VM Discovery fails, resulting in " Error: An architecture is required." when composing a LXD VM

Hello,

When adding LXD as a backend in MAAS it will scan all projects (even if one is selected), resulting in a failure. This failure ultimately results in an error when composing machines "Error: An architecture is required.".

 Machine: win10: {
  "iso": {
    "boot.priority": "10",
    "source": "/home/pjds/Downloads/Win10_20H2_v2_EnglishInternational_x64-distrobuilder.iso",
    "type": "disk"
  },
  "root": {
    "path": "/",
    "pool": "default",
    "size": "40GiB",
    "type": "disk"
  }
}

The issue is caused by this line in the LXD driver:

            if source:
                pool = client.storage_pools.get(device["pool"])
                volume = pool.volumes.get("custom", source)

This check will run on the ISO object, failing with a key error, bubbling up to a failure to discover the LXD backend.

Please find the rackd logs here: https://private-fileshare.canonical.com/~pjds/maas-rackd-lxd-issue.log

[To Reproduce]

1. Create a VM with an ISO mounted, as configured above
2. Install, setup MAAS
3. Setup the LXD backend
4. Attempt to compose a VM and observe the error "Error: An architecture is required."

[Workaround]

Scan machines in your LXD cluster with something like

for project in "charmcraft" "default" "demos" "infra" "sandbox" "sandboxing" "ws-sandbox"; do for machine in $(lxc list -f csv --project $project -c n); do echo "Machine: $machine: $(lxc config show --project $project $machine | yq .devices)"; done; done;

and dismount the ISO, remove, and re-add the LXD backend, this should resolve the issue.

Thanks,
Peter