====== machine-id ======

Read only root FS:
  * with preinit script (before real /sbin/init)
    * tmpfs overlay on ''/etc'' mounted
    * bind mount persistent ''/mnt/state/machine-id'' on ''/etc/machine-id''.
  * first boot works well
  * after reboot systemd complains:

<code>
systemd[1]: Starting Commit a transient machine-id on disk...
systemd-machine-id-setup[611]: /etc/machine-id is not on a temporary file system.
systemd-machine-id-commit.service: Main process exited, code=exited, status=1/FAILURE
systemd-machine-id-commit.service: Failed with result 'exit-code'.
systemd[1]: Failed to start Commit a transient machine-id on disk.
</code>

[[https://www.linux.org/docs/man8/systemd-machine-id-commit.html|SYSTEMD-MACHINE-ID-COMMIT(1)]]

It runs because of conditions defined in unit file:

<file ini systemd-machine-id-commit.service>
[Unit]
DefaultDependencies=no
Conflicts=shutdown.target
Before=shutdown.target
After=local-fs.target first-boot-complete.target
ConditionPathIsReadWrite=/etc
ConditionPathIsMountPoint=/etc/machine-id

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=systemd-machine-id-setup --commit
</file>


This unit is designed to be run when ''/etc/machine-id'' is in transient state (e.g. bind mounted to tmpfs by systemd),
so ''ConditionPathIsMountPoint=/etc/machine-id'' is fulfilled in our case.

As workarund ''ConditionFirstBoot=yes'' is added:
<file ini /etc/systemd/system/systemd-machine-id-commit.service.d/bind-mount-fix.conf>
[Unit]
ConditionPathIsReadWrite=|/etc
ConditionPathIsReadWrite=|/mnt/state/machine-id
ConditionFirstBoot=yes
ConditionPathIsMountPoint=/etc/machine-id
</file>

Second approach is [[https://github.com/systemd/systemd/issues/39438|persisting /etc/machine-id on system which is bootstrapped from immutable /usr, with tmpfs on / #39438]]:

<file ini /etc/systemd/system/systemd-machine-id-commit.service.d/bind-mount-fix.conf>
[Unit]
ConditionPathIsReadWrite=
ConditionPathIsReadWrite=/var/mutable/etc/
ConditionPathIsMountPoint=
ConditionPathIsMountPoint=/var/mutable/etc/machine-id

[Service]
ExecStart=
ExecStart=systemd-machine-id-setup --commit --root=/var/mutable/
</file>

More about problem:
[[https://github.com/systemd/systemd/issues/14131|Various use-cases for first boot/machine-id are broken #14131]]