From telemetry we see regular spikes of events where search engines have changed, when they have in fact not changed.

Bug 1876178 did some work on this in the past, but we only focussed on configuration engines, as that's where the issue was thought to be.

Mostly the unnecessary events come from the result of configuration updates or add-on updates, where there's an internal SearchUtils.MODIFIED_TYPE.CHANGED notification, for which a search engine may have updated some fields, but not the ones that the telemetry covers.

Hence we should only send out the event telemetry for any engine when the relevant fields have actually changed.

With the previous patch on this bug, these preventions are no longer necessary. Whilst we could keep them, it means that the behaviour of configuration engines are different to other engines. It might also prevent internal updates happening, which we might need sometimes (e.g. for bug 2043189).

In future we might want to do something more extensive with not sending out unnecessary internal notifications, however these are generally not hot paths, so that might be something we think about at an opportune time.

https://hg.mozilla.org/mozilla-central/rev/a96fcbc15931
https://hg.mozilla.org/mozilla-central/rev/ee60662cb7b4