[systemd-devel] preset enables everything by default

[Michael Marineau]

I didn't catch this behavior when it was first introduced since
originally it was much harder to trigger systemd's "empty /etc" logic
but now that it only requires /etc/machine-id to be missing it is
quite easy, booting a new instance from an image for example. By
default applying presets enables everything unless there is a preset
config that defines otherwise. I found this to be rather surprising,
booting a fresh machine reported all sorts of failures by trying to
start oodles of unconfigured services. Also the options are only
"enable" and "disable" so the existing pattern of pre-preconfiguring a
reference host and then creating an image (EC2 AMIs for example) won't
work very well since the preset defaults will clobber what the user
enabled/disabled. (assuming the user properly clears machine-id before
creating the image which may be rare, in all likelihood many people
just get away with having non-unique machine ids)

This behavior is probably ok in the case of interactively using
systemctl preset and preset-all when it is known that the user
explicitly asked the system to do something and can see what it did.
In the case of the system booting I would expect "do nothing" to be
the default when no preset file explicitly sates otherwise.

Is there a particular reason for the existing behavior? Would
switching the default to disable be reasonable or should the automatic
at boot mode gain a third "do nothing" option? Not sure where the
safest and least-surprising behavior lies while continuing to provide
this preset functionality.

Personally I've always found the enable/disable terminology to be
incredibly misleading to begin with since it only refers to
configuration in /etc and units can be equally activated in /usr. If
disable and mask were equivalent then the distro's "presets" would
just be whatever is in /usr and there won't be a need for this extra
preset mechanism to initialize /etc.

Cheers,
Mike