Your local package database and/or metadata may become corrupted if an update process is interrupted, for example if you hard power off during an update.
Metadata corruption will generally manifest in a message similar to:
error: could not open file /var/lib/pacman/local/$pkgname-$pkgver-$pkgrel/desc: No such file or directory
error: could not open file /var/lib/pacman/local/$pkgname-$pkgver-$pkgrel/files: No such file or directory
error: could not open file /var/lib/pacman/local/$pkgname-$pkgver-$pkgrel/mtree: No such file or directory
during a package operation (install, update, etc.).
Note: $pkgname is the package name, $pkgver the version, $pkgrel the release
Fixing this issue involves manually creating a replacement file to allow package operations to continue, then putting the correct metadata back in place.
Step 1
Note the package for which metadata has been corrupted. It's the $pkgname part of the path.
Step 2
Create a temporary replacement file for any files which are currently missing, e.g.:
sudo touch /var/lib/pacman/local/$pkgname-$pkgver-$pkgrel/{desc,files,mtree}
This path must exactly match the text from the error message. For example, for
error: could not open file /var/lib/pacman/local/smbclient-4.10.2-0/desc: No such file or directory
you must type:
sudo touch /var/lib/pacman/local/smbclient-4.10.2-0/desc
Step 3
Reinstall the package.
The replacement file is blank and must be replaced with the correct content. The easiest way to do this is simply reinstall the package:
sudo pacman -S $pkgname
where $pkgname is the package you noted down previously.
For example, for
error: could not open file /var/lib/pacman/local/smbclient-4.10.2-0/desc: No such file or directory
you must type:
sudo pacman -S smbclient
If you see errors about other missing metadata files simply repeat Step 2.
Step 4
Continue with your life and be glad you're running Linux where an interrupted update doesn't simply kill your OS and require a reinstallation (unlike another OS I could mention...).