Add reload systemd to run unit generator for st2ctl

[nzlosh]

This PR is part of StackStorm/st2-packages#706 to use st2.confto dynamically generate st2api, st2auth and st2stream socket files.

For this to work seamlessly for most users, an update to st2ctl is required to perform a systemctl daemon-reload which will regenerator the .socket files and pickup any modifications to st2.conf.

Fixes: #3356 StackStorm/st2-packages#686 #2676

[arm4b]

Considering SYSTEMD_RELOADED state is not saved in any file storage or passed externally, what's the idea here?

Will it run the systemctl daemon-reload on every new st2ctl run?

From that point, is the daemon-reload that triggers system-generator then?

[nzlosh]

Yes, you've understood the logic correctly. The SYSTEMD_RELOADED is to guard against reloading systemd for every daemon process st2ctl will restart in a single invocation. The systemctl daemon-reload triggers the execution of the generators. This way the socket files are regenerated with each st2ctl restart.

While this may seem inefficient, because the likelihood of st2.conf being changed on each restart is low, the logic required to test st2.conf last modified time and compare it with the age the daemon process being restarted would have added unnecessary complexity. The daemon-reload is ~200ms on my dev machine which is insignificant compare with the time it takes to restart all st2 daemon (12.848s on my dev machine).

For the sake of science, here's the time it takes to restart each of the concerned daemons:

root@u1804:~# time systemctl restart st2api
real    0m4.014s
user    0m0.004s
sys     0m0.003s

root@u1804:~# time systemctl restart st2auth
real    0m1.879s
user    0m0.007s
sys     0m0.000s

root@u1804:~# time systemctl restart st2stream
real    0m1.483s
user    0m0.003s
sys     0m0.004s

[arm4b]

Thanks, @nzlosh!

Great item for including in the v3.6.0 👍 Marked it as such.

[CLAassistant]

All committers have signed the CLA.

[arm4b]

This PR comes with the StackStorm/st2-packages#706 and should be merged only once the change in packaging is ready.
In this situation, we merged an unfinished feature which is not ideal.