gdbus: Fix the ordering of signals

Consider the following example:

/foo
  properties: "A", "B"

/bar
  properties: "C", "D"

If during a given mainloop iteration, property "A" of object '/foo' is
changed, then properties "C" and "D" of '/bar', lastly "B" of '/foo',
the current code will emit the PropertiesChanged signals in following
order: "A", "B", "C", "D".

This may confuse applications that have a dependency on the order of
those signals.

This fixes the ordering, so in the example, the order becomes:
"C", "D", "A", B". This is considered not to be a problem, as
applications may use the flag G_DBUS_PROPERTY_CHANGED_FLAG_FLUSH, so
property changed signals are emitted as soon as possible.

The solution is for each object, to reschedule the signals every time a
signal is emitted.
2 files changed