Comment 0 for bug 1889556

Failure to install new grub core to the specified device does not correctly prevent upgrade to incompatible modules (LP: #1889509)

$ sudo debconf-get-selections |grep sda
grub-pc grub-pc/install_devices_disks_changed multiselect /dev/sda
grub-pc grub-pc/install_devices multiselect /dev/sda

$ mount|grep nvme
/dev/nvme0n1p1 on / type ext4 (rw,relatime,discard,data=ordered)

$ ls /dev/sda
ls: cannot access '/dev/sda': No such file or directory

$ sudo env DEBIAN_FRONTEND=noninteractive apt full-upgrade -y
...
Get:10 http://us-west-2.ec2.archive.ubuntu.com/ubuntu xenial-updates/main amd64 grub-pc-bin amd64 2.02~beta2-36ubuntu3.26 [891 kB]
...
Installing for i386-pc platform.
grub-install: error: cannot find a GRUB drive for /dev/sda. Check your device.map.
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-4.4.0-1111-aws
Found linux image: /boot/vmlinuz-4.4.0-1109-aws
Found initrd image: /boot/initrd.img-4.4.0-1109-aws
done
...

# update-grub failed during the install but the return code is 0
$ echo $?
0

# The package is installed without apparent error, but the instance will fail to reboot (LP: #1889509)
$ dpkg -l|grep grub-pc
ii grub-pc 2.02~beta2-36ubuntu3.26 amd64 GRand Unified Bootloader, version 2 (PC/BIOS version)
ii grub-pc-bin 2.02~beta2-36ubuntu3.26 amd64 GRand Unified Bootloader, version 2 (PC/BIOS binaries)

# If I reboot it will fail to boot:
Booting from Hard Disk 0...
error: symbol `grub_calloc' not found.
Entering rescue mode...
grub rescue> _

---

Xenial in AWS (us-west-2 ami-060d1be0dd4526759 built on 20200611)
The debconf for grub was not set to the correct device when cloud-init first ran (LP: #1877491) or when the fix for that was applied (LP: #1889555)
The fact that grub-install fails during the upgrade but does not fail the package install (and cause a rollback) means that how we have a mismatch between grub core and modules which breaks boot.