This was something supported only in ClockServer. Zope's ZServer had an
API to control the shutdown sequence, by allowing servers to implement a
clean_shutdown_control method that was be called at every phase of the
shutdown:

https://github.com/zopefoundation/ZServer/blob/0259e288/src/Lifetime/__init__.py#L20-L38

ClockServer was implementing clean_shutdown_control method:
https://lab.nexedi.com/nexedi/erp5/blob/07a136ec3/product/ClockServer/ClockServer.py#L57

and as described in the README ( https://lab.nexedi.com/nexedi/erp5/blob/07a136ec3/product/ClockServer/README#L32 ),
it was expected to be configured to call TimerService's process_shutdown.

It seems we were using a mix of ClockServer + TimerService at the time,
but this was maybe never used.

In 49400d40 (we don't use ClockServer. we use TimerService instead.,
2013-10-07) we removed ClockServer. Since this day this code is not used.

In WSGI mode, there's no "clean_shutdown_control" API, so this