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Date: Tue, 23 Aug 2016 10:15:44 -0700 (PDT)
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Subject: How do I get an enctry in a protected object to block until a certain
 item arrives from a producer task?
From: john@peppermind.com
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Xref: news.eternal-september.org comp.lang.ada:31519
Date: 2016-08-23T10:15:44-07:00
List-Id: <comp.lang.ada>

Hi! I've also asked this on Stackoverflow because I'm really stuck with thi=
s. I have a protected object that contains an ordered hash map. It stored e=
vents by their id as they are provided by some producer task. Now one or mo=
re consumer tasks can obtain these events, but I need a call that blocks th=
e consumer until a given id has arrived while the producer continues produc=
ing. So a simple barrier doesn't work.

I can't find a directly applicable design pattern in my Ada books for this,=
 although I imagine this is some kind of FAQ.

Here is the relevant code snippet:

   package Reply_Storage is new Ada.Containers.Indefinite_Ordered_Maps
     (Key_Type     =3D> Command_Id_Type,
      Element_Type =3D> Event_Type);

   protected type Reply_Queue is
      procedure Put (Event : Event_Type);
      entry Take (Id : Command_Id_Type; Event : out Event_Type);
   private
      Storage : Reply_Storage.Map;
   end Reply_Queue;

   protected body Reply_Queue is
      procedure Put (Event : Event_Type) is
         Id : Command_Id_Type :=3D Event_Command_Id (Event);
      begin
         Storage.Insert (Id, Event);
      end Put;
      entry Take (Id : Command_Id_Type; Event : out Event_Type)
       when not Storage.Is_Empty is
      begin
         if Storage.Contains(Id) then
           Event :=3D Storage.Element (Id);
            Storage.Delete (Id);
         end if;
      end Take;
   end Reply_Queue;

Basically, instead of not Storage.Is_Empty, I'd not the condition Storage.C=
ontains(Id). The consumer should block until the map has received the event=
 with the given id. But I don't know how. :/