[systemd-devel] udev rule with SYSTEMD_WANTS not working if included in initramfs

[Marcin Szewczyk]

Hi,

I have a problem with a udev rule which should start a service if a
device (ttyS0) is detected. It uses the SYSTEMD_WANTS env.

When the rule is present in /etc/udev/rules.d/ but not in the initramfs
-- the service is properly activated.

When the rule is present in both /etc/udev/rules.d/ and the initramfs --
the service is not activated (loaded inactive dead). In this case
systemd's log says[1]:

    systemd[1]: sys-devices-pnp0-00:03-tty-ttyS0.device: sys-devices-pnp0-00:03-tty-ttyS0.device lost dependency Wants=foobar.service
    systemd[1]: foobar.service: foobar.service lost dependency WantedBy=sys-devices-pnp0-00:03-tty-ttyS0.device
    systemd[1]: foobar.service: foobar.service lost dependency ReferencedBy=sys-devices-pnp0-00:03-tty-ttyS0.device
    systemd[1]: sys-devices-pnp0-00:03-tty-ttyS0.device: sys-devices-pnp0-00:03-tty-ttyS0.device lost dependency References=foobar.service

The rule worked for me with systemd v232 on an older Debian. Now with
Debian Buster and systemd v241 it doesn't. I think it may be associated
with a change introduced by a commit[2] described as:

    core: remove SYSTEMD_WANTS udev property configured dependencies at the right moment

…and more generally with PR #7186 [3].

What does "the right moment" mean? Do I violate some implicit assumption
by adding the udev rule to initramfs? BTW, it happens automatically
because initramfs-tools copy all files from /etc/udev/rules.d/ to
initramfs. Having the rule at such an early stage was not my objective.
Should I move the rules file to some other location so it's not copied
to initramfs? Should I add some other criterion to disable the rule
during initramfs and allow it to execute during a later booting phase?

Nevertheless, I am most interested in understanding why it does not
work. Maybe the failure just correlates with the rule's presence in the
initramfs but the direct cause is different?


[1]: https://github.com/systemd/systemd/commit/c999cf385aee08295dc204d98585cb4001d08ade
[2]: https://github.com/systemd/systemd/commit/38b9b72ec10fa8875b1b7c93bc7ca6b8a2d5486e
[3]: https://github.com/systemd/systemd/pull/7186

-- 
Marcin Szewczyk
http://wodny.org