Demo of a race-free GDBus daemon that exits when idle

Background: https://bugs.freedesktop.org/show_bug.cgi?id=11454

Inspired by http://cgit.freedesktop.org/systemd/systemd/tree/src/libsystemd/sd-bus/bus-util.c?id=91077af69e4eb5f7631e25a2bc5ae8e3c9c08178

Use sd_notify(STOPPING=1) to ensure systemd queues start requests

This is what the above does - systemd is tracking our process state, and thus ensures mutual exclusion.

Do the asynchronous release of the name

Needs a GDBus patch to clean up.

Ignore SIGTERM as systemd seems to send that quickly for some reason

TODO: Debug why this is

Need a clean tristate for RUNNING/FLUSHING/EXITING

The systemd event loop has special support for exiting, which GMainLoop doesn't. This demo app uses a tristate.

Clients should always use the well-known name

One thing that wasn't obvious to me at the start, but due to the way DBus policies work, any attempt to send a message to the non-current owner of the name will be denied (which happens when the service is exiting and has released the name).

Thus, clients should always send messages to the well-known name.

Save our state on disk

A stateful service needs to actually save things on disk, this app demos how to do that in an async/timed fashion.