Systemd not adding "...wants" link to match WantedBy in unit file

Systemd not adding "...wants" link to match WantedBy in unit file

Ubuntu 16.04LTS
watchdog package version 5.14-3ubuntu0.16.04.1
systemd package version 229-4ubuntu21.15

I can use "systemctl start watchdog.service" to start up the watchdog
daemon. But I'm trying to cause watchdog.service to start at boot-up with
"systemctl enable watchdog.service".

The default unit file that comes in the package is:

$ cat /lib/systemd/system/watchdog.service
[Unit]
Description=watchdog daemon
Conflicts=wd_keepalive.service
After=multi-user.target
OnFailure=wd_keepalive.service

[Service]
Type=forking
EnvironmentFile=/etc/default/watchdog
ExecStartPre=/bin/sh -c '[ -z "${watchdog_module}" ] || [ "${watchdog_module}" = "none" ] || /sbin/modprobe $watchdog_module'
ExecStart=/bin/sh -c '[ $run_watchdog != 1 ] || exec /usr/sbin/watchdog $watchdog_options'
ExecStopPost=/bin/sh -c '[ $run_wd_keepalive != 1 ] || false'

[Install]

And, because there is nothing in the [Install] section, I understand that
"systemctl enable watchdog.service" won't do anything.

So, I added the proper override file:

$ cat /etc/systemd/system/watchdog.service.d/override.conf
[Install]
WantedBy=multi-user.target

systemd seems to see the override file:

$ systemctl cat watchdog.service
# /lib/systemd/system/watchdog.service
[Unit]
Description=watchdog daemon
Conflicts=wd_keepalive.service
After=multi-user.target
OnFailure=wd_keepalive.service

[Service]
Type=forking
EnvironmentFile=/etc/default/watchdog
ExecStartPre=/bin/sh -c '[ -z "${watchdog_module}" ] || [ "${watchdog_module}" = "none" ] || /sbin/modprobe
ExecStart=/bin/sh -c '[ $run_watchdog != 1 ] || exec /usr/sbin/watchdog $watchdog_options'
ExecStopPost=/bin/sh -c '[ $run_wd_keepalive != 1 ] || false'

[Install]

# /etc/systemd/system/watchdog.service.d/override.conf
[Install]
WantedBy=multi-user.target

Then I run: systemctl enable watchdog.service

And this seems to work. But my understanding is that there should be a link in
/etc/systemd/system/multi-user.target.wants to the watchdog.service unit file.
But this link is not created.

As a result, when I reboot, I get:

$ systemctl status watchdog.service
● watchdog.service - watchdog daemon
   Loaded: loaded (/lib/systemd/system/watchdog.service; static; vendor preset: enabled)
  Drop-In: /etc/systemd/system/watchdog.service.d
           └─override.conf
   Active: inactive (dead)

Also,

$ systemctl show watchdog.service -p WantedBy
WantedBy=

It's like systemd doesn't see the WantedBy line.

I also tried a complete override of /lib/systemd/system/watchdog.service with
/etc/systemd/system/watchdog.service. I even tried adding the WantedBy line
directly into the /lib/systemd/system/watchdog.service file (which I know is
not the right way to override). But in all cases, systemd won't recognize the
WantedBy= line and won't create the link in .../multi-user.target.wants.

If I manually create the link:

ln -s /lib/systemd/system/watchdog.service \
  /etc/systemd/system/multi-user.target.wants/watchdog.service

and then reboot, watchdog comes up just fine.

But my understanding is that the link should be autogenerated based on the
WantedBy parameter in the [Install] section of the unit file.

So, why isn't that happening? What am I doing wrong?

Also, in case it's helpful /etc/default/watchdog is set to start watchdog
at boot time. I'm not sure how that relates to the above, if at all.

$ cat /etc/default/watchdog
# Start watchdog at boot time? 0 or 1
run_watchdog=1
# Start wd_keepalive after stopping watchdog? 0 or 1
run_wd_keepalive=1
# Load module before starting watchdog
#watchdog_module="none"
watchdog_module="iTCO_wdt"
# Specify additional watchdog options here (see manpage).
#watchdog_options="-v"