http_server: Ready to support Async 2.0 gem

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
http_server plugin helper doesn't work with Async 2.x gem due to using obsolete usage.
This PR updates it to follow current documented way:
https://github.com/socketry/async-http/blob/0a65acd7cf7486e1877f0da86580e1692cd8207b/readme.md#usage
It's applicable both Async 2.x & Async 1.x.
But this PR still stay on Async 1.x because io-event gem which is required by Async 2.x can't build on Windows.

• TODO
  • Fix Build error with Ruby 3.1 on Windows
    • Build error with Ruby 3.1 on Windows socketry/io-event#111
  • Solve "private method `raise' called for nil (NoMethodError)" error if stop Async HTTP server socketry/async-http#176
  • Replace cool.io gem to something
    • cool.io introduces IO::Buffer which conflicts with Ruby core classes. It affects the behavior of the Async HTTP server.
    • Ref. Async HTTP Server does not respond on Windows if cool.io gem is loaded socketry/async-http#175

Docs Changes:

Release Note:

[ioquatix]

How are you using cool.io? Can you replace it with Async?

[ioquatix]

By the way, I'm sorry that this is complicated, I did not intend for this to become a big problem. I'm happy to advise or help out.

[ashie]

How are you using cool.io? Can you replace it with Async?

Thanks for your comment!
We have an idea to migrate to Async for quite some time:
https://github.com/fluent/fluentd/wiki/Roadmap-and-Development-tasks#v1
but we haven't started migrating it yet because Fluentd depends on cool.io deeply, it's our fundamental event loop mechanism.

We may need to start considering it now.

[ioquatix]

Is it exposed to user code or is it all internal implementation?

[ashie]

Is it exposed to user code or is it all internal implementation?

Partially wrapped by our API but Coolio's classes are exposed to plugins.
Probably many built-in & third-party plugins use them.
e.g.)

fluentd/lib/fluent/plugin/in_tail.rb

Lines 821 to 849 in 7f13c7a

	class StatWatcher < Coolio::StatWatcher
	def initialize(path, log, &callback)
	@callback = callback
	@log = log
	super(path)
	end
	def on_change(prev, cur)
	@callback.call
	rescue
	@log.error $!.to_s
	@log.error_backtrace
	end
	end
	class TimerTrigger < Coolio::TimerWatcher
	def initialize(interval, log, &callback)
	@log = log
	@callback = callback
	super(interval, true)
	end
	def on_timer
	@callback.call
	rescue => e
	@log.error e.to_s
	@log.error_backtrace
	end
	end

[ioquatix]

I see, it's a tricky change then.

One option would be to change cool.io to rename IO::Buffer to it's own namespace, e.g. cool.io::IOBuffer (which would make more sense anyway).