update-done: add minimal tool to manage system updates for /etc and /var, if /usr has changed

In order to support offline updates to /usr, we need to be able to run
certain tasks on next boot-up to bring /etc and /var in line with the
updated /usr. Hence, let's devise a mechanism how we can detect whether
/etc or /var are not up-to-date with /usr anymore: we keep "touch
files" in /etc/.updated and /var/.updated that are mtime-compared with
/usr. This means:

Whenever the vendor OS tree in /usr is updated, and any services that
shall be executed at next boot shall be triggered, it is sufficient to
update the mtime of /usr itself. At next boot, if /etc/.updated and/or
/var/.updated is older than than /usr (or missing), we know we have to
run the update tools once. After that is completed we need to update the
mtime of these files to the one of /usr, to keep track that we made the
necessary updates, and won't repeat them on next reboot.

A subsequent commit adds a new ConditionNeedsUpdate= condition that
allows checking on boot whether /etc or /var are outdated and need
updating.

This is an early step to allow booting up with an empty /etc, with
automatic rebuilding of the necessary cache files or user databases
therein, as well as supporting later updates of /usr that then propagate
to /etc and /var again.

1 files changed, 1 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
index cdb2ac9dbf..bcebb96e0c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -109,6 +109,7 @@
 /systemd-tty-ask-password-agent
 /systemd-uaccess
 /systemd-udevd
+/systemd-update-done
 /systemd-update-utmp
 /systemd-user-sessions
 /systemd-vconsole-setup