arch-nspawn: don't delete the guest gpg configuration

It's important to ensure the guest has up to date data because updating
a chroot after quite some time can potentially rely on updated
archlinux-keyring, something which the host machine either kept up to
date on or manually fixed, but it kills automation to mess around with
chroot configs like that. Alternatively, signed packages added with -I
need to work, and we assume the host is configured to accept these.

That is *not* a good reason to completely nuke whatever is in the guest,
though. A guest might have been manually configured to accept keys which
aren't accepted by the host; one example of this happening in practice,
is archlinux32 when building 32-bit packages from an archlinux host.

The right solution is to append to, rather thna overwrite, the chrooted
guest's pacman keyring.

To do this, we will use gpg's native facility to dump the keyring from
one GNUPGHOME and import it into another. We'd use pacman-key's --import
option directly, but this doesn't support passing custom options like
--import-options import-local-sigs
Finally use pacman-key's native facility to import the trust status from
the host.

While we are at it, fix a bug where we didn't respect the host's
pacman.conf settings for the GpgDir. While it isn't wildly likely a user
will choose to customize this, it is a valid and supported use case and
we must think about this ourselves.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Levente Polyak <anthraxx@archlinux.org>

arch-nspawn.in

@@ -80,7 +80,8 @@ build_mount_args() {
 }
 
 copy_hostconf () {
-	cp -a /etc/pacman.d/gnupg "$working_dir/etc/pacman.d"
+	gpg --homedir "$working_dir"/etc/pacman.d/gnupg/ --no-permission-warning --quiet --batch --import --import-options import-local-sigs "$(pacman-conf GpgDir)"/pubring.gpg >/dev/null 2>&1
+	pacman-key --gpgdir "$working_dir"/etc/pacman.d/gnupg/ --import-trustdb "$(pacman-conf GpgDir)" >/dev/null 2>&1
 	printf 'Server = %s\n' "${host_mirrors[@]}" >"$working_dir/etc/pacman.d/mirrorlist"
 
 	[[ -n $pac_conf ]] && cp "$pac_conf" "$working_dir/etc/pacman.conf"