coreos-teardown-initramfs-network: also propagate hostname

This is a forward port of 1f8184e (coreos#156). If the admin specified
a hostname in the ip= karg static networking config then we'll
want to make sure that gets applied to the real root (persistently)
as well.

Ideally in the future there will be better support for this in
NetworkManager itself as the `network-legacy` dracut module did
at least provide more support for setting the hostname via ip=
kargs than `network-manager` currently does. The discussion
about this problem is in [1]. The fix for that will most likely
implicate changes to the propagation code introduced here.

Fixes: coreos/fedora-coreos-tracker#466

[1] https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/419

dracut/30ignition/coreos-teardown-initramfs-network.sh

@@ -2,8 +2,10 @@
 # -*- mode: shell-script; indent-tabs-mode: nil; sh-basic-offset: 4; -*-
 # ex: ts=8 sw=4 sts=4 et filetype=sh
 
-# Load dracut library. Using getargbool() from dracut-lib.
+# Load dracut libraries. Using getargbool() and getargs() from
+# dracut-lib and ip_to_var() from net-lib
 type getargbool &>/dev/null || . /lib/dracut-lib.sh
+type ip_to_var &>/dev/null  || . /lib/net-lib.sh
 
 # Use tmpfiles.d to schedule SELinux relabel of files
 schedule_selinux_relabel() {
@@ -36,6 +38,36 @@ propagate_initramfs_networking() {
     fi
 }
 
+# Propagate the ip= karg hostname if desired. The policy here is:
+#
+#     - IF a hostname is specified in static networking ip= kargs
+#     - AND no hostname was set via Ignition (realroot `/etc/hostname`)
+#     - THEN we make the last hostname specified in an ip= karg apply
+#       permanently by writing it into `/etc/hostname`
+#
+# This may no longer be needed when the following bug is fixed:
+# https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/419
+propagate_initramfs_hostname() {
+    # run in a subshell so we don't pollute our environment
+    hostname=$(
+        last_nonempty_hostname=''
+        # Inspired from ifup.sh from the 40network dracut module. Note that
+        # $hostname from ip_to_var will only be nonempty for static networking.
+        for iparg in $(getargs ip=); do
+            ip_to_var $iparg
+            [ -n "$hostname" ] && last_nonempty_hostname="$hostname"
+        done
+        echo -n "$last_nonempty_hostname"
+    )
+    if [ -n "$hostname" ]; then
+        echo "info: propagating initramfs hostname (${hostname}) to the real root"
+        echo $hostname > /sysroot/etc/hostname
+        schedule_selinux_relabel /etc/hostname
+    else
+        echo "info: no initramfs hostname information to propagate"
+    fi
+}
+
 down_interface() {
     echo "info: taking down network device: $1"
     # On recommendation from the NM team let's try to delete the device
@@ -83,6 +115,7 @@ main() {
         echo 'info: coreos.no_persist_ip karg detected'
         echo "info: skipping propagating initramfs settings"
     else
+        propagate_initramfs_hostname
         propagate_initramfs_networking
     fi