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 D.14.2 Group Execution Time Budgets

{AI95-00354-01} This clause describes a language-defined package to assign execution time budgets to groups of tasks. 

Static Semantics

{AI95-00354-01} The following language-defined library package exists: 
with System;
package Ada.Execution_Time.Group_Budgets is
  type Group_Budget is tagged limited private;
  type Group_Budget_Handler is access
       protected procedure (GB : in out Group_Budget);
  type Task_Array is array (Positive range <>) of
  Min_Handler_Ceiling : constant System.Any_Priority :=
  procedure Add_Task (GB : in out Group_Budget;
                      T  : in Ada.Task_Identification.Task_Id);
  procedure Remove_Task (GB: in out Group_Budget;
                         T  : in Ada.Task_Identification.Task_Id);
  function Is_Member (GB : Group_Budget;
                      T : Ada.Task_Identification.Task_Id) return Boolean;
  function Is_A_Group_Member
     (T : Ada.Task_Identification.Task_Id) return Boolean;
  function Members (GB : Group_Budget) return Task_Array;
  procedure Replenish (GB : in out Group_Budget; To : in Time_Span);
  procedure Add (GB : in out Group_Budget; Interval : in Time_Span);
  function Budget_Has_Expired (GB : Group_Budget) return Boolean;
  function Budget_Remaining (GB : Group_Budget) return Time_Span;
  procedure Set_Handler (GB      : in out Group_Budget;
                         Handler : in Group_Budget_Handler);
  function Current_Handler (GB : Group_Budget)
     return Group_Budget_Handler;
  procedure Cancel_Handler (GB        : in out Group_Budget;
                            Cancelled : out Boolean);
  Group_Budget_Error : exception;
    --  not specified by the language
end Ada.Execution_Time.Group_Budgets;
 {AI95-00354-01} The type Group_Budget represents an execution time budget to be used by a group of tasks. The type Group_Budget needs finalization (see 7.6). A task can belong to at most one group. Tasks of any priority can be added to a group.
 {AI95-00354-01} An object of type Group_Budget has an associated nonnegative value of type Time_Span known as its budget, which is initially Time_Span_Zero. The type Group_Budget_Handler identifies a protected procedure to be executed by the implementation when the budget is exhausted, that is, reaches zero. Such a protected procedure is called a handler.{budget} {exhaust (a budget)} {handler (group budget) [partial]}
 {AI95-00354-01} An object of type Group_Budget also includes a handler, which is a value of type Group_Budget_Handler. The handler of the object is said to be set if it is not null and cleared otherwise. The handler of all Group_Budget objects is initially cleared. {set (group budget object) [partial]} {clear (group budget object) [partial]}
Discussion: Type Group_Budget is tagged. This makes it possible to share a handler between several events. In simple cases, 'Access can be used to compare the parameter with a specific group budget object (this works because a tagged type is a by-reference type). In more complex cases, a type extension of type Group_Budget can be declared; a double type conversion can be used to access the extension data. An example of how this can be done can be found for the similar type Timing_Event, see D.15.

Dynamic Semantics

 {AI95-00354-01} The procedure Add_Task adds the task identified by T to the group GB; if that task is already a member of some other group, Group_Budget_Error is raised.
 {AI95-00354-01} The procedure Remove_Task removes the task identified by T from the group GB; if that task is not a member of the group GB, Group_Budget_Error is raised. After successful execution of this procedure, the task is no longer a member of any group.
 {AI95-00354-01} The function Is_Member returns True if the task identified by T is a member of the group GB; otherwise it return False.
 {AI95-00354-01} The function Is_A_Group_Member returns True if the task identified by T is a member of some group; otherwise it returns False.
 {AI95-00354-01} The function Members returns an array of values of type Task_Identification.Task_Id identifying the members of the group GB. The order of the components of the array is unspecified.
 {AI95-00354-01} The procedure Replenish loads the group budget GB with To as the Time_Span value. The exception Group_Budget_Error is raised if the Time_Span value To is non-positive. Any execution of any member of the group of tasks results in the budget counting down, unless exhausted. When the budget becomes exhausted (reaches Time_Span_Zero), the associated handler is executed if the handler of group budget GB is set. Nevertheless, the tasks continue to execute.
 {AI95-00354-01} The procedure Add modifies the budget of the group GB. A positive value for Interval increases the budget. A negative value for Interval reduces the budget, but never below Time_Span_Zero. A zero value for Interval has no effect. A call of procedure Add that results in the value of the budget going to Time_Span_Zero causes the associated handler to be executed if the handler of the group budget GB is set.
 {AI95-00354-01} The function Budget_Has_Expired returns True if the budget of group GB is exhausted (equal to Time_Span_Zero); otherwise it returns False.
 {AI95-00354-01} The function Budget_Remaining returns the remaining budget for the group GB. If the budget is exhausted it returns Time_Span_Zero. This is the minimum value for a budget.
 {AI95-00354-01} The procedure Set_Handler associates the handler Handler with the Group_Budget GB; if Handler is null, the handler of Group_Budget is cleared, otherwise it is set.
 {AI95-00354-01} A call of Set_Handler for a Group_Budget that already has a handler set replaces the handler; if Handler is not null, the handler for Group_Budget remains set.
 {AI95-00354-01} The function Current_Handler returns the handler associated with the group budget GB if the handler for that group budget is set; otherwise it returns null.
 {AI95-00354-01} The procedure Cancel_Handler clears the handler for the group budget if it is set. Cancelled is assigned True if the handler for the group budget was set prior to it being cleared; otherwise it is assigned False.
 {AI95-00354-01} The constant Min_Handler_Ceiling is the minimum ceiling priority required for a protected object with a handler to ensure that no ceiling violation will occur when that handler is invoked.
 {AI95-00354-01} The precision of the accounting of task execution time to a Group_Budget is the same as that defined for execution-time clocks from the parent package.
 {AI95-00354-01} As part of the finalization of an object of type Group_Budget all member tasks are removed from the group identified by that object.
 {AI95-00354-01} If a task is a member of a Group_Budget when it terminates then as part of the finalization of the task it is removed from the group.
 {AI95-00354-01} For all the operations defined in this package, Tasking_Error is raised if the task identified by T has terminated, and Program_Error is raised if the value of T is Task_Identification.Null_Task_Id.
 {AI95-00354-01} An exception propagated from a handler invoked when the budget of a group of tasks becomes exhausted has no effect.

Erroneous Execution

 {AI95-00354-01} {erroneous execution (cause) [partial]} For a call of any of the subprograms defined in this package, if the task identified by T no longer exists, the execution of the program is erroneous. 

Implementation Requirements

 {AI95-00354-01} For a given Group_Budget object, the implementation shall perform the operations declared in this package atomically with respect to any of these operations on the same Group_Budget object. The replacement of a handler, by a call of Set_Handler, shall be performed atomically with respect to the execution of the handler. 
Reason: This prevents various race conditions. In particular it ensures that if the budget is exhausted when Set_Handler is changing the handler then either the new or old handler is executed and the exhausting event is not lost.
47  {AI95-00354-01} Clearing or setting of the handler of a group budget does not change the current value of the budget. Exhaustion or loading of a budget does not change whether the handler of the group budget is set or cleared.
48  {AI95-00354-01} A Group_Budget_Handler can be associated with several Group_Budget objects. 

Extensions to Ada 95

{AI95-00354-01} {extensions to Ada 95} The package Execution_Time.Group_Budgets is new. 

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