[ruby-core:119665] [Ruby master Feature#20782] Introduction of Happy Eyeballs Version 2 (RFC8305) in TCPSocket.new

["hsbt (Hiroshi SHIBATA) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20782 has been updated by hsbt (Hiroshi SHIBATA).


@matz I propose @shioimm as a Ruby committer. She is best person to maintain HEv2 and Socket related feature. It's good time to commit https://github.com/ruby/ruby/pull/11653 by herself.

I'll support and mentor her.

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Feature #20782: Introduction of Happy Eyeballs Version 2 (RFC8305) in TCPSocket.new
https://bugs.ruby-lang.org/issues/20782#change-110321

* Author: shioimm (Misaki Shioi)
* Status: Assigned
* Assignee: naruse (Yui NARUSE)
----------------------------------------
This is an implementation of Happy Eyeballs version 2 (RFC 8305) in `TCPSocket.new`.
See https://github.com/ruby/ruby/pull/11653

### Background
Prior to this implementation, I implemented Happy Eyeballs Version 2 (HEv2) for `Socket.tcp` in https://github.com/ruby/ruby/pull/9374. 
HEv2 is an algorithm defined in [RFC 8305](https://datatracker.ietf.org/doc/html/rfc8305), aimed at improving network connectivity. 
For more details on the specific cases that HEv2 helps, please refer to https://bugs.ruby-lang.org/issues/20108.

### Proposal & Outcome
This proposal implements the same HEv2 algorithm in `TCPSocket.new`.
Since `TCPSocket.new` is used more widely than `Socket.tcp`, this change is expected to broaden the impact of HEv2's benefits. 
Like `Socket.tcp`, I have also added `fast_fallback` keyword argument to `TCPSocket.new`. 
This option is set to true by default, enabling the HEv2 functionality. 
However, users can explicitly set it to false to disable HEv2 and use the previous behavior of `TCPSocket.new`.

It should be noted that HEv2 is enabled only in environments where pthreads are available.
This specification follows the approach taken in https://bugs.ruby-lang.org/issues/19965, where name resolution can be interrupted. 
(In environments where pthreads are not available, the `fast_fallback` option is ignored.)

### Performance
Below is the benchmark of 100 requests to `www.ruby-lang.org` with the fast_fallback option set to true and false, respectively. 
While there is a slight performance degradation when HEv2 is enabled, the degradation is smaller compared to that seen in `Socket.tcp` .

```ruby
require 'socket'
require 'benchmark'

hostname = "www.ruby-lang.org"
port = 80
n = 100

Benchmark.bmbm do |x|
  x.report("fast_fallback: true") do
    n.times { TCPSocket.new(hostname, port).close }
  end

  x.report("fast_fallback: false") do
    n.times { TCPSocket.new(hostname, port, fast_fallback: false).close }
  end
end
```

```
~/s/build ❯❯❯ ../install/bin/ruby ../ruby/test.rb
Rehearsal --------------------------------------------------------
fast_fallback: true    0.017588   0.097045   0.114633 (  1.460664)
fast_fallback: false   0.014033   0.078984   0.093017 (  1.413951)
----------------------------------------------- total: 0.207650sec

                           user     system      total        real
fast_fallback: true    0.020891   0.124054   0.144945 (  1.473816)
fast_fallback: false   0.018392   0.110852   0.129244 (  1.466014)
```



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