Grub doesn’t fully revert Zsys snapshots

[Maurice Volaski]

To reproduce this bug, take the following steps

Install Ubuntu 20.04.3 using a ZFS-based root.
Boot into it and get past the initial startup screens.

Install a package like htop to force the creation of a ZSys snapshot and for additional confirmation in the user space, create a file on the desktop.

You should see the file and htop should be able to run.

Reboot and hit escape to enter the grub screen. Choose the history menu and choose the snapshot you just created. Then hit enter and then choose the option to revert system and user data and then enter again.

Most of the time, this will work fine, but not absolutely *every* time. Every so often, the system will hang after the last step (forcing you to hard reboot), and the snapshots may or may not have been reverted.

One thing I noticed while doing this on a fresh install is that other part of the system (the upgrade software) creates its own snapshots. It's not clear if these have to be there to trip the system up into not reverting. I think I reproduced once without them.

Note this is different from bug https://bugs.launchpad.net/ubuntu/+source/zsys/+bug/1949490, which applies to ordinarily created snapshots.