Tracd --daemonize does not work without --pidfile with systemd

[ak]

The following systemd config is expected to work, since systemd would try to guess the PID (see ​man systemd.service).

[Unit]
Description=Trac Server
After=network.target

[Service]
Type=forking
ExecStart=/usr/bin/tracd -d -p 8000 -e /tmp

[Install]
WantedBy=multi-user.target

$ sudo systemctl status tracd
● tracd.service - Trac Server
   Loaded: loaded (/lib/systemd/system/tracd.service; enabled)
   Active: inactive (dead) since Mon 2014-11-03 00:11:32 CET; 4s ago
  Process: 1010 ExecStart=/usr/bin/tracd -d -p 8000 -e /tmp (code=exited, status=0/SUCCESS)
 Main PID: 1011 (code=exited, status=0/SUCCESS)

However, the following configuration works:

tracd.service

@@ -6 +6 @@
 Type=forking
-ExecStart=/usr/bin/tracd -d -p 8000 -e /tmp
+ExecStart=/usr/bin/tracd -d -p 8000 --pidfile=/run/tracd.pid -e /tmp
+PIDFile=/run/tracd.pid

The status output may be a hint:

$ sudo systemctl status -l tracd
● tracd.service - Trac Server
   Loaded: loaded (/lib/systemd/system/tracd.service; enabled)
   Active: active (running) since Mon 2014-11-03 00:10:07 CET; 11s ago
  Process: 975 ExecStart=/usr/bin/tracd -d -p 8000 --pidfile=/run/tracd.pid -e /tmp (code=exited, status=0/SUCCESS)
 Main PID: 980 (tracd)
   CGroup: /system.slice/tracd.service
           └─980 /usr/bin/python /usr/bin/tracd -d -p 8000 --pidfile=/run/tracd.pid -e /tmp

Nov 03 00:10:07 host systemd[1]: tracd.service: Supervising process 980 which is not our child. We'll most likely not notice when it exits.

Why does tracd exit when simply not using a pidfile?

[Christian Boos]

tracd uses the "double fork" technique, so that:

• ​zombie processes are avoided (actually it seems we miss the waitpid() step in our implementation…)
• ​make sure the server process can't have a controlling terminal

But I'm surprised that systemd can't cope with this, as the above seems pretty common practice.

Indeed, it looks like systemd ​discourages the use of the double fork, so we could perhaps think about adding special support for systemd notification system, ​sd_notify. It seems pretty trivial to emulate that in Python, see e.g. ​pysystemd-daemon.

The idea would be to use [Service] Type=notify, and in daemonize()​ we could check if there's a NOTIFY_SOCKET environment variable set. In this case, we avoid the forks altogether, unset NOTIFY_SOCKET and do a few sd_notify calls.

[krichter@…]

Either systemd or tracd removes the parent of the PID file (when both the PIDFile directive and --pidfile argument are specified) which makes it necessary to add a systemd unit which creates the parent directory with higher privileges (usually /run/ or /var/run/ are owned by root) and make the systemd unit depend on it as a workaround.

[Ryan J Ollos]

Moved ticket assigned to next-stable-1.0.x since maintenance of 1.0.x is coming to a close. Please move the ticket back if it's critical to fix on 1.0.x.

[Ryan J Ollos]

Milestone renamed