Fix empty fs_uuid on multi-device Btrfs#440
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On a multi-device Btrfs filesystem `grub-probe --target=device /` prints
one device per line (e.g. /dev/vda3 and /dev/vdb). The whole multi-line
result was then passed to `grub-probe --device "${root_device}"
--target=fs_uuid`, which cannot handle several devices at once and
returns an empty string. The blkid fallback fails for the same reason,
so grub-btrfs aborts with "Cannot determine UUID".
Pipe the device probe through `head -n1` and keep only the first member
device. Every member of a Btrfs pool reports the same pool-wide fs UUID,
so the first device is sufficient. For single-device setups `head -n1`
is a no-op, so existing configurations are unaffected.
The same one-line-per-device behaviour applies to a /boot that lives on
a multi-device Btrfs, so the identical guard is added to boot_device.
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Problem
On a multi-device Btrfs filesystem,
grub-probe --target=device /prints one device per line, e.g.:The whole multi-line value is then passed to
grub-probe --device "${root_device}" --target=fs_uuid, which cannot handle several devices at once and returns an empty string. Theblkidfallback fails for the same reason (it gets a multi-line device argument), so41_snapshots-btrfsaborts with:This makes grub-btrfs unusable on any multi-device Btrfs (RAID0/1/10, single-profile with added devices, etc.). The affected line was introduced in #378.
Fix
Pipe the device probe through
head -n1and keep only the first member device. Every member of a Btrfs pool reports the same pool-wide fs UUID, so the first device is sufficient. For single-device setupshead -n1is a no-op, so existing configurations are unaffected.The identical one-line-per-device behaviour applies to a
/bootthat lives on a multi-device Btrfs, so the same guard is added toboot_devicefor consistency.Testing
Reproduced and verified on Fedora Server 43, root on a 2-device Btrfs (
/dev/vda3+/dev/vdb),/booton a separate single-device XFS partition. Before the fixgrub-mkconfigaborted with the "Cannot determine UUID" error; after the fix the correct fs UUID is resolved andgrub-btrfs.cfgis generated as expected.bash -npasses.