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------------------------------------------------------------------------------
--                                                                          --
--                  GNAT RUN-TIME LIBRARY (GNARL) COMPONENTS                --
--                                                                          --
--                 S Y S T E M . T A S K I N G . Q U E U I N G              --
--                                                                          --
--                                  B o d y                                 --
--                                                                          --
--         Copyright (C) 1992-2009, Free Software Foundation, Inc.          --
--                                                                          --
-- GNARL is free software; you can  redistribute it  and/or modify it under --
-- terms of the  GNU General Public License as published  by the Free Soft- --
-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
--                                                                          --
-- As a special exception under Section 7 of GPL version 3, you are granted --
-- additional permissions described in the GCC Runtime Library Exception,   --
-- version 3.1, as published by the Free Software Foundation.               --
--                                                                          --
-- You should have received a copy of the GNU General Public License and    --
-- a copy of the GCC Runtime Library Exception along with this program;     --
-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
-- <http://www.gnu.org/licenses/>.                                          --
--                                                                          --
-- GNARL was developed by the GNARL team at Florida State University.       --
-- Extensive contributions were provided by Ada Core Technologies, Inc.     --
--                                                                          --
------------------------------------------------------------------------------
 
--  This version of the body implements queueing policy according to the policy
--  specified by the pragma Queuing_Policy. When no such pragma is specified
--  FIFO policy is used as default.
 
with System.Task_Primitives.Operations;
with System.Tasking.Initialization;
with System.Parameters;
 
package body System.Tasking.Queuing is
 
   use Parameters;
   use Task_Primitives.Operations;
   use Protected_Objects;
   use Protected_Objects.Entries;
 
   --  Entry Queues implemented as doubly linked list
 
   Queuing_Policy : Character;
   pragma Import (C, Queuing_Policy, "__gl_queuing_policy");
 
   Priority_Queuing : constant Boolean := Queuing_Policy = 'P';
 
   procedure Send_Program_Error
     (Self_ID    : Task_Id;
      Entry_Call : Entry_Call_Link);
   --  Raise Program_Error in the caller of the specified entry call
 
   function Check_Queue (E : Entry_Queue) return Boolean;
   --  Check the validity of E.
   --  Return True if E is valid, raise Assert_Failure if assertions are
   --  enabled and False otherwise.
 
   -----------------------------
   -- Broadcast_Program_Error --
   -----------------------------
 
   procedure Broadcast_Program_Error
     (Self_ID      : Task_Id;
      Object       : Protection_Entries_Access;
      Pending_Call : Entry_Call_Link;
      RTS_Locked   : Boolean := False)
   is
      Entry_Call : Entry_Call_Link;
   begin
      if Single_Lock and then not RTS_Locked then
         Lock_RTS;
      end if;
 
      if Pending_Call /= null then
         Send_Program_Error (Self_ID, Pending_Call);
      end if;
 
      for E in Object.Entry_Queues'Range loop
         Dequeue_Head (Object.Entry_Queues (E), Entry_Call);
 
         while Entry_Call /= null loop
            pragma Assert (Entry_Call.Mode /= Conditional_Call);
 
            Send_Program_Error (Self_ID, Entry_Call);
            Dequeue_Head (Object.Entry_Queues (E), Entry_Call);
         end loop;
      end loop;
 
      if Single_Lock and then not RTS_Locked then
         Unlock_RTS;
      end if;
   end Broadcast_Program_Error;
 
   -----------------
   -- Check_Queue --
   -----------------
 
   function Check_Queue (E : Entry_Queue) return Boolean is
      Valid   : Boolean := True;
      C, Prev : Entry_Call_Link;
 
   begin
      if E.Head = null then
         if E.Tail /= null then
            Valid := False;
            pragma Assert (Valid);
         end if;
      else
         if E.Tail = null
           or else E.Tail.Next /= E.Head
         then
            Valid := False;
            pragma Assert (Valid);
 
         else
            C := E.Head;
 
            loop
               Prev := C;
               C := C.Next;
 
               if C = null then
                  Valid := False;
                  pragma Assert (Valid);
                  exit;
               end if;
 
               if Prev /= C.Prev then
                  Valid := False;
                  pragma Assert (Valid);
                  exit;
               end if;
 
               exit when C = E.Head;
            end loop;
 
            if Prev /= E.Tail then
               Valid := False;
               pragma Assert (Valid);
            end if;
         end if;
      end if;
 
      return Valid;
   end Check_Queue;
 
