[systemd-devel] Hosts without /etc/machine-id on boot

[Didier Roche]

Hey,

Some other topic related to "empty /etc" discussions: when preparing 
some generic distro images, we are have the desire to ensure that all 
new instances will get a different /etc/machine-id file.
As part of the empty /etc at boot, we first thought that removing 
/etc/machine-id would be sufficient, however, the instance then doesn't 
generate a new machine-id file and complain heavily.

The new debug message of systemd 216+ helped shading some lights on it: 
http://cgit.freedesktop.org/systemd/systemd-stable/diff/src/core/machine-id-setup.c?h=v216-stable&id=896050eeb3acbf4106d71204a5173b4984cf1675, 
and adding debug statement in machine_id_setup() from 
src/core/machine-id-setup.c just before "open(etc_machine_id, 
O_RDWR|O_CREAT|O_CLOEXEC|O_NOCTTY, 0444)" explains what happens with 
/proc/mounts:

[    2.119041] systemd[1]: rootfs / rootfs rw
[    2.126775] systemd[1]: /dev/disk/by-uuid/ec8166e5-d5ed-45ec-b350-6cf5773904ac / ext4 ro,relatime,data=ordered


It's clear then that at this stage of the boot process / is readonly.
The error message (and code) will say that in this case, what is 
supported is an empty /etc/machine-id. After reboot, the consequence is 
that /etc/machine-id is mounted as a tmpfs:

tmpfs on /etc/machine-id type tmpfs (ro,relatime,size=204948k,mode=755)

However, this means is that each boot of this instance will result in a 
different machine-id, which isn't what is desired in the empty /etc case 
after a factory reset. I know that there is the utility 
systemd-machine-id-setup that we are running on systemd postinst in 
debian/ubuntu, but that doesn't cover the factory reset one.

Is there anything obvious that I'm missing to cover that case or 
anything in the pipe?
Cheers,
Didier
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