fstab-generator: add support for volatile boots

This adds support for a new kernel command line option "systemd.volatile=" that
provides the same functionality that systemd-nspawn's --volatile= switch
provides, but for host systems (i.e. systems booting with a kernel).

It takes the same parameter and has the same effect.

In order to implement systemd.volatile=yes a new service
systemd-volatile-root.service is introduced that only runs in the initrd and
rearranges the root directory as needed to become a tmpfs instance. Note that
systemd.volatile=state is implemented different: it simply generates a
var.mount unit file that is part of the normal boot and has no effect on the
initrd execution.

The way this is implemented ensures that other explicit configuration for /var
can always override the effect of these options.  Specifically, the var.mount
unit is generated in the "late" generator directory, so that it only is in
effect if nothing else overrides it.

1 files changed, 21 insertions, 0 deletions

diff --git a/units/systemd-volatile-root.service.in b/units/systemd-volatile-root.service.in
new file mode 100644
index 0000000000..cc4e604e4c
--- /dev/null
+++ b/units/systemd-volatile-root.service.in
@@ -0,0 +1,21 @@
+#  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=Enforce Volatile Root File Systems
+Documentation=man:systemd-volatile-root.service(8)
+DefaultDependencies=no
+Conflicts=shutdown.target
+After=sysroot.mount
+Before=initrd-root-fs.target shutdown.target
+Conflicts=shutdown.target
+AssertPathExists=/etc/initrd-release
+
+[Service]
+Type=oneshot
+RemainAfterExit=yes
+ExecStart=@rootlibexecdir@/systemd-volatile-root yes /sysroot