   -------------------
   -- Count_Waiting --
   -------------------
 
   --  Return number of calls on the waiting queue of E
 
   function Count_Waiting (E : Entry_Queue) return Natural is
      Count   : Natural;
      Temp    : Entry_Call_Link;
 
   begin
      pragma Assert (Check_Queue (E));
 
      Count := 0;
 
      if E.Head /= null then
         Temp := E.Head;
 
         loop
            Count := Count + 1;
            exit when E.Tail = Temp;
            Temp := Temp.Next;
         end loop;
      end if;
 
      return Count;
   end Count_Waiting;
 
   -------------
   -- Dequeue --
   -------------
 
   --  Dequeue call from entry_queue E
 
   procedure Dequeue (E : in out Entry_Queue; Call : Entry_Call_Link) is
   begin
      pragma Assert (Check_Queue (E));
      pragma Assert (Call /= null);
 
      --  If empty queue, simply return
 
      if E.Head = null then
         return;
      end if;
 
      pragma Assert (Call.Prev /= null);
      pragma Assert (Call.Next /= null);
 
      Call.Prev.Next := Call.Next;
      Call.Next.Prev := Call.Prev;
 
      if E.Head = Call then
 
         --  Case of one element
 
         if E.Tail = Call then
            E.Head := null;
            E.Tail := null;
 
         --  More than one element
 
         else
            E.Head := Call.Next;
         end if;
 
      elsif E.Tail = Call then
         E.Tail := Call.Prev;
      end if;
 
      --  Successfully dequeued
 
      Call.Prev := null;
      Call.Next := null;
      pragma Assert (Check_Queue (E));
   end Dequeue;
 
   ------------------
   -- Dequeue_Call --
   ------------------
 
   procedure Dequeue_Call (Entry_Call : Entry_Call_Link) is
      Called_PO : Protection_Entries_Access;
 
   begin
      pragma Assert (Entry_Call /= null);
 
      if Entry_Call.Called_Task /= null then
         Dequeue
           (Entry_Call.Called_Task.Entry_Queues
             (Task_Entry_Index (Entry_Call.E)),
           Entry_Call);
 
      else
         Called_PO := To_Protection (Entry_Call.Called_PO);
         Dequeue (Called_PO.Entry_Queues
             (Protected_Entry_Index (Entry_Call.E)),
           Entry_Call);
      end if;
   end Dequeue_Call;
 
   ------------------
   -- Dequeue_Head --
   ------------------
 
   --  Remove and return the head of entry_queue E
 
   procedure Dequeue_Head
     (E    : in out Entry_Queue;
      Call : out Entry_Call_Link)
   is
      Temp : Entry_Call_Link;
 
   begin
      pragma Assert (Check_Queue (E));
      --  If empty queue, return null pointer
 
      if E.Head = null then
         Call := null;
         return;
      end if;
 
      Temp := E.Head;
 
      --  Case of one element
 
      if E.Head = E.Tail then
         E.Head := null;
         E.Tail := null;
 
      --  More than one element
 
      else
         pragma Assert (Temp /= null);
         pragma Assert (Temp.Next /= null);
         pragma Assert (Temp.Prev /= null);
 
         E.Head := Temp.Next;
         Temp.Prev.Next := Temp.Next;
         Temp.Next.Prev := Temp.Prev;
      end if;
 
      --  Successfully dequeued
 
      Temp.Prev := null;
      Temp.Next := null;
      Call := Temp;
      pragma Assert (Check_Queue (E));
   end Dequeue_Head;
 
   -------------
   -- Enqueue --
   -------------
 
   --  Enqueue call at the end of entry_queue E, for FIFO queuing policy.
   --  Enqueue call priority ordered, FIFO at same priority level, for
   --  Priority queuing policy.
 
   procedure Enqueue (E : in out Entry_Queue; Call : Entry_Call_Link) is
      Temp : Entry_Call_Link := E.Head;
 
   begin
      pragma Assert (Check_Queue (E));
      pragma Assert (Call /= null);
 
