overlay: 15fcos: add systemd unit to migrate to systemd-resolved

This systemd unit migrates the /etc/resolv.conf file on systems to
point to ../run/systemd/resolve/stub-resolv.conf if users haven't
set up a custom resolv.conf. It will only run on Fedora 33 systems
and will only execute once (a single migration).

Fixes: coreos/fedora-coreos-tracker#646
(cherry picked from commit 56b0ceb)

overlay.d/15fcos/usr/lib/systemd/system-preset/45-fcos.preset

@@ -8,3 +8,5 @@ enable coreos-reset-stub-resolv-selinux-context.path
 # Run once on startup to prevent some race conditions with
 # NetworkManager and systemd-resolved starting up.
 enable coreos-reset-stub-resolv-selinux-context.service
+# Enable the migration service to systemd-resolved
+enable coreos-migrate-to-systemd-resolved.service

overlay.d/15fcos/usr/lib/systemd/system/coreos-migrate-to-systemd-resolved.service

@@ -0,0 +1,17 @@
+# This unit is used to migrate existing systems running default
+# configurations to systemd-resolved as part of the Fedora 33 change.
+# https://fedoraproject.org/wiki/Changes/systemd-resolved
+[Unit]
+Description=CoreOS Migrate to Systemd Resolved
+Documentation=https://github.com/coreos/fedora-coreos-tracker/issues/646
+# Run before NetworkManager and systemd-resolved start
+Before=NetworkManager.service
+Before=systemd-resolved.service
+
+[Service]
+Type=oneshot
+RemainAfterExit=true
+ExecStart=/usr/libexec/coreos-migrate-to-systemd-resolved
+
+[Install]
+WantedBy=multi-user.target

overlay.d/15fcos/usr/libexec/coreos-migrate-to-systemd-resolved

@@ -0,0 +1,35 @@
+#!/usr/bin/bash
+set -euo pipefail
+
+# This script will migrate a FCOS system running the default configuration
+# to systemd-resolved, which is standard for Fedora 33. For non OSTree Fedora
+# systems the RPM post scriptlet takes care of the transition.
+# https://fedoraproject.org/wiki/Changes/systemd-resolved
+# https://github.com/coreos/fedora-coreos-tracker/issues/646
+
+# If we're not on f33 yet then hard exit.
+source /usr/lib/os-release
+if [ "$VERSION_ID" == "32" ]; then
+    echo 'Delaying systemd-resolved migration until Fedora 33'
+    exit 0
+fi
+
+# Here we have a bit of logic:
+#
+#   - Does /etc/resolv.conf exist?
+#      - It should since tmpfiles will create it otherwise
+#      - If it doesn't, do nothing.
+#   - Is /etc/resolv.conf currently being managed by NetworkManager?
+#      - This is the default configuration in <F33.
+#      - If yes, then create symlink to stub-resolv.conf
+#   
+if [ -f /etc/resolv.conf ]; then
+    if [ "$(head -n 1 /etc/resolv.conf)" == '# Generated by NetworkManager' ]; then
+       echo 'Detected that NetworkManager is managing resolv.conf'
+       echo 'Migrating to systemd-resolved for Fedora 33 change'
+       ln -fsv ../run/systemd/resolve/stub-resolv.conf /etc/resolv.conf
+    fi
+fi
+
+# Only run this on Fedora 33 once
+systemctl disable coreos-migrate-to-systemd-resolved.service