Periodically send status to systemd

[ekohl]

Description

With systemd it's possible to send a status line. This will show up when users run systemctl status puma.service.

Most of this code is based on puma-plugin-systemd.

At the moment this is completely untested, but I had written some code that I wanted to share. If anyone wants to finish this before I do, they should feel welcome to.

Closes #2604

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[nateberkopec]

Rather than create threads ourselves, puma should provide an API that a puma plugin can use to manage this process.

[ekohl]

That sounds like a good idea because it should also be used to stop. Just like the thread for the watchdog. It may be more important in the test suite, but right now it may be leaking the thread.

And there is actually for plugins some code for plugins with threads:

puma/lib/puma/plugin.rb

Lines 60 to 71 in eb59ed0

	def add_background(blk)
	@background << blk
	end
	def fire_background
	@background.each_with_index do |b, i|
	Thread.new do
	Puma.set_thread_name "plgn bg #{i}"
	b.call
	end
	end
	end

Perhaps that should be utilized instead?

[nateberkopec]

TIL 😆 Yup that's fine too.

[nateberkopec]

Happy to make this happen, but we should take the approach of providing APIs necessary for a plugin to provide this functionality.

[ekohl]

This is still very much a draft since I didn't actually test this yet, but I took a stab at rewriting the systemd integration as a plugin that is automatically loaded. I wonder if this is a good direction to take. If so, I can clean up the patch and split it in two (first one that transforms it into a plugin, then one that adds status watching).

[nateberkopec]

To me this looks great. I think if you rebase and add tests it's good to go.

[ekohl]

I've rebased it to resolve conflicts, but I don't have time to write tests right now. If anyone does have the time, I'd highly welcome it.

[nateberkopec]

Adding the contrib-wanted label.

[dentarg]

Closing in favour of #3006