Comment 1 for bug 1752722

This does not appear to affect Artful and systemd 234. Example session:

~# cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=17.10
DISTRIB_CODENAME=artful
DISTRIB_DESCRIPTION="Ubuntu 17.10"

~# uname -a
Linux ip-100-65-137-86 4.13.0-36-generic #40-Ubuntu SMP Fri Feb 16 20:07:48 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

~# systemctl --version
systemd 234
+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ +LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN2 +IDN default-hierarchy=hybrid

~# systemctl daemon-reload

~# systemctl list-unit-files test-mask.service

0 unit files listed.

~# cat <<EOF > /lib/systemd/system/test-mask.service
> [Unit]
> Description=dummy service
>
> [Service]
> Type=oneshot
> ExecStart=/bin/true
>
> [Install]
> WantedBy=multi-user.target
>
> EOF

~# systemctl daemon-reload

~# systemctl list-unit-files test-mask.service; echo '**'; systemctl is-enabled test-mask.service; echo '**'; systemctl status test-mask.service
UNIT FILE STATE
test-mask.service disabled

1 unit files listed.
**
disabled
**
● test-mask.service - dummy service
   Loaded: loaded (/lib/systemd/system/test-mask.service; disabled; vendor preset: enabled)
   Active: inactive (dead)

~# systemctl enable test-mask.service
Created symlink /etc/systemd/system/multi-user.target.wants/test-mask.service → /lib/systemd/system/test-mask.service.

~# systemctl list-unit-files test-mask.service; echo '**'; systemctl is-enabled test-mask.service; echo '**'; systemctl status test-mask.service
UNIT FILE STATE
test-mask.service enabled

1 unit files listed.
**
enabled
**
● test-mask.service - dummy service
   Loaded: loaded (/lib/systemd/system/test-mask.service; enabled; vendor preset: enabled)
   Active: inactive (dead)

~# systemctl list-unit-files test-mask.service; echo '**'; systemctl is-enabled test-mask.service; echo '**'; systemctl status test-mask.service
UNIT FILE STATE
test-mask.service masked

1 unit files listed.
**
masked
**
● test-mask.service
   Loaded: masked (/dev/null; bad)
   Active: inactive (dead)

~# systemctl unmask test-mask.service
Removed /etc/systemd/system/test-mask.service.

~# mkdir -p /etc/systemd/system/test-mask.service.d

~# cat <<EOF > /etc/systemd/system/test-mask.service.d/override.conf
> # Arbitrary override for the sake of demonstration
> [Unit]
> After=ssh.service
>
> EOF

~# systemctl daemon-reload

~# systemctl cat test-mask.service
# /lib/systemd/system/test-mask.service
[Unit]
Description=dummy service

[Service]
Type=oneshot
ExecStart=/bin/true

[Install]
WantedBy=multi-user.target

# /etc/systemd/system/test-mask.service.d/override.conf
# Arbitrary override for the sake of demonstration
[Unit]
After=ssh.service

~# systemctl list-unit-files test-mask.service; echo '**'; systemctl is-enabled test-mask.service; echo '**'; systemctl status test-mask.service
UNIT FILE STATE
test-mask.service enabled

1 unit files listed.
**
enabled
**
● test-mask.service - dummy service
   Loaded: loaded (/lib/systemd/system/test-mask.service; enabled; vendor preset: enabled)
  Drop-In: /etc/systemd/system/test-mask.service.d
           └─override.conf
   Active: inactive (dead)

~# systemctl mask test-mask.service
Created symlink /etc/systemd/system/test-mask.service → /dev/null.

~# systemctl list-unit-files test-mask.service; echo '**'; systemctl is-enabled test-mask.service; echo '**'; systemctl status test-mask.service
UNIT FILE STATE
test-mask.service masked

1 unit files listed.
**
masked
**
Warning: test-mask.service changed on disk. Run 'systemctl daemon-reload' to reload units.
● test-mask.service
   Loaded: masked (/dev/null; bad)
   Active: inactive (dead)