[systemd-devel] Launch a mount unit from udev rule via ENV{SYSTEMD_WANTS}

[Jacopo]

I'be been having issues lately trying to automatically remount an external
USB drive that is mounted at boot from an fstab entry:
LABEL=data-ssd  /opt/data-ssd  ext4  defaults,nofail,users  0  2

I'm using systemd 244 and after some investigation I learned about the
possibility to launch a systemd unit from a udev rule via
ENV{SYSTEMD_WANTS} (see here
https://github.com/systemd/systemd/issues/22589#issuecomment-1047940003 and
https://github.com/systemd/systemd/pull/11373#issuecomment-594014841)

In particular the second suggestion is exactly what I need, but the only
way I could make it work was launching a dummy.service that "Wants" the
corresponding mount unit:

udev rule:
ACTION=="add", SUBSYSTEM=="block", KERNEL=="sd*", SUBSYSTEMS=="usb",
ENV{DEVTYPE}=="partition", IMPORT{builtin}="blkid",
ENV{ID_FS_TYPE}=="ext4", ENV{ID_FS_LABEL_ENC}=="data-ssd",
ENV{SYSTEMD_WANTS}+="dummy.service"

dummy.service:
[Unit]
Description=Dummy service
Wants=opt-data\x2dssd.mount

[Service]
Type=simple
ExecStart=/bin/echo "I'm dummy"

[Install]
WantedBy=local-fs.target

I also tested using ACTION!="remove" in the udev rule, but same result.

As I mentioned in the github issue before being redirected here, if I query
with udevadm, ENV{SYSTEMD_WANTS} is printed out only with in the
non-working case (.mount unit launched directly from the udev rule):

# udevadm info --query=property --path=/sys/class/block/sda1
[...]
SYSTEMD_WANTS=opt-data\x2dssd.mount
[...]

whereas it disappears if launching the dummy.service first.

Thanks
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