of *charm.URL.

There is an overhead for parsing the stored URL, which we are paying
whether we access it or not.

Include the application name to point user in the correct direction.
With bundle handling, any number of applications could cause the
problem.  LP 1931739.

Help with debugging later by using the required logger in ChangeConfig
if the model doesn't have one.

Fix for LP 1950237. Old controllers have the charm channel data, but do
not include it in FullStatus results.  It is available in
ApplicationInfo.  Covers cases where bundles are exported from old
controllers, edited to include a channel, and deployed with newer
clients.  There may be others.

Existing model data now ensures that the channel is provided, to
correctly determine if the bundle is attempting an upgrade by channel,
assume a stable channel if none is provided in the bundle.

Unit tests updated to include data as current controllers will return.

Include the application name so the person deploying the bundle has a
starting point to fix the error.

…y with 2.9 < controllers

Allow the order of arguments to change from the expected alphabeticial
ordering.  Update the response appropriately too.

…ller

juju#13556

Fix 1950237 by using ApplicationsInfo to find existing channels, rather than the status output, which doesn't contain the data in pre-2.9 controllers. This will only be done for pre 2.9 controllers, as is the case when the defaultCharmSchema is charmstore. Nicely, ApplicationsInfo is a bulk api call. However this necessitated requiring applications to have a channel from the bundle. To keep compatibility assume stable if channel not specified in the bundle.

Fix 1931739 by updating the error messages to include the application name causing the failure. Helps finding the problem in the bundle, where multiple applications exist.

Now that ChangeConfig is internal to juju, ensure there is always a logger.

## QA steps

Please try various combinations of juju clients and controllers with deploying bundles and redeploying them with --dry-run to ensure no changes. Below are reproducers for the bugs resolved.

```console
$ sudo snap refresh juju --channel 2.8/stable
# using the snap juju:
$ juju bootstrap localhost two-eight
$ juju deploy cs:~ubuntu-mirror-charm-maintainers/ubuntu-mirror
Located charm "cs:~ubuntu-mirror-charm-maintainers/ubuntu-mirror-9".
Deploying charm "cs:~ubuntu-mirror-charm-maintainers/ubuntu-mirror-9".
$ juju deploy cs:~ubuntu-mirror-charm-maintainers/ubuntu-mirror --channel candidate ubuntu-mirror-candidate
Located charm "cs:~ubuntu-mirror-charm-maintainers/ubuntu-mirror-9".
Deploying charm "cs:~ubuntu-mirror-charm-maintainers/ubuntu-mirror-9".
$ juju export-bundle > /tmp/bundle.yaml

# edit the bundle to include the candidate channel for ubuntu-mirror-candidate

# with the 2.9 client:
$ juju deploy /tmp/bundle.yaml --dry-run
No changes to apply.
```

## Bug reference

https://bugs.launchpad.net/juju/+bug/1950237
https://bugs.launchpad.net/juju/+bug/1931739

Increment juju to 2.9.23

was at the time (version 2.8) that stored *charm.URL instead of *string.

juju#13546

We have long stored in the unit Mongo document, an actual `*charm.URL` reference.

This of course is fraught with danger, as upstream changes to the library can change what is stored, and potentially render old documents incompatible with an updated type.

In addition, we instituted `GetBSON` and `SetBSON` helpers to handle persistence and retrieval, which were made non-trivial by the introduction of CharmHub and multiple possible formats for this URL.

We have ended up in a situation where we are paying a small overhead to parse this URL upon every access, even when we might not use the field.

Here we update the document to store a string, and only parse the URL at need. The patch is scoped only to what is necessary to change the one field in the unit document. We still do unnecessary conversion, and could benefit from changing the application document as well, including all the sundry methods that accept `*charm.URL`, but immediately stringify it for use.

Upgrades had one usage of the doc for a version 2.8 step. This is replaced with the old version of the doc still storing a `*charm.URL`.

## QA steps

- Bootstrap a 2.7 controller and `juju install ubuntu -n 3`.
- With this patch, `juju upgrade-controller --build-agent`.
- Run `juju upgrade-model` twice. It will jump to the latest 2.8, then this patch.
- Ensure that the logs show no errors relating to unit documents.

## Documentation changes

None.

## Bug reference

N/A

channel request." Found with merge of 2.9 into develop.

commit "Fix tests related to channel/revision changes for client
compatibility with 2.9 < controllers"