ft: notify-reload support

[hauleth]

• ft: send monotonic timestamp when restarting
• ft: rename auto_formatter to install_kmsg
• feat: add support for notify-reload and automatic reload handling

[Munksgaard]

For us systemd newbies, what does notify-reload do?

[hauleth]

@Munksgaard It is new service type in systemd 253 that provides a way to have synchronous service reloads. In short it makes systemctl reload my-app.service. This change makes it that way that by default your application handles SIGHUP signals from the OS, and if there is such signal, then it will automatically run init:restart/0, but with some additional things to handle communication with systemd. So if you want to have soft-restart (without killing ERTS) this is the way.

Theoretically this code can also be changed to support stuff like hot-code reloads via systemctl reload, but as these require some additional work. I may implement that in the future, but for now it is set just to init:restart/0.

In short - now operator can decide between hard restart (systemctl restart) and soft-restart (systemctl reload) without touching Erlang codebase (assuming that systemd library is used).

[Munksgaard]

Interesting! What would I need to do to support this in my app?

[hauleth]

You mean the soft-restarts? When I will release this then the answer is - nothing. It will be supported out of the box.

For the hot code reloads - I have some ideas, but first I want to release 1.0, but before I can do that, I need to manage day to day work.

[Munksgaard]

That sounds amazing.

Thanks for the detailed answers, and thanks for the work you do on erlang-systemd ❤️