is_callable(environment)?

[tmoran@bix.com (Tom Moran)]

An app has a library level task that would like to use a select with a
terminate alternative, but can't since the task must periodically poll
hardware via a delay alternative.  Using
ada.task_identification.is_callable as in
  main_program : task_id;
begin
  accept start_up do
     main_program := start_up'caller;
  end start_up;
...
  loop
     exit when not is_callable(main_program);
     select ...
appears to work on the three compilers I've tried so far, but I wonder
if I'm just very lucky, or if most compilers take the allowance of
C.7.1(21) and allow 'caller to return a task_id of the environment
task, and if is_callable on the environment task becomes false when it
reaches the end of the main program and before its dependent tasks
have terminated and library level controlled objects have finalized.