Automated backups as explained in documentation overwrites SYSTEMD_WANTS

[httpdev]

In the documentation for setting up automated backups, the service to autostart is defined by ENV{SYSTEMD_WANTS}= instead of ENV{SYSTEMD_WANTS}+=. This will overwrite other autostart services attached by earlier udev rules.

[ThomasWaldmann]

Is that the case?

The docs state that the value is ADDED:

https://www.freedesktop.org/software/systemd/man/latest/systemd.device.html#SYSTEMD_WANTS=

[httpdev]

I was curious because the doc does say that it is added, so I ran a test: add another udev rule with higher priority (79-test-a.rules) that sets ENV{SYSTEMD_WANTS}=test-a.service, then check which of the services are run.

Output of udevadm monitor --property with ENV{SYSTEMD_WANTS}= in 80-backup.rules (old behavior)

UDEV  [238217.279129] add      /devices/pci0000:80/0000:80:14.0/usb4/4-6/4-6.1/4-6.1:1.0/host8/target8:0:0/8:0:0:0/block/sda/sda1 (block)
ACTION=add
DEVPATH=/devices/pci0000:80/0000:80:14.0/usb4/4-6/4-6.1/4-6.1:1.0/host8/target8:0:0/8:0:0:0/block/sda/sda1
SUBSYSTEM=block
DEVNAME=/dev/sda1
DEVTYPE=partition
...
ID_PART_ENTRY_DISK=8:0
.LOCAL_ifNum=00
SYSTEMD_WANTS=automatic-backup.service        <----------------
...
TAGS=:systemd:
CURRENT_TAGS=:systemd:

With ENV{SYSTEMD_WANTS}+= in 80-backup.rules (pull request):

UDEV  [238388.798162] add      /devices/pci0000:80/0000:80:14.0/usb4/4-6/4-6.1/4-6.1:1.0/host8/target8:0:0/8:0:0:0/block/sda/sda1 (block)
ACTION=add
DEVPATH=/devices/pci0000:80/0000:80:14.0/usb4/4-6/4-6.1/4-6.1:1.0/host8/target8:0:0/8:0:0:0/block/sda/sda1
SUBSYSTEM=block
DEVNAME=/dev/sda1
DEVTYPE=partition
...
ID_PART_ENTRY_DISK=8:0
.LOCAL_ifNum=00
SYSTEMD_WANTS=test-a.service automatic-backup.service        <----------------
...
TAGS=:systemd:
CURRENT_TAGS=:systemd:

So it appears that SYSTEMD_WANTS is in fact overridden when using "=" and the documentation is wrong or at least misleading. I think the intention might have been to say that the contents of SYSTEMD_WANTS are added but only after all rules have been processed. In that process, SYSTEMD_WANTS can be overridden by other rules. Just to be sure, I have also verified that the test-a.service is not called in the first example.