Problem

If you machine is force shutdown during the process of pacman -Syu, it’s possible the packages are in partial update state.
To fix this problem, we need to do:

1. Find out how bad it is. (How many packages are corrupted)
2. Re-install these corrupted packages.

Solution

Setup the living-os

Insert your living-os sticker (You can make this sticker folloing the arch wiki).

Step:

• Mount the root partition of your host os into /mnt
• Connect to the Internet using iwctl.

Check the corrupted package list

Let’s say your pacman in the host os is corrupted, so that you need to use the pacman inside living os.
The command looks like pacman --root /mnt ....

To get the corrupted packages list in the host-os, issue

sudo LC_ALL=C pacman --root /mnt -Qkk >> list_packages.txt 2>&1
awk '/mtree/ {print}' ./list_packages.txt > filter_packages.txt
awk -F ':' '{print $1}' ./filter_packages.txt > collect_packages.txt
grep -v "error" collect_packages.txt > packages.txt

And all the corrupted packages are in packages.txt

Fix the pacman inside host-os

First, we need to update the keyring inside living os using pacman -Sy archlinux-keyring to resolve the outdated gpg keys.
Then, you can use pacman --root /mnt -S $(< packages.txt) to re-install all the corrupted packages.

If you need to modify mirrorlist, then you should modify the mirrorlist in living os, and use pacman -Syy to update databse.

Chroot into the host-os and do a fully-package upgrade again

You need to chroot into the host-os using arch-chroot /mnt
And issue pacman -Syu to upgrade all packages.
If pacman says “there is nothing to do”, then it done.

Fix the bootloader

To fix the bootloader

mount /dev/<root_partition> /mnt
mount /dev/<EFI_system_partition> /mnt/boot
arch-chroot /mnt
grub-mkconfig -o /boot/grub/grub.cfg
grub-install --target=x86_64-efi --efi-directory=/boot --bootloader-id=GRUB
pacman -S linux