#22208

description v12 has a new hostname field in machine struct. It also bumps the machine version from v3 to v4. To allow 3.6 to 4.0.6 migration, we have to add this change so that 4.0.6 knows how to deserialize machine v4.

The PR to set hostname in DB will come afterwards. It's important this PR lands first to make migration work.

The 3.6 PR is here #22207

## Checklist

<!-- If an item is not applicable, use `~strikethrough~`. -->

- [ ] Code style: imports ordered, good names, simple structure, etc
- [ ] Comments saying why design decisions were made
- [ ] Go unit tests, with comments saying what you're testing
- [ ] [Integration tests](https://github.com/juju/juju/tree/main/tests), with comments saying what you're testing
- [ ] [doc.go](https://discourse.charmhub.io/t/readme-in-packages/451) added or updated in changed packages

## QA steps

Unit test

## Documentation changes

N/A

## Links

<!-- Link to all relevant specification, documentation, bug, issue or JIRA card. -->

<!-- Replace #19267 with an issue reference or link, otherwise remove this line. -->
**Issue:** Fixes #18641.

<!-- Place JIRA number in both places below. -->
**Jira card:** [JUJU-7682](https://warthogs.atlassian.net/browse/JUJU-7682)


[JUJU-7682]: https://warthogs.atlassian.net/browse/JUJU-7682?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

#22217

This PR persists the machine hostname when we import a 3.6 model. 

Requires #22207 to test as it contains setting the hostname in the description machine struct.

## Checklist


- [x] Code style: imports ordered, good names, simple structure, etc
- [ ] Comments saying why design decisions were made
- [x] Go unit tests, with comments saying what you're testing
- [ ] [Integration tests](https://github.com/juju/juju/tree/main/tests), with comments saying what you're testing
- [ ] [doc.go](https://discourse.charmhub.io/t/readme-in-packages/451) added or updated in changed packages

## QA steps


#### Test Case 1 - Migrate machine

Outcome: migrate from 3.6 to 4.0.6 works. 4.0.6 sets the machine hostname.

Checkout this branch. Bootstrap 4.0 controller.

```
juju bootstrap localhost 406-dst --build-agent
```

Checkout #22207. Bootstrap source controller. Add model, deploy charm, add machine.

```
juju bootstrap localhost 3622-src --build-agent
juju add-model moveme
juju deploy ubuntu u1
juju add-machine
```

Wait to stabilize. 

```
# take note of the machine hostname
juju status --format json | jq '.machines | .[] | .hostname'

"juju-6fe27a-0"
"juju-6fe27a-0"

```

Migrate to destination

```
juju migrate moveme 406-dst
juju switch 406-dst:admin/moveme
```

```
Check hostname are carried over

juju status --format json | jq '.machines | .[] | .hostname'

"juju-6fe27a-0"
"juju-6fe27a-0"

```

#### Test Case 2 - Migrate machine hosting a container

Outcome: hostnames of host machine and its container are migrated. Unfortunately, an existing bug fails to import the layer link device #22224.

Apply this patch as a workaround to verify hostnames are set. It gives us enough time to query the records in DB.

```
diff --git a/domain/network/modelmigration/import_linklayerdevices.go b/domain/network/modelmigration/import_linklayerdevices.go
index 01330b6..c789eb04c9 100644
--- a/domain/network/modelmigration/import_linklayerdevices.go
+++ b/domain/network/modelmigration/import_linklayerdevices.go
@@ -7,6 +7,7 @@ import (
 "context"
 "fmt"
 "strings"
+ "time"
 
 "github.com/juju/description/v12"
 
@@ -65,6 +66,10 @@ func (i *importOperation) Setup(scope modelmigration.Scope) error {
 
 // Execute the import of the link layer devices contained in the model.
 func (i *importOperation) Execute(ctx context.Context, model description.Model) error {
+ i.logger.Infof(ctx, "[linklayerimport] sleeping for 3 mins...")
+ <-time.After(3 prdesc time.Minute)
+ i.logger.Infof(ctx, "[linklayerimport] woke up from sleep...")
+
 if err := i.importLinkLayerDevices(ctx, model.LinkLayerDevices(), model.IPAddresses()); err != nil {
 return errors.Capture(err)
 }
```



Checkout this branch. Bootstrap 4.0 destination controller

```
juju bootstrap localhost 40-dst-new --build-agent
```

Checkout #22207. Bootstrap source controller. Add model and machines.

```
juju bootstrap localhost 36-src-new --debug
juju add-model moveme
juju add-machine
# create container on machine 0
juju add-machine lxd:0
```

Wait until machines are started... Check machine and container hostname. Note them down

```
juju status --format json \n| jq '.machines[]
 | (.hostname,
 .containers[]?.hostname)'

"juju-6fe27a-0"
"juju-6fe27a-0-lxd-0"
```

Then migrate

```
juju migrate moveme 40-dst-new
```

We have 3 minutes to check the records in DB before the next import operation is run. 

```
juju switch 40-dst-new
juju ssh -m controller controller/0

repl (controller)> .switch model-move

repl (model-move)> select name, hostname from machine
name hostname 
0 juju-6fe27a-0 
0/lxd/0 juju-6fe27a-0-lxd-0 
```

Eventually migration fails with 

```
 migration model data transfer failed, failed to import model into target controller: execute operation import link layer devices: importing link layer devices: converting device "lxdbr0" on machine "0": converting address "10.136.55.1/24": no subnet found, nor created
```

However, this occurs in link layer devices import ([ref](https://github.com/juju/juju/blob/f3d234040cc8ad825373628d03e0b6d875a8bb28/domain/modelmigration/import.go#L83)) which is executed after machines import. So we know that the machine import was successful. 


Ideally, if the link layer device import succeeded, you should be able to juju status to see the hostnames set.

## Documentation changes

<!-- How it affects user workflow (CLI or API). -->

## Links

**Issue:** Fixes #18641.

**Jira card:** [JUJU-7682](https://warthogs.atlassian.net/browse/JUJU-7682)


[JUJU-7682]: https://warthogs.atlassian.net/browse/JUJU-7682?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