How to have systemd start Tomcat on port 80? - linux

Question

As described here and here, I'm trying (on Ubuntu 16.04) to start Tomcat 9.0.2 on port 80, by having systemd pre-initialize a socket on that privileged port and then pass it to Tomcat which runs as non-privileged user "tomcat" (I know there are other solutions, but I'm interested in making this one work). For this, I have the following socket unit (the "port" setting was already changed to 80 on server.xml):
/etc/sytemd/system/tomcat.socket
[Unit]
Description=Tomcat server socket
[Socket]
ListenStream=80
And the following service unit:
/etc/sytemd/system/tomcat.service
[Unit]
Description=Apache Tomcat Web Application Container
After=syslog.target network.target
[Service]
Type=forking
Environment=CATALINA_PID=/opt/tomcat/9.0.2/temp/tomcat.pid
Environment=CATALINA_HOME=/opt/tomcat/9.0.2
ExecStart=/opt/tomcat/9.0.2/bin/startup.sh
ExecStop=/opt/tomcat/9.0.2/bin/shutdown.sh
User=tomcat
Group=tomcat
UMask=0007
[Install]
Requires=tomcat.socket
When running sudo systemctl start tomcat.service, the command outputs no error message, but then systemctl status tomcat.service displays a failed status:
tomcat.service - Apache Tomcat Web Application Container
Loaded: loaded (/etc/systemd/system/tomcat.service; static; vendor preset: enabled)
Active: failed (Result: exit-code) since Tue 2017-12-19 21:46:53 -05; 45min ago
Process: 10207 ExecStop=/opt/tomcat/9.0.2/bin/shutdown.sh (code=exited, status=1/FAILURE)
Process: 10175 ExecStart=/opt/tomcat/9.0.2/bin/startup.sh (code=exited, status=0/SUCCESS)
Main PID: 10189 (code=exited, status=0/SUCCESS)
As can be seen in the journald logs below, Tomcat does start, but shutdown.sh is immediately run and makes it exit. Apparently, Tomcat still tries to create a socket on port 80, which fails because user tomcat is not superuser, and proceeds to exit right away.
systemd[1]: Starting Apache Tomcat Web Application Container...
startup.sh[10175]: Existing PID file found during start.
startup.sh[10175]: Removing/clearing stale PID file.
startup.sh[10175]: Tomcat started.
systemd[1]: Started Apache Tomcat Web Application Container.
sudo[10172]: pam_unix(sudo:session): session closed for user root
shutdown.sh[10207]: PID file found but no matching process was found. Stop aborted.
systemd[1]: tomcat.service: Control process exited, code=exited status=1
systemd[1]: tomcat.service: Unit entered failed state.
systemd[1]: tomcat.service: Failed with result 'exit-code'.
If Tomcat 9 is capable of receiving a preinitialized socket from systemd, how can I make that work? Or is Tomcat unable to do so?