[systemd-devel] socket activation systemd holding a reference to an accepted socket

[Ben Woodard]

Is it intentional that systemd holds a reference to a socket it has just accepted even though it just handed the open socket over to a socket.activated service that it has just started.

For example given the following unit files:

/etc/systemd/system/test.socket:
[Unit]
Description=test service

[Socket]
ListenStream=804
Accept=yes

[Install]
WantedBy=sockets.target

------------------
/etc/systemd/system/test at .service:
[Unit]
Description=test per-service unit

[Service]
ExecStart=-/home/ben/Work/sysdtest/sysdtest
------------------
The sysdtest is just a program which essentially daemonizes itself and then sleeps for a few seconds. So it doesn’t have an open reference to the file descriptor that was accepted. Yet the socket stays open and connected to the client until the daemon like process exits or the timeout for the service instance is reached because systemd holds onto a reference to the file descriptor.

Is this intentional? The behavior differs from xinetd which didn’t hold onto any references to the newly accepted socket. This allowed the socket to close as soon as the socket activated process daemonized. Holding onto a reference to the socket FD surprised me enough that I verified that it was in fact systemd holding the reference by using lsof:

[ben at localhost sysdtest]$ sudo lsof -i :804
COMMAND  PID USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
systemd    1 root   19u  IPv6 384026      0t0  TCP localhost.localdomain:804->localhost.localdomain:46220 (ESTABLISHED)
systemd    1 root   30u  IPv6 376208      0t0  TCP *:804 (LISTEN)
telnet  1021  ben    3u  IPv4 384025      0t0  TCP localhost.localdomain:46220->localhost.localdomain:804 (ESTABLISHED)

-ben