This service is essentially a random sleep() call that old software that
is unable to properly handle modern hardware initialisation being
asynchronous can use to *mostly* get by.
Over the years is has caused so many issues in NixOS, like introducing
long delays in the boot process[1], complete deadlocks[2] and even failures
when reloading services or activating a new configuration[3].
systemd has been discouraging its use since 2011 (15 years ago!), then
it officially deprecated it in 2018 and since 2020 it started to show on
every boot a huge wall of text calling out all the offending services
that still use it.
Around 2021 we managed to fix around 15 NixOS modules[4] that were
relying on systemd-udev-settle and practically ZFS[5] was the only
remaining one. However, since then, people have actually started to
bring it back with new services[6][7][8]. This is not acceptable.
So, to prevent any more (lazy) uses of systemd-udev-settle, we stop
providing the systemd-udev-settle.service unit entirely. For existing
modules that unfortunately still need it, we replace it with the command
`udevadm settle --timeout=180`, which is all that the service does.
Hopefully this will also increase the awareness that it's bad and
something to be fixed.
Note: I tested this change using
- `nixosTests.zfs`,
- `nixosTests.ifstate`,
- `nixosTests.misc`,
- `nixosTests.openvswitch`,
- `nixosTests.predictable-interface-names`
- `nixosTests.nvidia-container-toolkit`
and making sure that none of the 165 packages that provide upstream
units (via the `systemd.packages` option) had a dependency on
systemd-udev-settle.service.
[1]: https://github.com/NixOS/nixpkgs/pull/25311
[2]: https://github.com/NixOS/nixpkgs/issues/107341
[3]: https://github.com/NixOS/nixpkgs/pull/113804
[4]: https://github.com/NixOS/nixpkgs/issues/73095
[5]: https://github.com/openzfs/zfs/issues/10891
[6]: https://github.com/NixOS/nixpkgs/pull/257525#discussion_r1442702970
[7]: https://github.com/NixOS/nixpkgs/pull/460075
[8]: https://github.com/NixOS/nixpkgs/pull/284507
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