Revert to filepath traversal if subvolume list fails

[markylaing]

#12258 Added an optimisation to the btrfs storage driver to use btrfs subvolume list instead of traversing the file structure when listing subvolumes.

Unfortunately this may fail due to a permission error when using btrfs on btrfs. This commit reverts to using the previous method if btrfs subvolume list fails.

Closes #12555

[tomponline]

thanks @markylaing do you think it would be practical to add a test for this?

[tomponline]

@markylaing I wonder if it would be better to directly use the old mechanism if d.state.OS.RunningInUserNS is true, as this is what we've done in other places in the code that need to take into account nesting.

Rather than falling back on error and masking the btrfs command error.

[markylaing]

thanks @markylaing do you think it would be practical to add a test for this?

I'll give it a go

[tomponline]

As discussed lets use d.state.OS.RunningInUserNS instead of falling back

[markylaing]

As discussed lets use d.state.OS.RunningInUserNS instead of falling back

I've made this change. I'm finding testing it to be quite tricky, we need a set of utils for getting the LXD build running inside a container. The container itself can't be the busybox test image either. We don't seem to have any tests for nested containers as far as I can see. Shall I create a separate issue for it?

[tomponline]

As discussed lets use d.state.OS.RunningInUserNS instead of falling back

I've made this change. I'm finding testing it to be quite tricky, we need a set of utils for getting the LXD build running inside a container. The container itself can't be the busybox test image either. We don't seem to have any tests for nested containers as far as I can see. Shall I create a separate issue for it?

I wonder if we should rename https://github.com/canonical/lxd-ci/blob/main/tests/docker to test for both docker and LXD container nesting functionality.

Because this is based on using snap you are then free to install the snap inside the guest and use a full ubuntu container.

See https://github.com/canonical/lxd-ci/blob/main/tests/vm_nesting.sh for something similar but for VMs

[tomponline]

thanks!