F37 proposal: Preset All Systemd Units on First Boot (Self-Contained Change proposal)

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https://fedoraproject.org/wiki/Changes/Preset_All_Systemd_Units_on_First_Boot

This document represents a proposed Change. As part of the Changes
process, proposals are publicly announced in order to receive
community feedback. This proposal will only be implemented if approved
by the Fedora Engineering Steering Committee.

== Summary ==
Systemd will execute the equivalent of '''systemctl preset-all''' when
an unconfigured system is booted
([https://www.freedesktop.org/software/systemd/man/machine-id.html#First%20Boot%20Semantics
"First Boot"] condition). This means that units will be enabled or
disabled according to the preset configuration. We currently do the
equivalent of '''systemctl preset-all --preset-mode=enable-only''',
and this will be extended to also disable units, i.e. '''systemctl
preset-all --preset-mode=full'''. Any units which are manually
symlinked but presets say they shouldn't (which is against the
packaging guidelines for packaged units) will be disabled.

Note that this applies to "first boot" only, i.e. to boot from an
image without ''/etc'' fully populated. In does not apply to systems
that were installed using Anaconda.

== Owner ==
* Name: [[User:jlebon| Jonathan Lebon]]
* Name: [[User:Zbyszek| Zbigniew Jędrzejewski-Szmek]]
* Email: zbyszek at in.waw.pl, jlebon at redhat.com


== Detailed Description ==
Our [https://docs.fedoraproject.org/en-US/packaging-guidelines/Scriptlets/#_systemd
guidelines] say that units that are packaged in rpms must be enabled
through macros (''%systemd_post'') and the preset system. Almost all
packages conform to this, so effectively their enablement state
follows the preset config. When the system is installed, or more
precisely when ''systemd.rpm'' is installed, we do ''preset-all''. But
for historical reasons, when booting an unconfigured system ("first
boot") we only 'enable' units in this fashion. In Fedora and RHEL
CoreOS, some symlinks are created in the golden image, but should be
disabled in the local image after local preset configuration has been
inserted. To make this work, the call in systemd will be changed to
execute the equivalent of ''preset-all --preset-mode=full'', making
enablement during "first boot" more like enablement during an Anaconda
installation.


== Benefit to Fedora ==
* CoreOS can insert local preset configuration through Ignition and
this configuration will be applied on the first boot.
* Users can do something similar with local preset configuration on
distributed images.
* The system is made a bit simpler and easier to understand, because
we can say that "units are enabled/disabled after installation as
specified by the preset system".
* Users can call ''systemctl preset-all'' at any time to apply
preset-configuration. If no local changes to configuration have been
made, ''preset-all'' would make no changes to unit state. If units
have been enabled or disabled, ''preset-all'' would return unit
enablement to the pristine state after installation.

== Scope ==
* Proposal owners:
** implement patch for systemd to configure preset-all mode on first
boot (https://github.com/systemd/systemd/pull/15205)
** build systemd with this mode changed to ''--preset-mode=full''
** provide pull requests for two packages which have been identified
to not use the preset system for enablement to conform to the
packaging guidelines
(https://bugzilla.redhat.com/show_bug.cgi?id=2070862,
https://bugzilla.redhat.com/show_bug.cgi?id=2070726)

* Other developers: review and merge the pull requests
* Release engineering: N/A
* Policies and guidelines: none, this change is about following the
guidelines more closely
* Trademark approval: N/A (not needed for this Change)
* Alignment with Objectives:

== Upgrade/compatibility impact ==
None.

== How To Test ==
* On a newly installed system, with arbitrary set of packages: call
''systemctl preset-all''. This should result in no changes.
* On a system which is booted from an unconfigured image (e.g. the new
Server VM image should qualify, see
[[Changes/Supplement-server-by-kvm-vm-image]]): before the first boot,
enable some units manually that are disabled in presets. After
booting, those units should be disabled again.

== User Experience ==
In general this change will be a noop for users, because it only
applies to "first boot", i.e. to the case when a system is booted from
a distributable image without local configuration and is configured
when initially booted. In case where Anaconda is used to install
images, /etc is populated before the first boot and the "first boot"
condition never applies, thus this change is irrelevant. On systems
installed from a "golden image" such as Fedora CoreOS, units will
follow the preset configuration more closely. Thanks to the fixes to
make packages conform to packaging guidelines, users can call
'''preset-all''' to return the system to defaults.

== Dependencies ==


== Contingency Plan ==
* Contingency mechanism: (What to do?  Who will do it?) Systemd
maintainers: flip back the default in systemd, rebuild.
* Contingency deadline: N/A (not a System Wide Change) This can be
done at any time up to the release.
* Blocks release? No.

== Documentation ==
N/A (not a System Wide Change)





-- 
Ben Cotton
He / Him / His
Fedora Program Manager
Red Hat
TZ=America/Indiana/Indianapolis
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