[RFC kernel-helper] proxmox-boot: find root dev for grub default entry as update-grub does

[Stoiko Ivanov <s.ivanov@proxmox.com>]

Reading the ID that update-grub/grub-mkconfig uses for generating
unique ids for boot-entries from the existing grub.cfg breaks if the
following update-grub call changes the device id, effectively
rendering the pin setting useless.

Reported in our community-forum in context of renamed /dev/nvmeXnYpZ:
https://forum.proxmox.com/threads/186882/

Reproduced in a VM, by attaching a second disk/partition to a
single-disk vdev in rpool (/dev/sda3 becomes /dev/sda3_/dev/sdb3 for
grub).

Instead of relying on potentially outdated information aquire the
root device id the way update grub does it (in /etc/grub.d/10_linux)

I did shortly consider using the displayed title - but this is
explicitly discouraged in the grub-documentation:
https://www.gnu.org/software/grub/manual/grub/grub.html#Simple-configuration
(although I did not manage to get the menu entries to be displayed in
anything by English)

Fixes: 83a41a1 ("fix #3761: proxmox-boot: add pin/unpin for kernel-version")
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
---
Notes:
sending as RFC, as I'm a bit reluctant to re-create more of update-grub in
our wrapper for it - and given the options would consider:
"Advanced options for Proxmox VE GNU/Linux>Proxmox VE GNU/Linux, with Linux $kver"
actually more readable, but am not sure if there is an option to really
end up with this not being stable, as laid out in the grub-docs

 src/proxmox-boot/functions | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/proxmox-boot/functions b/src/proxmox-boot/functions
index a650c6f..54fc9d9 100755
--- a/src/proxmox-boot/functions
+++ b/src/proxmox-boot/functions
@@ -130,11 +130,11 @@ set_grub_default() {
 	if [ -z "${kver}" ]; then
 		rm -f "${GRUB_PIN_SNIPPET}"
 	else
-		# grub menu entry ids contain the internal root-device id (e.g. for zfs the GUID of
-		# the pool printed in hex) as this is independent of the ESP (or grub location)
-		# take it from /boot/grub/grub.cfg
-		root_devid=$(sed -rn "s/.*gnulinux-advanced-(.+)['] \{$/\1/p" \
-			/boot/grub/grub.cfg)
+		# grub menu entry ids contain the internal root-device id
+		# follow /etc/grub.d/10_linux in how to aquire it
+		. /usr/share/grub/grub-mkconfig_lib
+		GRUB_DEVICE=$(grub-probe --target=device /)
+		root_devid=$(grub_get_device_id "${GRUB_DEVICE}")
 		entry="gnulinux-advanced-${root_devid}>gnulinux-${kver}-advanced-${root_devid}"
 		echo "GRUB_DEFAULT=\"${entry}\"" > "${GRUB_PIN_SNIPPET}"
 	fi
-- 
2.47.3