Add support for GRUB patches from SUSE

[StollD]

Some GRUBs out there (Fedora, openSUSE) have an option that makes all paths relative to the default subvolume of the filesystem. This can be used to include /boot in your snapshots and roll them back without having to regenerate grub.cfg.

However, enabling that option will break grub-btrfs, because loading the kernel from a different snapshot requires the paths to be absolute. To make this work, GRUB has to be told explicitly to access the root subvolume when booting to a snapshot.

I hope this is acceptable, because it's essentially a workaround for a downstream patch of GRUB.

[Schievel1]

hm, if that is a downsteam patch some distros apply to grub, couldn't this PR be applied as a downstream patch for those distros as well? I mean when the package is created.

[StollD]

I can only speak about Fedora, but grub-btrfs is not packaged there. I've been installing it with make install, so I figured I would propose this here. Especially since I don't know if it might apply to any other distro out there.

[Schievel1]

I have been considering this for a while now, but in the end I think grub-btrfs should stay distro agnostic.
Nevertheless this would work for me if you name the variable a bit different (not mentioning a distro but instead describe what it actually does) and maybe add a few lines about that feature in the readme.

[StollD]

I just noticed that I never actually linked the patchset I am talking about, so I'd like to apologize for that.

• The patches are in here, prefixed with grub2-btrfs: https://code.opensuse.org/package/grub2/tree/master
• The patch that modifies how grub.cfg is generated is: https://code.opensuse.org/package/grub2/blob/master/f/grub2-btrfs-05-grub2-mkconfig.patch

The SUSE_BTRFS_SNAPSHOT_BOOTING variable is not defined by me, it is defined by the patchset itself and enables the behaviour I described. I can't change its name, and I wouldn't want to document it in the readme because it is not defined by grub-btrfs.

I can add a comment or reword the commit to better explain the purpose of that variable though, whatever you prefer.

[Schievel1]

ooh so this is not something the grub-btrfs user has to set in the config, it is available to all scripts in /etc/grub.d?

[StollD]

Yeah, it is set in /etc/default/grub