How do I pass a cloud-init config when creating an LXD container?

I've been reading through the LXD documentation and blogs and stackexchange answers that say LXD is supposed to have some kind of init syntax that allows you to pass cloud-config data to your containers as they're created, but none of the many examples have worked for me. I've tried following examples that set up a profile, examples that pass data in using --config, examples that pass yaml data directly in, and examples that set the data after init but before starting. Nothing has worked. I'm pasting in what I currently have... Could someone please tell me what I'm doing wrong and how to fix it?

The most basic example I can think of:

lxc delete -f x

cat << EOF >config.yml
#cloud-config
output: {all: '| tee -a /var/log/my-cloud-init-output.log'}
package_update: true
package_upgrade: true
package_reboot_if_required: true
packages:
  - tree
runcmd:
  - touch /run/cloud-config-did-run
EOF

lxc launch ubuntu: x --config=user.user-data="$(cat config.yml)"
sleep 5
lxc exec x -- bash -c "ls /run"
lxc exec x -- bash -c "tree /etc"

This should output to /var/log/my-cloud-init-output.log, install tree and also make an empty file in /run. The output to /var/log/my-cloud-init-output.log works, but nothing else does. The logs don't show anything special.

It appears you need to wait longer. Since you have both package_update and package_upgrade specified as true, you're essentially run apt-get update; apt-get upgrade (which can take more than 5 seconds) before tree can get installed or the runcmd executes.

Your exact commands work for me if I substitute the sleep 5 with

lxc exec x -- bash -c "cloud-init status --wait"
sleep 2  # because of the `package_reboot_if_required: true`