, started and then the execution of the Main program will block until the Counter task completes (important detail).

The trickiness starts to arrive when you talk about dynamically allocating task objects, and combine that with something like an infinite loop, such as one might find in a server program, e.g.:

    procedure Main is
        -- Socket set up omitted
                Client_Socket : Socket_Type;
                Request_Handler : Handler_Ptr := new Handler;
                -- Block until we receive a new inbound connection
                Accept_Socket (Server_Socket, Client_Socket, Server_Addr);
                -- Dereference the Handler_Ptr and call `Process` on the Handler
                -- task
                Request_Handler.all.Process (Client_Socket);
        end loop;
    end Main;

(The code above is an abbreviated version of echomultitask_main.adb which can be found here).

The issue with this code is that we’re allocating a new Handler task for every in-bound connection, and we have no means of ever cleaning them up properly. If we were to create an Array of Handler_Ptr, we still would have to find some mechanism (which exists) to check the status of each Handler to determine if we should clean it up. Problem being, we’d have to loop through all the active tasks, checking for a “terminated” status, in order to deallocate them. It’d be much better if a task could tell us when it’s finished, rather than us polling every one.

Fortunately in Ada 2005, a mechanism was added to make it easier to add “clean-up” to tasks: Ada.Task_Termination. The package allows you to set up a termination handler for the a specific task, which the runtime will call when that task terminates. Unfortunately however, the handler procedure that can be invoked when the task terminates will not be passed a pointer to the task itself, but rather the Task_Id (Ada.Task_Identification.Task_Id).

So close to being able to properly deallocate these dynamic tasks, but we need one more component, a protected object with a hash map inside of it:

    package Server.Handlers is
        protected Coordinator is
            procedure Track (Ptr : in Handler_Ptr);
            Active_Tasks : Handler_Containers.Map;
        end Coordinator;
    end Server.Handlers;

Then back in main.adb:

    Accept_Socket (Server_Socket, Client_Socket, Server_Addr);
    Request_Handler.all.Process (Client_Socket);
    -- Make sure the we keep track of the Request_Handler in order to properly
    -- deallocate it later
    Server.Handlers.Coordinator.Track (Request_Handler);

(The code above is an abbreviated version of which can be found here)

The singleton protected object Coordinator not only will give us protected (aka thread safe) access to the Active_Tasks map, but also gives us a protected object to hang our Ada.Task_Termination.Termination_Handler protected procedure off of:

    protected body Server.Handlers is
        protected body Coordinator is
            procedure Last_Wish (C : Ada.Task_Termination.Cause_Of_Termination;
                                T : Ada.Task_Identification.Task_Id;
                                X : Ada.Exceptions.Exception_Occurrence) is
                -- Deallocate our task identified by `T`
                -- and make sure we remove it from `Active_Tasks`
            end Last_Wish;
            procedure Track (Ptr : in Handler_Ptr) is
                -- Dereference our `Handler` task, and fish out its Task_Id
                Handler_Id : Ada.Task_Idenfitication.Task_Id := Ptr.all'Identity
                -- Add `Handler_Id` to our `Active_Tasks` map
                Active_Tasks.Insert (Handler_Id, Ptr);
                -- Set up the Last_Wish procedure to be executed after our task has
                -- terminated
                Ada.Task_Termination.Set_Specific_Handler (Handler_Id, Last_Wish'Access);
            end Track;
        end Coordinator;
    end Server.Handlers;

(The code above is an abbreviated version of echomultitask-worker.adb which can be found here)

This approach will allow us to safely create new dynamic tasks to handle the incoming requests, but will also make sure that the tasks are cleanly deallocated when they terminate.

If you’re interested in concurrency in Ada, I highly recommend purchasing Concurrent and Real-Time Programming in Ada by Alan Burns and Andy Wellings, it’s been tremendously helpful for my own concurrency exploration in Ada.