Use all results returned by getaddrinfo(3), if necessary

I have seen getaddrinfo(3) return IPv6 addresses before IPv4 addresses
when a hostname resolves to both. It is still more common for Redis to
listen only on a v4 address than a v6 address or both a v4 and a v6
address. When getaddrinfo(3) would returns a v6 address that Redis is
not listening on, the user would see ECONNREFUSED or similar without
this patch. This is clearly undesirable behavior.

Additionally, per the getaddrinfo(3) man page, we should try to use the
addresses it returns in the order they are returned.

Author: pietern

lib/redis/connection/ruby.rb

@@ -114,10 +114,9 @@ class TCPSocket < ::Socket
 
         include SocketMixin
 
-        def self.connect(host, port, timeout)
-          addr = ::Socket.getaddrinfo(host, nil, Socket::AF_UNSPEC)
-          sock = new(::Socket.const_get(addr[0][0]), Socket::SOCK_STREAM, 0)
-          sockaddr = ::Socket.pack_sockaddr_in(port, addr[0][3])
+        def self.connect_addrinfo(ai, port, timeout)
+          sock = new(::Socket.const_get(ai[0]), Socket::SOCK_STREAM, 0)
+          sockaddr = ::Socket.pack_sockaddr_in(port, ai[3])
 
           begin
             sock.connect_nonblock(sockaddr)
@@ -134,6 +133,25 @@ def self.connect(host, port, timeout)
 
           sock
         end
+
+        def self.connect(host, port, timeout)
+          addrinfo = ::Socket.getaddrinfo(host, nil, Socket::AF_UNSPEC, Socket::SOCK_STREAM)
+
+          # From the man page for getaddrinfo(3):
+          #
+          #   Normally, the application should try using the addresses in the
+          #   order in which they are returned. The sorting function used
+          #   within getaddrinfo() is defined in RFC 3484 [...].
+          #
+          addrinfo.each_with_index do |ai, i|
+            begin
+              return connect_addrinfo(ai, port, timeout)
+            rescue SystemCallError
+              # Raise if this was our last attempt.
+              raise if addrinfo.length == i+1
+            end
+          end
+        end
       end
 
       class UNIXSocket < ::Socket