flow.cfg: In Config_manager doc header suggest a strategy for applications for handling dynamic update requests.

[ygoldfeld]

Filed by @ygoldfeld pre-open-source:

Class template Config_manager doc header has a section "Relationship with the rest of your program/threads."

It should suggest an ordering of things. Something like:

• Make a class Updater which reacts to signals/requests/etc. to read dynamic update file D.
• In your application:
  ** Create Updater. Have it read baseline (apply_static_and_dynamic(B)) from file B; and possibly initial-dynamic (apply_dynamic(D)) from file D.
  ** Create and start each module M. Before M starts its threads, it must register_dynamic_change_listener() if relevant. Inside this listener, usually, true handling would be post()ed onto M's threads.
  ** Start Updater thread. When update signal comes in, call apply_dynamic(D) from that thread.
  ** SIGTERM/etc. happens. Then:
  ** Stop Updater thread. Once it is stopped, proceed:
  ** Stop each module M (stop its threads).
  ** Destroy modules M.
  ** Destroy Updater.
• The key is:
  ** Registration of listeners occurs before Updater can apply_dynamic().
  ** Listeners and their owners are fully alive until after the very last moment apply_dynamic() can still be called.
  ** Explicit unregistration of listeners is not necessary.
  ** Startup and shutdown are symmetrical.

Also, look through suggestions in existing comments about when to call unregister...(); it says stuff about "SHOULD" which is not really consistent with the above.

Lastly, make some suggestions about:

• Feature: Backup last-known-good file D.
• Feature: Crash rejection.