when using "AsIs" mode, golang controls how to connect to a "domain" address.

currently, golang use RFC-6555 to connect to the domains, although it is obsolete, but it is better than nothing. it solve the problem when one IP-type is unreachable. but regardless of the IP type, some IPs may not be available, thus RFC 8305 was created #4473 (comment)

RFC 6555 is obsoleted by RFC 8305, and python use RFC-8305 from v3.8.

///

anyway, It doesn't matter what RFC is implemented for the "AsIs" mode, because we should use built-in-dns to bypass GFW DNS spoofing,
and for "useIP/forceIP" mode, we don't even have RFC-6555, and only one random IP is selected.

even Xray-core cannot detect user network and for IPv4-only network, an IPv6 can be chosen!(we can use forceIPv4v6 for this problem but this makes IPv4 always a priority)

and also Iran-GFW blocked some range-IPs of meta(facebook, instagram,...) and some not, so with happy-eyeballs iranian-users can access instagram without using any server.(in 99% cases)

as a result, I implemented Happy Eyeballs RFC-8305 for "forceIP/useIP" mode.

///

because happy eyeballs only applies when sockopt-domainStrategy is "forceIP/useIP" and this options are in "sockopt" settings, so i put "happyEyeballs" settings in "sockopt" settings.

///

we can use happy eyeballs for all type of proxy: freedom, vless, ...

///

"streamSettings": {
  "sockopt": {
    "domainStrategy": "forceIP",
      "happyEyeballs": { 
              "enabled": true,
              "prioritizeIPv6": true, 
              "maxConcurrentTry": 4,
              "tryDelayMs": 250,
              "interleave": 1
       }
     }
   } 

///

enabled (bool): enable/disable happy eyeballs, default is false (but i think it should be enabled by default later)

prioritizeIPv6 ( bool): indicate "First Address Family" in RFC-8305, default is false

interleave (uint32): indicate "First Address Family count" in RFC-8305, default is 1

maxConcurrentTry (uint32): maximum concurrent attempt (this is only maximum and in most cases our concurrent attempts is less, unless all connection fail to connect) also we can always have a maximum of concurrent-attempt as many IPs as we have, and this option is useful when the number of IPs is too high, and we want to control the number of concurrent-attempts, default is 4.

tryDelayMs: delay time between each attempt, RFC-8305 recommend 250ms, so the default is 250ms

///

for example suppose our IP-list is [ip4-1, ip4-2, ip4-3, ip4-4, ip6-1, ip6-2, ip6-3, ip6-4]

when interleave is 1 and prioritizeIPv6 is false, the sorted-ip-list is:
[ip4-1, ip6-1, ip4-2, ip6-2, ip4-3, ip6-3, ip4-4, ip6-4]

and when for example interleave is 2 and prioritizeIPv6 is true:
[ip6-1, ip6-2, ip4-1, ip4-2, ip6-3, ip6-4, ip4-3, ip4-4]

then delay 250ms for each attempt until first connection is established.

the first-stablished-connection is winner connection and selected for sending/receiving data.