Subordinate charm deployment ignores global series settings

[Vladimir Grevtsev]

Problem statement:

When deploying a subordinate charm, it simply ignores a series explicitly being set in bundle (and machine of primary charm, as well). I have a locally stored bundle [1], and while deploying it, I'm getting a following error while trying to establish a relation:

ERROR cannot deploy bundle: cannot add relation between "nova-compute:neutron-plugin" and "neutron-openvswitch:neutron-plugin": cannot add relation "nova-compute:neutron-plugin neutron-openvswitch:neutron-plugin": principal and subordinate applications' series must match

(full log: https://paste.ubuntu.com/p/ZhzKpvWNT5/)

It looks like Juju ignores an original series of primary charm and takes just first one from subordinate's metadata [2]; if I manually swap a "xenial" with "bionic" in metadata, redeploy a subordinate and try to establish relation - all is fine: [3] (also, if try to deploy Xenial machines - all is fine too, this reproduces on Bionic machines)

Expected behaviour:
Subordinate charm should take its series from primary machine series, or, at least, respect a bundle "series: ..." settings.

[1] https://paste.ubuntu.com/p/KR3Cyt8qn2/
[2] https://paste.ubuntu.com/p/KNzhrxVVNr/
[3] https://paste.ubuntu.com/p/G9tHKyqPzk/

Environment:
Juju macOS 2.4.3-mojave-amd64 (same on Ubuntu 2.4.3-bionic-amd64)
Controller: https://paste.ubuntu.com/p/vgrGVvgCTp/

[Richard Harding (rharding)]

I think the bug here is that the subordinate, when resolving what series to use, should be respecting the bundle default ahead of it's own preference in the multi-series list. Since this is a local charm I think it's skipping some of that normal logic.

[Richard Harding (rharding)]

Marking as high, but the work around is to adjust the series: value for the application in the bundle.

[Ian Booth (wallyworld)]

If there's a default series declared for the bundle, we'll use that over and charm default series.
Thus the order of resolution becomes:
- application
- bundle
- charm default

https://github.com/juju/bundlechanges/pull/47