      --  Priority Queuing
 
      if Priority_Queuing then
         if Temp = null then
            Call.Prev := Call;
            Call.Next := Call;
            E.Head := Call;
            E.Tail := Call;
 
         else
            loop
               --  Find the entry that the new guy should precede
 
               exit when Call.Prio > Temp.Prio;
               Temp := Temp.Next;
 
               if Temp = E.Head then
                  Temp := null;
                  exit;
               end if;
            end loop;
 
            if Temp = null then
               --  Insert at tail
 
               Call.Prev := E.Tail;
               Call.Next := E.Head;
               E.Tail := Call;
 
            else
               Call.Prev := Temp.Prev;
               Call.Next := Temp;
 
               --  Insert at head
 
               if Temp = E.Head then
                  E.Head := Call;
               end if;
            end if;
 
            pragma Assert (Call.Prev /= null);
            pragma Assert (Call.Next /= null);
 
            Call.Prev.Next := Call;
            Call.Next.Prev := Call;
         end if;
 
         pragma Assert (Check_Queue (E));
         return;
      end if;
 
      --  FIFO Queuing
 
      if E.Head = null then
         E.Head := Call;
      else
         E.Tail.Next := Call;
         Call.Prev   := E.Tail;
      end if;
 
      E.Head.Prev := Call;
      E.Tail      := Call;
      Call.Next   := E.Head;
      pragma Assert (Check_Queue (E));
   end Enqueue;
 
   ------------------
   -- Enqueue_Call --
   ------------------
 
   procedure Enqueue_Call (Entry_Call : Entry_Call_Link) is
      Called_PO : Protection_Entries_Access;
 
   begin
      pragma Assert (Entry_Call /= null);
 
      if Entry_Call.Called_Task /= null then
         Enqueue
           (Entry_Call.Called_Task.Entry_Queues
              (Task_Entry_Index (Entry_Call.E)),
           Entry_Call);
 
      else
         Called_PO := To_Protection (Entry_Call.Called_PO);
         Enqueue (Called_PO.Entry_Queues
             (Protected_Entry_Index (Entry_Call.E)),
           Entry_Call);
      end if;
   end Enqueue_Call;
 
   ----------
   -- Head --
   ----------
 
   --  Return the head of entry_queue E
 
   function Head (E : Entry_Queue) return Entry_Call_Link is
   begin
      pragma Assert (Check_Queue (E));
      return E.Head;
   end Head;
 
   -------------
   -- Onqueue --
   -------------
 
   --  Return True if Call is on any entry_queue at all
 
   function Onqueue (Call : Entry_Call_Link) return Boolean is
   begin
      pragma Assert (Call /= null);
 
      --  Utilize the fact that every queue is circular, so if Call
      --  is on any queue at all, Call.Next must NOT be null.
 
      return Call.Next /= null;
   end Onqueue;
 
   --------------------------------
   -- Requeue_Call_With_New_Prio --
   --------------------------------
 
   procedure Requeue_Call_With_New_Prio
     (Entry_Call : Entry_Call_Link; Prio : System.Any_Priority) is
   begin
      pragma Assert (Entry_Call /= null);
 
      --  Perform a queue reordering only when the policy being used is the
      --  Priority Queuing.
 
      if Priority_Queuing then
         if Onqueue (Entry_Call) then
            Dequeue_Call (Entry_Call);
            Entry_Call.Prio := Prio;
            Enqueue_Call (Entry_Call);
         end if;
      end if;
   end Requeue_Call_With_New_Prio;
 
   ---------------------------------
   -- Select_Protected_Entry_Call --
   ---------------------------------
 
   --  Select an entry of a protected object. Selection depends on the
   --  queuing policy being used.
 
   procedure Select_Protected_Entry_Call
     (Self_ID : Task_Id;
      Object  : Protection_Entries_Access;
      Call    : out Entry_Call_Link)
   is
      Entry_Call  : Entry_Call_Link;
      Temp_Call   : Entry_Call_Link;
      Entry_Index : Protected_Entry_Index := Null_Entry; -- stop warning
 
   begin
      Entry_Call := null;
 
      begin
         --  Priority queuing case
 
         if Priority_Queuing then
            for J in Object.Entry_Queues'Range loop
               Temp_Call := Head (Object.Entry_Queues (J));
 
               if Temp_Call /= null
                 and then
                   Object.Entry_Bodies
                     (Object.Find_Body_Index
                       (Object.Compiler_Info, J)).
                          Barrier (Object.Compiler_Info, J)
               then
                  if Entry_Call = null
                    or else Entry_Call.Prio < Temp_Call.Prio
                  then
                     Entry_Call := Temp_Call;
                     Entry_Index := J;
                  end if;
               end if;
            end loop;
 
