[systemd-devel] Generic problems with dependencies for on-demand services on shutdown

[Andrey Borzenkov]

networkmanager crashes on shutdown every time. There is a fix to avoid
this crash, but it reveals some quite fundamental issue.

On startup we have

dbus.socket -> dbus.target -> services that are After dbus.target

So far so good. dbus.service is implicitly pulled in by first connect
to dbus.socket and everyone is happy

But on *shutdown* we apparently do not have any sort of valid
ordering. Which means dbus is snatched away unexpectedly at arbitrary
time. While of course services must be able to cope with it, they may
want to do some cleanup on clean shutdown, and we deprive them of
this.

Lennart, do you have clever idea how to express such dependency? This
seems to be inherent issue for any type of socket activation. Somehow
we *must* mark any service that requires foo.socket that spawns
foo.service as ordered After foo.service (or Before on shutdown). We
need some generic way to compute such dependency. Right now there is a
gap between dbus.service and dbus.target, where the former is
completely hidden from all consumers.

Same issue with D-Bus activation BTW. Anything started by D-Bus must
be ordered after D-Bus itself. This should happen dynamically. Ideally
is service foo triggers service bar via D-Bus (or socket or whatever),
service bar should be ordered After service foo. Do we have enough
information to do it?