Purging LXD snap doesn't remove LVM storage backed by loop device

[simondeziel]

# snap install lxd --channel latest/candidate
lxd (candidate) 6.2-7a6ecda from Canonical✓ installed
# lxd init --auto --storage-backend=lvm
# snap remove --purge lxd
2025-02-03T22:22:40Z INFO Waiting for "snap.lxd.daemon.service" to stop.
lxd removed
# snap install lxd --channel latest/candidate
lxd (candidate) 6.2-7a6ecda from Canonical✓ installed
# lxd init --auto --storage-backend=lvm
Error: Failed to create storage pool "default": A volume group already exists called "default"

This behavior goes back to a long time as even 4.0/stable is affected:

root@v1:~# snap install lxd --channel 4.0/stable
2025-02-17T20:15:09Z INFO Waiting for automatic snapd restart...
lxd (4.0/stable) 4.0.10-e664786 from Canonical✓ installed
root@v1:~# lxd init --auto --storage-backend=lvm
root@v1:~# LD_LIBRARY_PATH=/snap/lxd/current/lib/:/snap/lxd/current/lib/x86_64-linux-gnu/:/snap/lxd/current/lib/x86_64-linux-gnu/ceph:/snap/lxd/current/zfs-2.2/lib PATH=/snap/lxd/current/zfs-2.2/bin:/snap/lxd/current/bin:$PATH nsenter --mount=/run/snapd/ns/lxd.mnt -- lvs
  LV          VG      Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  LXDThinPool default twi-a-tz-- 2.65g             0.00   1.57                            
root@v1:~# LD_LIBRARY_PATH=/snap/lxd/current/lib/:/snap/lxd/current/lib/x86_64-linux-gnu/:/snap/lxd/current/lib/x86_64-linux-gnu/ceph:/snap/lxd/current/zfs-2.2/lib PATH=/snap/lxd/current/zfs-2.2/bin:/snap/lxd/current/bin:$PATH nsenter --mount=/run/snapd/ns/lxd.mnt -- vgs
  VG      #PV #LV #SN Attr   VSize VFree
  default   1   1   0 wz--n- 4.65g    0 
root@v1:~# snap remove --purge lxd
2025-02-17T20:19:22Z INFO Waiting for "snap.lxd.daemon.service" to stop.
lxd removed
root@v1:~# snap install lxd --channel 4.0/stable
lxd (4.0/stable) 4.0.10-e664786 from Canonical✓ installed
root@v1:~# LD_LIBRARY_PATH=/snap/lxd/current/lib/:/snap/lxd/current/lib/x86_64-linux-gnu/:/snap/lxd/current/lib/x86_64-linux-gnu/ceph:/snap/lxd/current/zfs-2.2/lib PATH=/snap/lxd/current/zfs-2.2/bin:/snap/lxd/current/bin:$PATH nsenter --mount=/run/snapd/ns/lxd.mnt -- lvs
  LV          VG      Attr       LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  LXDThinPool default twi-a-tz-- 2.65g             0.00   1.57                            
root@v1:~# LD_LIBRARY_PATH=/snap/lxd/current/lib/:/snap/lxd/current/lib/x86_64-linux-gnu/:/snap/lxd/current/lib/x86_64-linux-gnu/ceph:/snap/lxd/current/zfs-2.2/lib PATH=/snap/lxd/current/zfs-2.2/bin:/snap/lxd/current/bin:$PATH nsenter --mount=/run/snapd/ns/lxd.mnt -- vgs
  VG      #PV #LV #SN Attr   VSize VFree
  default   1   1   0 wz--n- 4.65g    0 
root@v1:~# lxd init --auto --storage-backend=lvm
Error: Failed to create storage pool 'default': Volume group "default" is already used by LXD

Note: this was initially lumped with #14918 but the LVM case is much older and different due to the interaction between PV, VG and LV in LVM.

[tomponline]

Could be related to #9278