Instead of operating directly on the tree for a stage we can operate
on a mount too. This is useful in the case where operating on the
directory tree of files isn't sufficient and the modifications need
to be made directly to the filesystems on the disk image that we are
creating.
One such example of this is we are having a problem right now where
the immutable bit being set on an OSTree deployment root doesn't
survive the `cp -a --reflink=auto` in the org.osbuild.copy stage when
being copied from the directory tree into the mounted XFS filesystem
we created on the disk image. Thus we have to workaround this loss
of attribute by applying the attribute directly on the mounted
filesystem from the disk.
In this change here we also add a check in osbuild/mounts.py to not
attempt a umount of the root of the mounts directory if that path
is no longer a mountpoint, which can happen when the umount -R
from the mounts/org.osbuild.ostree.deployment also removes the
overmount.
Here is an example of how this would be used:
```
- type: org.osbuild.chattr
options:
immutable: true
path: mount://root/
devices:
disk:
type: org.osbuild.loopback
options:
filename: disk.img
partscan: true
mounts:
- name: root
type: org.osbuild.xfs
source: disk
partition:
mpp-format-int: '{image.layout[''root''].partnum}'
target: /
- name: ostree.deployment
type: org.osbuild.ostree.deployment
options:
source: mount
deployment:
osname: fedora-coreos
ref: ostree/1/1/0
```
The initial mount on `/` is the filesystem from the root partition
on the disk. The second mount (of type org.osbuild.ostree.deployment)
then reconfigures things similar to how an OSTree system is set up.
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