Investigate why go-systemd doesn't send events we expect

[bcwaldon]

fleet would benefit from getting events when units are created/removed from go-systemd's dbus subscription functionality. Currently, we have to pull this state manually when we know we've created/removed a unit like so:

• fleet/agent/agent.go

  Line 238 in 50bbd56

  // We must explicitly refresh the payload state, as the dbus

• fleet/agent/agent.go

  Line 278 in 50bbd56

  // The dbus event systemd will not trigger an event telling

• fleet/agent/event.go

  Line 136 in 50bbd56

  state, _ = eh.agent.systemd.GetPayloadState(jobName)

[jonboulle]

In certain cases (see #540), UnitProperties change events are not getting sent in go-systemd either. This seems to be most trivially reproducible by rapidly stopping and starting a unit. An event is sent on dbus (1), but seemingly not picked up by go-systemd.

(1) e.g.

signal sender=:1.0 -> dest=(null destination) serial=36424 path=/org/freedesktop/systemd1/unit/foo_2eservice; interface=org.freedesktop.DBus.Properties; member=PropertiesChanged

[bcwaldon]

No longer necessary as we don't use dbus events anymore.