snapcore
Add a finalrd script to move filesystems
/mnt/run/base contains the snap file that is mounted on /. For
that reason it is not possible to unmount /mnt/run/base until we
have switched root to the shutdown ramfs.
At this point before we move /sysroot/mnt/run/base out of /sysroot
so that systemd-shutdown is able to unmount all filesystems.
systemd-shutdown allows hooks. But they are called right after
systemd-shutdown. That means we cannot run a hook after the root
switch and before systemd-shutdown. For that reason the
finalrd hook replaces systemd-shutdown in the ramfs by a wrapper.
This solution is not elegant. In the long term we should try to make
systemd-shutdown always move all sub mount points before trying to
unmount.
Also, there is still an error message when systemd tries to umount
/mnt/run/base before switching root. There does not seem to be
anyway to make mnt-run-base.mount perpetual.
Note there also snapcore/snapd#11119 to at least help systemd-shutdown to remount /mnt/run/base read-only. But there are still error messages.
We fixed it in systemd in https://github.com/systemd/systemd/pull/25168 . So it will be fixed in uc24. And along this there is https://github.com/snapcore/core-base/pull/93 and https://github.com/snapcore/core-base/pull/83 which we could backport. On snapd side there was also https://github.com/snapcore/snapd/pull/11119 But this PR should not be merged. So I am closing this.