os-prober: generates buggy btrfs entries in grub.cfg

[Achim Bohnet]

17.10 & 16.04: Choosing a grub menu entry that references a parallell installed ubuntu system on a btrfs FS does not work.

I've installed neonuser (xenial based distro) in parallel to Kubuntu artful.
It's mounted via

UUID=879cfb57-f580-4326-a2fe-8dc83e0013f4 /os-neon-user btrfs defaults,subvol=@ 0 2
UUID=879cfb57-f580-4326-a2fe-8dc83e0013f4 /os-neon-user/home btrfs defaults,subvol=@home 0 2

Running update-grub on artful generates grub.cf entries like for the Neon User system:

..
               linux /boot/vmlinuz-4.13.0-32-generic root=/dev/sda3
               initrd /boot/initrd.img-4.13.0-32-generic
..

with does not work.

Workaround: Edit the entry to

               linux /@/boot/vmlinuz-4.13.0-32-generic root=/dev/sda3 rootflags=subvol=@
                initrd /@/boot/initrd.img-4.13.0-32-generic

Boots the other ubuntu OS on the btrfs filesys.

I can reproduce the bug on two systems (desktop with xenial, laptop with artful)

The grub.cfg entry of the currently booted OS on a btrfs FS is okay. So update-grub handles
/ on btrfs okay. But os-prober on /whatever with btrfs does not.

Achim

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: os-prober 1.74ubuntu1
ProcVersionSignature: Ubuntu 4.13.0-36.40-generic 4.13.13
Uname: Linux 4.13.0-36-generic x86_64
NonfreeKernelModules: openafs wl
ApportVersion: 2.20.7-0ubuntu3.7
Architecture: amd64
CurrentDesktop: KDE
Date: Sun Feb 25 10:55:13 2018
EcryptfsInUse: Yes
InstallationDate: Installed on 2018-01-13 (42 days ago)
InstallationMedia: Kubuntu 17.10 "Artful Aardvark" - Release amd64 (20180105.2)
SourcePackage: os-prober
UpgradeStatus: No upgrade log present (probably fresh install)