D.14.2 Group Execution Time Budgets

     This clause describes a language-defined package to assign execution time budgets to groups of tasks.

     The following language-defined library exists:

      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.

      An object of type Group_Budget has an associated non-negative 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.

      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.

      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.

      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.

      The function Is_Member returns True if the task identified by T is a member of the group GB; otherwise it return False.

      The function Is_A_Group_Member returns True if the task identified by T is a member of some group; otherwise it returns False.

      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.

      The procedure Replenish loads the group budget GB with the Time_Span value To. 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. When the budget becomes exhausted (reaches Time_Span_Zero), the associated handler is executed if the handler of group budget GB is set; the tasks continue to execute.

      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.

      The function Budget_Has_Expired returns True if the budget of group GB is exhausted (equal to Time_Span_Zero); otherwise it returns False.

      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.

      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.

      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.

      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.

      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.

      The constant Min_Handler_Ceiling is the priority value that ensures that no ceiling violation would occur, were a handler to be executed.

      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.

      As part of the finalization of an object of type Group_Budget all member tasks are removed from the group identified by that object.

      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.

      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.

      An exception propagated from a handler invoked when the budget of a group of tasks becomes exhausted has no effect.

      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.

      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.