container scoped relations between 2 subordinates broken in 1.20.12

The fix for https://bugs.launchpad.net/juju-core/+bug/1382751 was backported to 1.20.12, but it breaks a use case, I think unintentionally.

The use case is relating 2 subordinates together in scoped relation.

The specific use case we have is a subordinate charm (https://jujucharms.com/u/ubuntuone-hackers/conn-check/trusty/18) that has a relation to the nrpe-external-master subordinate charm via it's usual scoped relation. The conn-check nrpe-external-master relation is also scoped: containter, as it makes sense intuitively that way (to me, anyway).

We've beeing using this successfully on 1.18 and 1.20 for a while now.

#juju conversation with Kapil:

<bloodearnest> https://bugs.launchpad.net/juju-core/+bug/1382751
<mup> Bug #1382751: non subordinate container scoped relations broken <regression> <relations> <subordinate> <juju-core:Fix Committed by menno.smits> <juju-core 1.20:Fix Released by menno.smits> <juju-core 1.21:Fix Released by natefinch> <https://launchpad.net/bugs/1382751>
<bloodearnest> so, this is a breaking change
<bloodearnest> in a point release
<hazmat> bloodearnest, ugh
<hazmat> that looks like unintended fallout
<bloodearnest> seems so
<bloodearnest> relating 2 subordinates has been supported for ages, I've been using for a good while
<hazmat> bloodearnest, is it a container scoped relation between the two subordinates?
<bloodearnest> hazmat: yes
<bloodearnest> hmm, I guess one side could be non-scoped, since the placement has already been done by the scoped relation
<hazmat> bloodearnest, it looks like this line + if eps[0].Scope == charm.ScopeContainer && subordinateCount != 1 {
<hazmat> should be subordinateCount >= 1
<hazmat> bloodearnest, is this with public charms, i think its a worth a bug report
<bloodearnest> hazmat: public charm, but not yet promulgated. It's the charm for the new conn-check utility we announce recently, related to nrpe-external-master to surface the checks as nagios results
<bloodearnest> I will file a report

Kapil suggests this line should be >=1, not != 1:

https://github.com/juju/juju/pull/1075/files#diff-f6be34387a141f652301c16cf7a5e36cR1569