add-unit deploys wrong agent version after controller is upgraded

[Menno Finlay-Smits]

When add-unit is used after the controller has been upgraded, the new unit uses the tools version of the controller, not the model agent version.

To reproduce:

* bootstrap a controller with 2.1
* deploy "ubuntu" in the default model
* use `juju upgrade-juju -m controller ...` to upgrade the controller to 2.2
* juju add-unit ubuntu
* wait
* the new machine for ubuntu/1 is running 2.2, not 2.1.x as per the model version

I tested with 2.1.2 and 2.2.3 (tip of current 2.2 branch).

[John A Meinel (jameinel)]

new machines created in a model should be trying to create agents according to *that model's* target 'agent version', not the controller's 'agent version'.

[Ian Booth (wallyworld)]

https://github.com/juju/juju/pull/7721

[William Grant (wgrant)]

Is there a way to fix existing affected models? We can't upgrade the whole model because it thinks the previous upgrade hasn't completed, as the agent versions don't all match.

[Anastasia (anastasia-macmood)]

I suspect that the fix for this has been already ported into 2.3 (develop) as part of one of the larger merges we have undertaken recently.