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 and no hostname
was specified via Ignition then we'll make sure the hostname from
the ip= karg gets applied to the real root (persistently).

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() {
@@ -39,6 +41,42 @@ 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() {
+    if [ -e '/sysroot/etc/hostname' ]; then
+        echo "info: hostname is defined in the real root"
+        echo "info: will not attempt to propagate initramfs hostname"
+        return 0
+    fi
+    # Detect if any hostname was provided via static ip= kargs
+    # 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
@@ -91,6 +129,7 @@ main() {
         echo "info: coreos.no_persist_ip karg detected"
         echo "info: skipping propagating initramfs settings"
     else
+        propagate_initramfs_hostname
         propagate_initramfs_networking
     fi