Add comment about AI_ADDRCONFIG

Author: pietern

lib/redis/connection/ruby.rb

@@ -135,6 +135,20 @@ def self.connect_addrinfo(ai, port, timeout)
         end
 
         def self.connect(host, port, timeout)
+          # Don't pass AI_ADDRCONFIG as flag to getaddrinfo(3)
+          #
+          # From the man page for getaddrinfo(3):
+          #
+          #   If hints.ai_flags includes the AI_ADDRCONFIG flag, then IPv4
+          #   addresses are returned in the list pointed to by res only if the
+          #   local system has at least one IPv4 address configured, and IPv6
+          #   addresses are returned only if the local system has at least one
+          #   IPv6 address configured. The loopback address is not considered
+          #   for this case as valid as a configured address.
+          #
+          # We do want the IPv6 loopback address to be returned if applicable,
+          # even if it is the only configured IPv6 address on the machine.
+          # Also see: https://github.com/redis/redis-rb/pull/394.
           addrinfo = ::Socket.getaddrinfo(host, nil, Socket::AF_UNSPEC, Socket::SOCK_STREAM)
 
           # From the man page for getaddrinfo(3):