subordinate can't relate to applications with different series

[Jason Hobbs]

I'd like the relate a subordinate service, ntp for example, to multiple applications, some of which are on xenial and some of which are on bionic. Since the ntp charm supports both xenial and bionic, it seems like this should be possible, but it isn't:

http://paste.ubuntu.com/p/Dh9dPzvwVC/

Instead, I need one subordinate application for each series, which complicates my model and operations.

I think this is also a problem for operating system upgrades. If a subordinate is shared between multiple applications, you couldn't upgrade one of those applications, because all of the applications have to be on the same series.

[Tim Penhey (thumper)]

I believe the basis for this is that a charm when stored in the database
has only a single entry for series, not a sequence.

If we updated that, the logic used could be expanded.

[Ian Booth (wallyworld)]

@thumper,

ERROR cannot add relation "ntp:juju-info bionic-ubuntu:juju-info": principal and subordinate applications' series must match

I think this error came from the days before multi-series charms. I think all we need to do now is inspect the charm metadata to see if it supports what it's being related to, especially so with subordinates where we don't actually create a unit until we relate. The fact we store a "nominal" series against the subordinate app when deploying we can make immaterial, so long as the supported series match ok.

[Ian Booth (wallyworld)]

https://github.com/juju/juju/pull/9431