Version 1.6 of ai05s/ai05-0202-1.txt

!standard C.7.3(8/2) 11-02-19 AI05-0202-1/03
!class binding interpretation 10-02-12
!status Amendment 2012 11-03-10
!status ARG Approved 7-0-1 10-02-20
!status work item 10-02-12
!status received 09-09-24
!priority Medium
!difficulty Easy
!qualifier Omission
!subject Task_Termination and Exceptions raised during finalization

Suppose a task has a termination handler set up by either Set_Specific_Handler or Set_Dependents_Fallback_Handler. Suppose the task executes the last statement of the task body normally, but an exception occurs during finalization (as part of the "actions specified in 7.6 for finalization of a master"). For instance, the task may have a local variable with a controlled part, and a user-defined Finalize routine propagates an exception.

What is passed as the Cause parameter to the termination handler? Normal, or Unhandled_Exception? I don't think the wording of C.7.3(8/2) gives a clear answer; it says Cause=>Normal if the task is terminating "due to completing the last statement of its body", and that is the reason the task is terminating, it's just that some other unhandled exception came up afterwards.

In a similar vein, suppose an exception Exc1 is propagated out of the task body, and in the course of finalizing local objects, another exception Exc2 is propagated (by a user-defined Finalize). Now Cause=>Unhandled_Exception is passed to the termination handler, but which exception does the Exception_Occurrence X refer to? Would Exception_Identity(X) be the identity of Exc1, Exc2, or Program_Error?

The type Termination_Handler identifies a protected procedure to be executed by the implementation when a task terminates. Such a protected procedure is called a handler. In all cases T identifies the task that is terminating. If the task terminates due to completing the last statement of its body, or as a result of waiting on a terminate alternative, {and the finalization of the task completes normally,} then Cause is set to Normal and X is set to Null_Occurrence. If the task terminates because it is being aborted, then Cause is set to Abnormal [and] {;} X is set to Null_Occurrence {if the finalization of the task completes normally}. If the task terminates because of an exception raised by the execution of its task_body, then Cause is set to Unhandled_Exception [and] {;} X is set to the associated exception occurrence {if the finalization of the task completes normally}. {Independent of how the task completes, if finalization of the task propagates an exception, then Cause is either Unhandled_Exception or Abnormal, and X is an exception occurrence that identifies the Program_Error exception.}

The only exception that can be propagated by a failed Finalize routine is Program_Error. Finalization of a protected object itself cannot propagate an exception (it will propagate Program_Error to any queued callers). Waiting for tasks cannot propagate an exception (but we might wait forever). Nothing else happens during a finalization.

To avoid reporting Normal in the case of failed finalization, we specify that failed finalization results in Unhandled_Exception and an occurrence that identifies Program_Error. We don't specify anything else about the occurrence, as there is no desire to overspecify the implementation and perhaps thereby introduce undesirable distributed overhead on behalf of this very unusual circumstance.

The type Termination_Handler identifies a protected procedure to be executed by the implementation when a task terminates. Such a protected procedure is called a handler. In all cases T identifies the task that is terminating. If the task terminates due to completing the last statement of its body, or as a result of waiting on a terminate alternative, and the finalization of the task completes normally, then Cause is set to Normal and X is set to Null_Occurrence. If the task terminates because it is being aborted, then Cause is set to Abnormal; X is set to Null_Occurrence if the finalization of the task completes normally. If the task terminates because of an exception raised by the execution of its task_body, then Cause is set to Unhandled_Exception; X is set to the associated exception occurrence if the finalization of the task completes normally. Independent of how the task completes, if finalization of the task propagates an exception, then Cause is either Unhandled_Exception or Abnormal, and X is an exception occurrence that identifies the Program_Error exception.