pid1: add "soft-reboot" reboot method

This adds a new mechanism for rebooting, a form of "userspace reboot"
hereby dubbed "soft-reboot". It will stop all services as in a usual
shutdown, possibly transition into a new root fs and then issue a fresh
initial transaction. The kernel is not replaced.

File descriptors can be passed over, thus opening the door for leaving
certain resources around between such reboots.

Usecase: this is an extremely quick way to reset userspace fully when
updating image based systems, without going through a full
hardware/firmware/boot loader/kernel/initrd cycle. It minimizes "grayout time"
for OS updates. (In particular when combined with kernel live patching)

1 files changed, 16 insertions, 0 deletions

diff --git a/units/systemd-soft-reboot.service b/units/systemd-soft-reboot.service
new file mode 100644
index 0000000000..35ba3a9fc4
--- /dev/null
+++ b/units/systemd-soft-reboot.service
@@ -0,0 +1,16 @@
+#  SPDX-License-Identifier: LGPL-2.1-or-later
+#
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+[Unit]
+Description=Reboot System Userspace
+Documentation=man:systemd-soft-reboot.service(8)
+DefaultDependencies=no
+Requires=shutdown.target umount.target final.target
+After=shutdown.target umount.target final.target
+SuccessAction=soft-reboot-force