Currently, at least in the EDS and Tracker backends, we're unconditionally emitting property change notifications for properties when their _update_[property name]() functions get called.

While this isn't technically wrong, it does mean that the IndividualAggregator ends up doing a lot of extra work re-linking personas when nothing's actually changed. This also results in spurious IndividualAggregator.individuals_changed_detailed signals, which break the tests when the patch in bug #656689 is applied.

At least for the linkable properties, we should ensure that we only emit change notifications when things have actually changed.

This fits in well with bug #657680, which suggests that we should never be calling Set.clear() on sets of values (such as sets of FieldDetails), since we could be breaking the iterator on that set which is currently being used in client code.

I guess our template for _update_*() functions should change to something like:

var new_bag_o_values = new HashSet<Foo> ();
var changed = (get_new_value_from_backend ().size != this._bag_o_values.size); // If the sets have different sizes, we don't need to perform a detailed check, since we know the property's changed

foreach (var v in get_new_value_from_backend ())
  {
    if (changed == false)
      {
        var old_v = this._bag_o_values.get (v);
        if (old_v == null || !v.equal (old_v))
          {
            changed = true;
          }
      }

    new_bag_o_values.add (v);
  }

if (changed == true)
  {
    this._bag_o_values = new_bag_o_values;
    this._bag_o_values_ro = new_bag_o_values.read_only_view;
    this.notify_property ("bag-o-values");
  }