guideline

Do not create nonterminating tasks unintentionally.

Explicitly shut down tasks that depend on library packages.

Confirm that a task is terminated before freeing it with Ada.Unchecked_Deallocation.

Consider using a select statement with a terminate alternative rather than an accept statement alone.

Consider providing a terminate alternative for every selective accept that does not require an else part or a delay .

Do not declare or create a task within a user-defined Finalize procedure after the environment task has finished waiting for other tasks.

This task will never terminate:

---------------------------------------------------------------------
task body Message_Buffer is
   ...
begin  -- Message_Buffer
   loop
      select
         when Head /= Tail => -- Circular buffer not empty
            accept Retrieve (Value :    out Element) do
               ...
            end Retrieve;
              
      or
         when not ((Head  = Index'First and then
                    Tail  = Index'Last) or else
                   (Head /= Index'First and then
                    Tail  = Index'Pred(Head))    )
                 => -- Circular buffer not full
            accept Store (Value : in     Element);
      end select;
   end loop;
...
end Message_Buffer;
---------------------------------------------------------------------

rationale

The implicit environment task does not terminate until all other tasks have terminated. The environment task serves as a master for all other tasks created as part of the execution of the partition; it awaits termination of all such tasks in order to perform finalization of any remaining objects of the partition. Thus, a partition will exist until all library tasks are terminated.

A nonterminating task is a task whose body consists of a nonterminating loop with no selective accept with terminate or a task that depends on a library package. Execution of a subprogram or block containing a task cannot complete until the task terminates. Any task that calls a subprogram containing a nonterminating task will be delayed indefinitely.

A task that depends on a library package cannot be forced to terminate using a selective accept construct with alternative and should be terminated explicitly during program shutdown. One way to explicitly shut down tasks that depend on library packages is to provide them with exit entries and have the main subprogram call the exit entry just before it terminates.