[systemd-devel] Is it possible to let systemd create a listening socket and yet be able to have that socket activate nothing, at least temporarily?

[Klaus Ebbe Grue]

Hi Lennart,

> Can't you run your upgrade script in idempotent way as a helper
> service that is pulled in by your main daemon and ordered before it,
> but conditions itself out if it already did its job? that's usually
> the most robust way, since then it's sufficient to just restart your
> daemon or reboot, and everything will always catch up correctly.

> i.e. if you have foo-daemon.socket + foo-daemon.service then define
> foo-upgrade.service that is pulled in from foo-daemon.service via
> `Wants=foo-upgrade.service` + `After=foo-upgrade.service`. And then
> add `ConditionFileExists=!/some/touch/file` to `foo-upgrade.service` to
> make it a NOP if things have already been updated, using a touch
> file. (some better, smarter condition check might work as well, see
> man pages of things systemd can check for you).

That is a great pattern!

A Type=oneshot seems useful for the upgrade process.

For the record, I have appended what ended up working.

Thanks!

Cheers,
Klaus

---

The following three unit files open a listening socket, run the myupgrade service,
and only then start the /usr/bin/mydaemon service. The socket listens while the
myupgrade service runs so that clients can connect. But the clients are queued
until myupgrade has finished and /usr/bin/mydaemon starts accepting
connections.

mydeamon.service:
[Unit]
Description=service that does something
After=mydaemon.socket
Requires=mydaemon.socket
After=myupgrade.service
Wants=myupgrade.service
[Install]
WantedBy=multi-user.target
[Service]
Type=simple
ExecStart=/usr/bin/mydaemon

myupgrade.service:
[Unit]
Description=service to run before mydaemon
[Service]
Type=oneshot
ExecStart=/usr/bin/bash -c "echo goodnight;sleep 60;echo goodmorning"

mydeamon.socket
[Unit]
Description=mydaemon listen socket
[Socket]
ListenStream=9999
[Install]
WantedBy=sockets.target

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