Update charm resources if necessary when updating a charm

[cderici]

Description

Updates the charm resources (if needed) when refreshing a charm.

Fixes #278

Type of change

• Logic changes in resources (the API interaction with Juju has been changed)

Environment

• Juju controller version: 2.9.46

• Terraform version: 1.5

QA steps

I used coredns on microk8s to test this, any charm with different resource revisions would work for the QA.
We start with the channel=1.27/stable revision=48 as follows:

provider "juju" {}

resource "juju_model" "tst278" {
  name = "tst278"

  cloud {
    name = "microk8s"
    region = "localhost"
  }
}

resource "juju_application" "tst" {
  name = "corednstest"

  model = juju_model.tst278.name

  charm {
    name = "coredns"
    channel = "1.27/stable"
    revision = 48
    series = "jammy"
  }
}

Apply this plan:

 $ terraform init && terraform plan && terraform apply --auto-approve

Check that the application has the resource revision 13:

juju resources coredns
Resource       Supplied by  Revision
coredns-image  charmstore   13

Now modify the charm details in the plan and change it to channel=1.28/stable revision=121, and re-apply.

You should see an update in-place for the application in the plan:

$ terraform plan
juju_model.tst278: Refreshing state... [id=4c930e9e-9ef7-496b-8d10-5127662a8ea3]
juju_application.tst: Refreshing state... [id=tst278:corednstest]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the
following symbols:
  ~ update in-place

Terraform will perform the following actions:

  # juju_application.tst will be updated in-place
  ~ resource "juju_application" "tst" {
        id          = "tst278:corednstest"
        name        = "corednstest"
        # (5 unchanged attributes hidden)

      ~ charm {
          ~ channel  = "1.27/stable" -> "1.28/stable"
            name     = "coredns"
          ~ revision = 48 -> 121
            # (1 unchanged attribute hidden)
        }
    }

Plan: 0 to add, 1 to change, 0 to destroy.

Now apply this with terraform apply --auto-approve and check the resource revision, it should be 60 🎉 :

juju resources coredns
Resource       Supplied by  Revision
coredns-image  charmstore   60

Additional notes

JUJU-4527

[hmlanigan]

Approved with a couple small things