"List of bases on invalid base" response is broken for charms with default track and no `latest` track

[Daniel Manrique]

When Juju does the initial "discovery" request to see which bases are available for a charm, it uses a bogus base. Charmhub then replies with an "invalid-charm-base" response showing the available bases. Example:

curl -XPOST -s https://api.charmhub.io/v2/charms/refresh -H 'Content-type: application/json' -d '{ "context": [], "actions": [{"name": "nginx-ingress-integrator", "base": {"name": "NA", "channel": "NA", "architecture": "amd64"} , "channel": "stable" , "action": "install", "instance-key": "a-test"}]}' | jq .

{
  "error-list": [],
  "results": [
    {
      "charm": null,
      "error": {
        "code": "invalid-charm-base",
        "extra": {
          "default-bases": [
            {
              "architecture": "amd64",
              "channel": "20.04",
              "name": "ubuntu"
            }
          ]
        },
        "message": "Instance key 'a-test' invalid 'base' in 'install' for charm_id=aQVXlrbsLWBTiAiQF0lilvUFolDOpKaE name=nginx-ingress-integrator"
      },
      "id": "aQVXlrbsLWBTiAiQF0lilvUFolDOpKaE",
      "instance-key": "a-test",
      "name": "nginx-ingress-integrator",
      "released-at": null,
      "result": "error"
    }
  ]
}

However, for a charm that has the following configuration:
1) A non-latest track
2) Said track is set as the default track
3) `latest` is closed

the response is revision-not-found instead, which confuses Juju and makes it say there is no revision to install.

I think the code that handles the invalid base and sends invalid-charm-base is unaware of default tracks, which should be preferred if they exist. Usual behavior:

1) if `latest` is specified, then use that (this will rightfully fail because there is no latest track).

2) if latest is not given, in the case of this charm which has a default track, return data for the *default* track instead.

3) (for completeness) If latest is not given, and the charm does NOT have a default track, then use "latest" by default (this is typical behavior fwiw).

The confusing thing for this charm is as mentioned that it has a default track *and* latest has been closed.

$ curl -XPOST -s https://api.charmhub.io/v2/charms/refresh -H 'Content-type: application/json' -d '{ "context": [], "actions": [{"name": "mysql", "base": {"name": "NA", "channel": "NA", "architecture": "amd64"} , "channel": "stable" , "action": "install", "instance-key": "a-test"}]}' | jq .
{
  "error-list": [],
  "results": [
    {
      "charm": null,
      "error": {
        "code": "revision-not-found",
        "extra": {
          "releases": [
            {
              "base": {
                "architecture": "amd64",
                "channel": "22.04",
                "name": "ubuntu"
              },
              "channel": "8.0/stable"
            },
            {
              "base": {
                "architecture": "amd64",
                "channel": "22.04",
                "name": "ubuntu"
              },
              "channel": "8.0/candidate"
            },
            {
              "base": {
                "architecture": "amd64",
                "channel": "22.04",
                "name": "ubuntu"
              },
              "channel": "8.0/beta"
            },
            {
              "base": {
                "architecture": "amd64",
                "channel": "22.04",
                "name": "ubuntu"
              },
              "channel": "8.0/edge"
            }
          ]
        },
        "message": "No revision was found in the Store."
      },
      "id": "F8CtvgcznoeRSupRrY2uEa67oQtBTF7x",
      "instance-key": "a-test",
      "name": "mysql",
      "released-at": null,
      "result": "error"
    }
  ]
}

For more info, this is the actual request Juju makes :

curl -XPOST -s https://api.charmhub.io/v2/charms/refresh -H 'Content-type: application/json' -d '{"context":[],"actions":[{"action":"install","instance-key":"ba0ea644-b34a-4a8c-83eb-4f3930ac8e83","name":"mysql","channel":"stable","base":{"architecture":"amd64","name":"NA","channel":"NA"}}],"fields":["bases","config-yaml","download","id","license","metadata-yaml","name","publisher","resources","revision","summary","type","version"]}

[Alex Lutay (taurus)]

Dear Daniel, please check https://chat.charmhub.io/charmhub/pl/ct994gu8ybdjxx7cthjiicpmcw

If possible, please rise the priority for this bugreport. Thank you in advance!