Revert "Add readiness coordination for systemd notifications"

[bastelfreak]

This reverts #87 + contains #106

[bastelfreak]

I guess I missed something during the revert, but I'm not sure what :(
edit: fixed it

[bastelfreak]

If we revert this, we also need to remove the readiness call from openvox-server: OpenVoxProject/openvox-server#610.

[Sharpie]

I think this moves us in the right direction as it cuts a bunch of complexity that isn't solving the reload problem.

However, the end result seems very similar to the initial PR #23, so it feels as though there is a missing piece.

[Sharpie]

Ok. I think I found the problem(s). It might be a combination of:

• When Type=notify-reload, SystemD will send SIGHUP by its self. Having ExecReload=kill -HUP $MAINPID causes it to send an additional SIGHUP and also return early from systemctl reload when kill finishes running. So, the app is stuck in the middle of handling that second reload when systemctl returns and isn't actually ready.

• When using Type=notify-reload, SystemD expects to see a MONOTONIC_USEC= line included after the RELOADING=1 line in the message sent to the socket. It ignores any RELOADING=1 that does not include this and subsequent READY=1, and just waits until TimeoutStartSec elapses when it then fails. Having ExecReload= set short-circuits this as kill returning is enough to satisfy the timer.

Docs:

Behavior of notify-reload is similar to notify, with one difference: the SIGHUP UNIX process signal is sent to the service's main process when the service is asked to reload and the manager will wait for a notification about the reload being finished.

When initiating the reload process the service is expected to reply with a notification message via sd_notify(3) that contains the "RELOADING=1" field in combination with "MONOTONIC_USEC=" set to the current monotonic time (i.e. CLOCK_MONOTONIC in clock_gettime(2)) in μs, formatted as decimal string. Once reloading is complete another notification message must be sent, containing "READY=1". Using this service type and implementing this reload protocol is an efficient alternative to providing an ExecReload= command for reloading of the service's configuration.

https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html