[ruby-core:116539] [Ruby master Feature#20108] Introduction of Happy Eyeballs Version 2 (RFC8305) in Socket.tcp

1 Feb 2024

      Issue #20108 has been updated by shugo (Shugo Maeda).

shioimm (Misaki Shioi) wrote in #note-10:
...
Regarding the content mentioned above, I have made the following changes to the commit:
https://github.com/ruby/ruby/pull/9374/commits/461b75830599408feca086f7f6719...
- Improved performance in the case where there is only one address family to be name resolved
- Improvement performance in the case where there is only one name-resolved IP address.
- Added fast_fallback option.
Thank you.  The fast_fallback option of Socket.tcp and Socket.tcp_fast_fallback look fine for me.

----------------------------------------
Feature #20108: Introduction of Happy Eyeballs Version 2 (RFC8305) in Socket.tcp
https://bugs.ruby-lang.org/issues/20108#change-106557

* Author: shioimm (Misaki Shioi)
* Status: Open
* Priority: Normal
----------------------------------------
This is an implementation of Happy Eyeballs version 2 (RFC 8305) in Socket.tcp.

### Background
Currently, `Socket.tcp` synchronously resolves names and makes connection attempts with `Addrinfo::foreach.`
This implementation has the following two problems.

1. In hostname resolution, the program stops until the DNS server responds to all DNS queries.
2. In a connection attempt, while an IP address is trying to connect to the destination host and is taking time, the program stops, and other resolved IP addresses cannot try to connect.

### Proposal
"Happy Eyeballs" ([RFC 8305](https://datatracker.ietf.org/doc/html/rfc8305)) is an algorithm to solve this kind of problem. It avoids delays to the user whenever possible and also uses IPv6 preferentially.
I implemented it into `Socket.tcp` by using `Addrinfo.getaddrinfo` in each thread spawned per address family to resolve the hostname asynchronously, and using `Socket::connect_nonblock` to try to connect with multiple addrinfo in parallel. 

See https://github.com/ruby/ruby/pull/9374

### Outcome

This change eliminates a fatal defect in the following cases.

#### Case 1. One of the A or AAAA DNS queries does not return

```ruby
require 'socket'

class Addrinfo
  class << self
    # Current Socket.tcp depends on foreach
    def foreach(nodename, service, family=nil, socktype=nil, protocol=nil, flags=nil, timeout: nil, &block)
      getaddrinfo(nodename, service, Socket::AF_INET6, socktype, protocol, flags, timeout: timeout)
        .concat(getaddrinfo(nodename, service, Socket::AF_INET, socktype, protocol, flags, timeout: timeout))
        .each(&block)
    end

    def getaddrinfo(_, _, family, *_)
      case family
      when Socket::AF_INET6 then sleep
      when Socket::AF_INET then [Addrinfo.tcp("127.0.0.1", 4567)]
      end
    end
  end
end

Socket.tcp("localhost", 4567)
```

Because the current `Socket.tcp` cannot resolve IPv6 names, the program stops in this case. It cannot start to connect with IPv4 address.
Though `Socket.tcp` with HEv2 can promptly start a connection attempt with IPv4 address in this case.

#### Case 2. Server does not promptly return ack for syn of either IPv4 / IPv6 address family

```ruby
require 'socket'

fork do
  socket = Socket.new(Socket::AF_INET6, :STREAM)
  socket.setsockopt(:SOCKET, :REUSEADDR, true)
  socket.bind(Socket.pack_sockaddr_in(4567, '::1'))
  sleep
  socket.listen(1)
  connection, _ = socket.accept
  connection.close
  socket.close
end

fork do
  socket = Socket.new(Socket::AF_INET, :STREAM)
  socket.setsockopt(:SOCKET, :REUSEADDR, true)
  socket.bind(Socket.pack_sockaddr_in(4567, '127.0.0.1'))
  socket.listen(1)
  connection, _ = socket.accept
  connection.close
  socket.close
end

Socket.tcp("localhost", 4567)
```

The current `Socket.tcp` tries to connect serially, so when its first name resolves an IPv6 address and initiates a connection to an IPv6 server, this server does not return an ACK, and the program stops.
Though `Socket.tcp` with HEv2 starts to connect sequentially and in parallel so a connection can be established promptly at the socket that attempted to connect to the IPv4 server.

In exchange, the performance of `Socket.tcp` with HEv2 will be degraded.

```
100.times { Socket.tcp("www.ruby-lang.org", 80) }
# Socket.tcp (Before) 0.123809
# Socket.tcp (After)  0.224684
```

This is due to the addition of the creation of IO objects, Thread objects, etc., and calls to `IO::select` in the implementation.

-- 
https://bugs.ruby-lang.org/