Support passing in inet6 to gen_tcp:connect

[acco]

Hey there,

As far as I can tell, the {socket_options, SocketOpts} are gen_tcp:connect_option:

https://github.com/Nordix/eredis/blob/master/src/eredis_client.erl#L455

Note it's not a list of tuples – one of the options is a bare atom!

-type connect_option() ::
          {fd, Fd :: non_neg_integer()} |
          inet:address_family() |
          {ifaddr, socket:sockaddr_in() | socket:sockaddr_in6() | inet:socket_address()} |
          {ip, inet:socket_address()} |
          {port, inet:port_number()} |
          {tcp_module, module()} |
          {netns, file:filename_all()} |
          {bind_to_device, binary()} |
          option().

Namely, inet:address_family():

-type address_family() :: inet | inet6 | local.

So, to setup inet6, one needs to be able to pass in {socket_options, [inet6]} to eredis.

However, these lines assume a keyword list:
https://github.com/Nordix/eredis/blob/master/src/eredis_client.erl#L438-L442

So, we get an ArgumentError: lists.erl:1510: :lists.ukeymerge_1/3

Happy to open a PR here. As I understand it, current behavior is just trying to merge in default ?SOCKET_OPTS to provided opts

[acco]

Ah I see. The address family is automatically parsed and used to setup SocketOptions:

https://github.com/Nordix/eredis/blob/master/src/eredis_client.erl#L433

Issue I ran into is a common one in Docker: inet:getaddrs(Hostname, inet6) will resolve for host.docker.internal, but you can't connect without special networking setup. inet:getaddrs(Hostname, inet) will resolve too, and connect by default.

One easy way around this is to flip the check so we check inet:getaddrs(Hostname, inet) first and see if that's valid an resolves. If it is, use ipv4. Otherwise, try ipv6.

After all, ipv4 is far more widely supported anyway.

[zuiderkwast]

IMO we could just let gen_tcp:connect do the DNS lookup and accept the inet and inet6 options. We should fix the code that assumes a list of tuples. We can probabaly use functions from proplists instead of lists.