Systemd service using screen: 'No screen session found' - minecraft

Question

I'm trying to setup a minecraft server for my kid and screen is giving me fits. I've been following this tutorial and I've already consulted this AskUbuntu answer. I'm running on Ubuntu 16.04.5 x64. I have created the following Service file and linked it to /etc/systemd/system/minecraft.service.
[Unit]
Description=Minecraft Server
Documentation=
Wants=network.target
After=network.target
[Service]
User=minecraft
Group=minecraft
Nice=5
EnvironmentFile=-/var/minecraft/unit.conf
KillMode=none
SuccessExitStatus=0 1
ProtectHome=true
ProtectSystem=full
PrivateDevices=true
NoNewPrivileges=true
PrivateTmp=true
InaccessibleDirectories=/root /sys /srv -/opt /media -/lost+found
ReadWriteDirectories=/var/minecraft/server /usr/bin/screen
WorkingDirectory=/var/minecraft/server
#ExecStartPre= /usr/bin/screen -dmS Minecraft
ExecStart= /usr/bin/screen -dm -S Minecraft '/usr/bin/java -Xmx1500M -Xms1024M -jar /var/minecraft/server/craftbukkit-1.13.jar'
ExecStop=/usr/bin/screen -S minecraft -p 0 -X stuff "stop^M"
[Install]
WantedBy=multi-user.target
When I monitor the service startup, I get the following in the journalctl logs:
Aug 05 14:55:41 spigot-1 systemd[1]: Started Minecraft Server.
Aug 05 14:55:41 spigot-1 screen[9869]: No screen session found.
Per the SO post I referenced, I already changed the permissions in /var/run/screen. This is the current setup:
root#spigot-1:/var/minecraft/server# ls -alR /var/run/screen/
/var/run/screen/:
total 0
drwxrwxrwx 4 root root 80 Aug 5 14:12 .
drwxr-xr-x 23 root root 860 Aug 5 14:11 ..
drwx------ 2 minecraft minecraft 40 Aug 5 14:55 S-minecraft
drwx------ 2 root root 40 Aug 5 14:50 S-root
/var/run/screen/S-minecraft:
total 0
drwx------ 2 minecraft minecraft 40 Aug 5 14:55 .
drwxrwxrwx 4 root root 80 Aug 5 14:12 ..
/var/run/screen/S-root:
total 0
drwx------ 2 root root 40 Aug 5 14:50 .
drwxrwxrwx 4 root root 80 Aug 5 14:12 ..
** Debug Notes **
You'll see the commented ExecStartPre command in there... all that happens if I try to start the screen session with that command is I get a second 'No screen session found' error.
I can launch the server from the command line using 'screen -dmS Minecraft [..]' and reattach. I can do this both as root and as the 'minecraft' user.
Any help would be greatly appreciated

Answer 1

I don't have Ubuntu to hand, but I tried something similar on Fedora 24 and had a similar error. To investigate I added an strace to the screen command and removed PrivateTmp so that I could get the trace output.
ExecStart=/bin/strace -o /tmp/s -f screen -dm -S me bash -c 'sleep 999'
In the output I found that at the end, screen was trying to open a pseudo-tty and failed to do so:
open("/dev/ptmx", O_RDWR) = -1 EACCES (Permission denied)
...
write(1, "No more PTYs.\r\nSorry, could not "..., 52) = 52
I added some ls -l /dev commands to the Unit to see what the effect of the PrivateDevices=true option had been. There was indeed a /dev/ptmx file, but unlike my real /dev it was a symbolic link to /dev/pts/ptmx, instead of just being the special character device. For some reason on my system I have:
$ ls -l /dev/ptmx /dev/pts/ptmx
crw-rw-rw- 1 root tty 5, 2 Aug 6 14:29 /dev/ptmx
c--------- 1 root root 5, 2 Jun 27 08:29 /dev/pts/ptmx
(This is probably a bug depending on the version of systemd and udev.)
So when /dev/ptmx is replaced in the private namespace by a symbolic link to /dev/pts/ptmx, you can no longer open a new pty. A simple fix for testing was sudo chmod a+rw /dev/pts/ptmx which indeed made the Unit work and start the screen process.
This may not be the case for you, but you could undertake a similar debug method to try to find the problem.