Ada: behavior like pthread_barrier_wait?

Question

I'm trying to implement a barrier in Ada which has similar functionality to C's pthread_barrier_wait. Ada 2012 has Ada.Synchronous_Barriers but that's not available on my system (gnu-gnat on debian lenny).

More specifically, how can I get all waiting tasks to be released from a barrier at ~ the same time and, ideally, have one of these tasks do something special, without using Ada 2012? Below is a very sub-optimal implementation. What might be a better approach?

with Ada.Text_IO; use Ada.Text_IO;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;

procedure foobar is
   protected Synchronizer is
      entry Ready_For_Action; -- prepares for tasks to wait at barrier
      entry Wait_For_Release; -- barrier
      -- do work here
      entry Done;             -- signals that all tasks are done
      entry Wait_For_Others;  -- prepares for prepare to wait at barrier
   private
      ready, active: Natural := 0;  
      -- two state variables seem to be needed as entry conditions can't
      --    safely modify the condition variable as that influences wait
      --    state in other tasks
   end Synchronizer;

   NUM_OBJECTS: constant := 3;

   protected body Synchronizer is
      entry Ready_For_Action when active = 0 is
      begin
         ready := ready + 1;
      end Ready_For_Action;
      --
      entry Wait_For_Release when ready = NUM_OBJECTS is
      begin
         active := active + 1;
      end Wait_For_Release;
      --
      entry Done when active = NUM_OBJECTS is
      begin
         ready := ready - 1;
      end Done;
      --
      entry Wait_For_Others when ready = 0 is
      begin
         active := active - 1;
      end wait_for_others;
      --
   end Synchronizer;

   task type Foo(N: Natural);

   task body Foo is
      id: Natural := N;
   begin
      for iter in 1..3 loop
         Synchronizer.Ready_For_Action;
         Synchronizer.Wait_For_Release;
         -- task N doing something special
         if id = 1 then new_line; end if;
         -- do stuff here
         delay 0.1;
         put(id); new_line;
         -- re-sync
         Synchronizer.Done;
         Synchronizer.Wait_For_Others;
      end loop;
   end Foo;
   Task1: Foo(1);
   Task2: Foo(2);
   Task3: Foo(3);
begin
   Null;
end foobar;

Program output:

$ ./foobar 
  3
  1
  2

  3
  1
  2

  3
  2
  1

Answer 1

Maybe the 'count attribute on entries would be useful - is this the kind of thing you're looking for? Using task IDs to make one do something different seems sensible (or if it is sufficiently different you could just make a new task type).

No_Of_Tasks : Natural := 3;
   --
protected Barrier is
   entry Continue;
private
   Released : Boolean := False;
end Barrier
   --
protected body Barrier is 
   entry Continue when (Released or else Continue'count = No_Of_Tasks)
      Released := Continue'count > 0; -- the last task locks the barrier again
   end Continue                       
end Barrier                           

Answer 2

This extends Leon's answer to achieve the desired functionality. It uses a single barrier object and flags one arbitrary task to do something special.

EDIT: Incorporated Jacob's insight to further simplify barrier and achieve original goal

with Ada.Text_IO; use Ada.Text_IO;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;

procedure bar2 is
   NUM_TASKS: constant := 3;

   protected Barrier is
      entry Wait_For_Release(the_chosen_one: out Boolean);
   private
      released: Boolean := False;
   end Barrier;

   protected body Barrier is
      entry Wait_For_Release(the_chosen_one: out Boolean) 
         when (Released or else Wait_For_Release'count = NUM_TASKS) is
      begin
         the_chosen_one := False;
         if Wait_For_Release'count = NUM_TASKS-1 then
            the_chosen_one := True;
            released := True;
         elsif Wait_For_Release'count = 0 then
            released := False;
         end if;
      end Wait_For_Release;
   end Barrier;

   task type Foo(N: Natural);
   task body Foo is
      id: Natural := N;
      the_chosen_one: Boolean;
   begin
      for iter in 1..5 loop
         Barrier.Wait_For_Release(the_chosen_one);
         if the_chosen_one then
            new_line; 
         end if;
         put(id);     -- do stuff here
      end loop;
   end Foo;

   Task1: Foo(1);
   Task2: Foo(2);
   Task3: Foo(3);
begin
   Null;
end bar2;

Sample output:

$ ./bar 

          1          2          3
          3          1          2
          1          2          3
          1          3          2
          3          2          1