Description of problem:

tangd.socket After= lines cause it to try to start before multi-user.target, causing a dependency loop failure.

Version-Release number of selected component (if applicable):

tang-7-1.el8.x86_64


How reproducible:

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/system_design_guide/configuring-automated-unlocking-of-encrypted-volumes-using-policy-based-decryption_system-design-guide

Specifically:

# systemctl enable tangd.socket

then rebooting, we hit the issue 

# systemctl status tangd.socket

● tangd.socket - Tang Server socket
   Loaded: loaded (/usr/lib/systemd/system/tangd.socket; enabled; vendor preset>
   Active: inactive (dead)
   Listen: [::]:80 (Stream)
 Accepted: 0; Connected: 0;

# cat /var/log/messages | grep tang:
--------------------------------
Aug 23 13:09:05 localhost systemd[1]: Started tangd-update.path.
Aug 23 13:10:05 localhost systemd[1]: sockets.target: Found ordering cycle on tangd.socket/start
Aug 23 13:10:05 localhost systemd[1]: sockets.target: Found dependency on tangd-update.service/start
Aug 23 13:10:05 localhost systemd[1]: sockets.target: Job tangd.socket/start deleted to break ordering cycle starting with sockets.target/start
Aug 23 13:10:06 localhost systemd[1]: Started tangd-update.path.
--------------------------------


Steps to Reproduce:
# yum install tang
# systemctl enable tangd.socket
# reboot

Actual results:

# systemctl status tangd.socket:

● tangd.socket - Tang Server socket
   Loaded: loaded (/usr/lib/systemd/system/tangd.socket; enabled; vendor preset>
   Active: inactive (dead)
   Listen: [::]:80 (Stream)
 Accepted: 0; Connected: 0;


Expected results:

# systemctl status tangd.socket:

● tangd.socket - Tang Server socket
   Loaded: loaded (/usr/lib/systemd/system/tangd.socket; enabled; vendor preset>
   Active: active (listening) since Fri 2019-08-23 14:51:08 EDT; 2s ago
   Listen: [::]:80 (Stream)
 Accepted: 0; Connected: 0;
   CGroup: /system.slice/tangd.socket

Aug 23 14:51:08 localhost.localdomain systemd[1]: Listening on Tang Server sock>


Additional info:

I noticed that the default socket in the administration guide differs from the one shipped with the tang package (ignore the port on the admin guide socket):

Admin Guide:
--------------------------------
[Unit]
Description=Tang Server socket
Requires=tangd-update.path
Requires=tangd-keygen.service

[Socket]
ListenStream=7500
Accept=true

[Install]
WantedBy=multi-user.target
--------------------------------

RHEL 8:
--------------------------------
[Unit]
Description=Tang Server socket
Requires=tangd-keygen.service
Requires=tangd-update.service
Requires=tangd-update.path
After=tangd-keygen.service
After=tangd-update.service

[Socket]
ListenStream=80
Accept=true

[Install]
WantedBy=multi-user.target
--------------------------------

After removing the After= lines from: /usr/lib/systemd/system/tangd.socket to read:
--------------------------------
[Unit]
Description=Tang Server socket
Requires=tangd-keygen.service
Requires=tangd-update.service
Requires=tangd-update.path

[Socket]
ListenStream=80
Accept=true

[Install]
WantedBy=multi-user.target
--------------------------------

The socket started on reboot as intended. 

The cause of this is because tangd.socket is part of multi-user.target, and when enabled you can see it create the symlink:

# systemctl enable tangd.socket
Created symlink /etc/systemd/system/multi-user.target.wants/tangd.socket → /usr/lib/systemd/system/tangd.socket.

However neither tangd-update.service, tangd-update.path, or tangd-keygen.service are part of that target, which leads me to believe they do not require dbus to run, and therefore start before dbus. By calling tangd.socket to start immediately 'After=' one of these services, it then tries to start it outside of the multi-user.target, "skipping the line" so to speak, since these other services start before dbus, causing the issue. 

Specifically, multi-user.target needs basic.target, and basic.target needs sockets.target.. which needs dbus. You can't run a socket without sockets.target, and you can't run sockets.target without dbus, as far as I'm aware. In this case there's no reason to call After= in the socket because  the services are started before multi-user.target