Bad error message (ERROR: relation not found on) `juju relate` with incompatible interfaces

[Pietro Pasotti]

I am developing a simple tester charm that deploys a webserver, and I encountered a strange issue when attempting to deploy and relate with traefik.
"Juju gives ERROR no relations found"
and it looks as if the relation creation fails; no relations appear on juju status.

Repro:

charmcraft pack ./tests/integration/testers/tcp from https://github.com/canonical/traefik-k8s-operator/tree/tcp-route (commit d42bc82bfa26fe07e5446ae68bb5d0d5fda3f1ea)

juju deploy tcp-requirer-mock...charm
juju deploy traefik-k8s --channel beta
juju relate traefik-k8s tcp-requirer-mock

juju relate traefik-k8s:ingress-per-unit tcp-requirer-mock:ingress-per-unit
ERROR no relations found

[John A Meinel (jameinel)]

Looking at the source:
The actual charm has one interface:
https://github.com/canonical/traefik-k8s-operator/blob/d42bc82bfa26fe07e5446ae68bb5d0d5fda3f1ea/metadata.yaml#L43
```
  ingress-per-unit:
    interface: ingress_per_unit
    description: |
      Provides ingress-like routing to the single units of the related Juju application
```

While the test mock charm has:
https://github.com/canonical/traefik-k8s-operator/blob/d42bc82bfa26fe07e5446ae68bb5d0d5fda3f1ea/tests/integration/testers/tcp/metadata.yaml#L8
```
requires:
  ingress-per-unit:
    interface: ingress-per-unit
    limit: 1
```

note that interface: `foo-bar` is not compatible with interface: `foo_bar`

We could certainly give a better error message here.

[Pietro Pasotti (ppasotti)]

*facepalms*

Thanks for sorting that out!

[Canonical Juju QA Bot (juju-qa-bot)]

This Medium-priority bug has not been updated in 60 days, so we're marking it Low importance. If you believe this is incorrect, please update the importance.