update-grub fails with degraded multi-device btrfs root filesystem

[Tv]

Here's what happens:

$ sudo update-grub
/usr/sbin/grub-probe: error: failed to get canonical path of .

Reading the shell scripts, this is grub-mkconfig running "/usr/sbin/grub-probe --target=device /".

Root filesystem is a (currently degraded) multi-device btrfs:

/dev/sda6 on / type btrfs (rw,degraded,subvol=@)

ProblemType: Bug
DistroRelease: Ubuntu 13.04
Package: grub-common 2.00-13ubuntu3
ProcVersionSignature: Ubuntu 3.8.0-19.29-generic 3.8.8
Uname: Linux 3.8.0-19-generic x86_64
ApportVersion: 2.9.2-0ubuntu8
Architecture: amd64
Date: Mon Sep 23 14:50:20 2013
InstallationDate: Installed on 2013-09-23 (0 days ago)
InstallationMedia: Ubuntu-Server 13.04 "Raring Ringtail" - Release amd64 (20130423.1)
MarkForUpload: True
ProcEnviron:
 SHELL=/bin/bash
 TERM=linux
 PATH=(custom, no user)
 LANG=en_US.UTF-8
SourcePackage: grub2
UpgradeStatus: No upgrade log present (probably fresh install)

[Tv (tv42)]

Alright, I restored the second hard drive, here's what happens when not degraded:

tv@btraid:~$ sudo update-grub
[sudo] password for tv:
Generating grub.cfg ...
/usr/sbin/grub-probe: error: cannot find a GRUB drive for /dev/sda6
/dev/sdb6. Check your device.map.
Found linux image: /boot/vmlinuz-3.8.0-19-generic
Found initrd image: /boot/initrd.img-3.8.0-19-generic
/usr/sbin/grub-probe: warning: Couldn't find physical volume `(null)'. Some modules may be missing from core image..
/usr/sbin/grub-probe: warning: Couldn't find physical volume `(null)'. Some modules may be missing from core image..
/usr/sbin/grub-probe: warning: Couldn't find physical volume `(null)'. Some modules may be missing from core image..
/usr/sbin/grub-probe: error: cannot find a GRUB drive for /dev/sda6
/dev/sdb6. Check your device.map.
/usr/sbin/grub-probe: warning: Couldn't find physical volume `(null)'. Some modules may be missing from core image..
/usr/sbin/grub-probe: warning: Couldn't find physical volume `(null)'. Some modules may be missing from core image..
/usr/sbin/grub-probe: warning: Couldn't find physical volume `(null)'. Some modules may be missing from core image..
Found memtest86+ image: /memtest86+.bin
done
tv@btraid:~$ sudo btrfs filesystem show
Label: 'btraid' uuid: a77a5af6-21c1-4469-8db5-1d166087a3b3
 Total devices 2 FS bytes used 882.72MB
 devid 2 size 18.13GB used 3.28GB path /dev/sdb6
 devid 1 size 18.13GB used 3.29GB path /dev/sda6

Btrfs v0.20-rc1
tv@btraid:~$

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Marcus Tomlinson (marcustomlinson)]

This release of Ubuntu is no longer receiving maintenance updates. If this is still an issue on a maintained version of Ubuntu please let us know.

[Launchpad Janitor (janitor)]

[Expired for grub2 (Ubuntu) because there has been no activity for 60 days.]