auto-compose LXD VM fails due to default root disk size "0" only when also specifying an extra storage disk

[Trent Lloyd]

When using a MAAS LXD KVM host, charms with additional storage disks fail to deploy by default.

Example: juju deploy postgresql --storage pgdata=10G

This only fails when you are trying to automatically compose a VM, but it fails not because of the additional disk but due to the root-disk size being specified as "0".

If you over-ride the root-disk size by running "juju set-model-constraints root-disk=8G" then it works as expected. It also works as expected if you are using an existing composed machine or when deploying a charm without an additional storage disk.

For whatever reason the logic seems not to understand "0" as auto-allocate the size when there is an additional storage disk and it needs to compose a new VM.

The error is:
0 pending pending focal failed to start machine 0 (failed to acquire node: No available machine matches constraints: [('agent_name', ['6ca452ff-aaef-45a6-8d5c-2889d908c609']), ('arch', ['amd64']), ('interfaces', ['1:space=1']), ('storage', ['root:0,0:10']), ('zone', ['default'])] (resolved to "arch=amd64/generic interfaces=1:space=1 storage=root:0,0:10 zone=default")), retrying in 10s (10 more attempts)

MAAS 3.2.0-11989-g.84a255c14
LXD 5.0.0-b0287c1

Can be reproduced with a MAAS lab setup as follows:
https://github.com/canonical/maas-multipass/blob/main/maas.yml (https://maas.io/tutorials/build-a-maas-and-lxd-environment-in-30-minutes-with-multipass-on-ubuntu#5-launch-the-maas-and-lxd-multipass-environment)

[Canonical Juju QA Bot (juju-qa-bot)]

This Medium-priority bug has not been updated in 60 days, so we're marking it Low importance. If you believe this is incorrect, please update the importance.

[Trent Lloyd (lathiat)]

This is being hit in support labs by a few people, I hit it again now and one of my colleagues recently hit it (accidentally filed duplicate 2031440). Can we re-triage this?

Note: No one can update bug Importance, so the request from the juju-qa-bot is invalid. It should instead suggest to change the Status I guess.