         --  FIFO queueing case
 
         else
            for J in Object.Entry_Queues'Range loop
               Temp_Call := Head (Object.Entry_Queues (J));
 
               if Temp_Call /= null
                 and then
                   Object.Entry_Bodies
                     (Object.Find_Body_Index
                       (Object.Compiler_Info, J)).
                          Barrier (Object.Compiler_Info, J)
               then
                  Entry_Call := Temp_Call;
                  Entry_Index := J;
                  exit;
               end if;
            end loop;
         end if;
 
      exception
         when others =>
            Broadcast_Program_Error (Self_ID, Object, null);
      end;
 
      --  If a call was selected, dequeue it and return it for service
 
      if Entry_Call /= null then
         Temp_Call := Entry_Call;
         Dequeue_Head (Object.Entry_Queues (Entry_Index), Entry_Call);
         pragma Assert (Temp_Call = Entry_Call);
      end if;
 
      Call := Entry_Call;
   end Select_Protected_Entry_Call;
 
   ----------------------------
   -- Select_Task_Entry_Call --
   ----------------------------
 
   --  Select an entry for rendezvous. Selection depends on the queuing policy
   --  being used.
 
   procedure Select_Task_Entry_Call
     (Acceptor         : Task_Id;
      Open_Accepts     : Accept_List_Access;
      Call             : out Entry_Call_Link;
      Selection        : out Select_Index;
      Open_Alternative : out Boolean)
   is
      Entry_Call  : Entry_Call_Link;
      Temp_Call   : Entry_Call_Link;
      Entry_Index : Task_Entry_Index := Task_Entry_Index'First;
      Temp_Entry  : Task_Entry_Index;
 
   begin
      Open_Alternative := False;
      Entry_Call       := null;
      Selection        := No_Rendezvous;
 
      if Priority_Queuing then
         --  Priority queueing case
 
         for J in Open_Accepts'Range loop
            Temp_Entry := Open_Accepts (J).S;
 
            if Temp_Entry /= Null_Task_Entry then
               Open_Alternative := True;
               Temp_Call := Head (Acceptor.Entry_Queues (Temp_Entry));
 
               if Temp_Call /= null
                 and then (Entry_Call = null
                   or else Entry_Call.Prio < Temp_Call.Prio)
               then
                  Entry_Call  := Head (Acceptor.Entry_Queues (Temp_Entry));
                  Entry_Index := Temp_Entry;
                  Selection := J;
               end if;
            end if;
         end loop;
 
      else
         --  FIFO Queuing case
 
         for J in Open_Accepts'Range loop
            Temp_Entry := Open_Accepts (J).S;
 
            if Temp_Entry /= Null_Task_Entry then
               Open_Alternative := True;
               Temp_Call := Head (Acceptor.Entry_Queues (Temp_Entry));
 
               if Temp_Call /= null then
                  Entry_Call := Head (Acceptor.Entry_Queues (Temp_Entry));
                  Entry_Index := Temp_Entry;
                  Selection := J;
                  exit;
               end if;
            end if;
         end loop;
      end if;
 
      if Entry_Call /= null then
         Dequeue_Head (Acceptor.Entry_Queues (Entry_Index), Entry_Call);
 
         --  Guard is open
      end if;
 
      Call := Entry_Call;
   end Select_Task_Entry_Call;
 
   ------------------------
   -- Send_Program_Error --
   ------------------------
 
   procedure Send_Program_Error
     (Self_ID    : Task_Id;
      Entry_Call : Entry_Call_Link)
   is
      Caller : Task_Id;
   begin
      Caller := Entry_Call.Self;
      Entry_Call.Exception_To_Raise := Program_Error'Identity;
      Write_Lock (Caller);
      Initialization.Wakeup_Entry_Caller (Self_ID, Entry_Call, Done);
      Unlock (Caller);
   end Send_Program_Error;
 
end System.Tasking.Queuing;