[modbus] Change bridge to offline during reinitialization

[mrbig]

I noticed that things connected to a modbus bridge won't be reinitialized when the bridge changes. Digging down to openhab core I found out that connected things are only notified when the bridge changes status.

This small PR changes the bridge to OFFLINE if it was ONLINE when a change happened.

This is a split of the original PR #4220 as requested by @davidgraeff .

[davidgraeff]

LGTM

[ssalonen]

Ah in the original PR the bridge was switched to offline in dispose: https://github.com/openhab/openhab2-addons/pull/2246#discussion_r206529831 and openhab/openhab2-addons@e23af94

I think this bridge status notification was exactly the reason for that in the first place but I forgot it during review when Kai asked me to remove it as "extra".

Perhaps the dispose method would be a cleaner place for this? The comment is probably still necessary to avoid accidentally removing it in the future.

Alternatively one can ask whether framework should in fact update thing OFFLINE when disposing it?

[davidgraeff]

Alternatively one can ask whether framework should in fact update thing OFFLINE when disposing it?

Because a config change makes the thing handler by default go through the dispose/init cycle that would cause a lot of status changes. That's probably why the framework is not doing it.

[ssalonen]

@davidgraeff that makes sense then to leave it to binding developers.

Thinking more about this:

For this particular PR, it actually makes more sense to update status right in dispose to "notify" child things as soon as possible that bridge status has changed. Most likely children would become offline as well as they should

But on the other hand it would have a downside of extra status updated considering what davidgraeff just mentioned :/

[davidgraeff]

I'm sure that @mrbig has tested this particluar change. I'm not sure what the framework will do when the status is updated in dispose(), there are a lot of special cases in the bridge-status-change method.

Let's rather have this change in. If it can be made faster / more responsive in a later PR, that's fine.

[mrbig]

I can confirm @davidgraeff : when only the config was changes on a running bridge, then the initialize has been called in the ONLINE state. For the connected things to get notified I had to forcefully move the bridge to an OFFLINE state, eg by setting an invalid config first.

[ssalonen]

Davidgraeff: I take your word on the details/special cases and agree it makes sense to move incrementally and conservatively here, and merge this as is