Consider adding mcast support again?

[complyue]

I see #381 has been concluded, but as I'm re-implementing in Haskell a swarm control infrastructure managing clusters of number crunching servers originally implemented in Python, I miss basic UDP mcast support very much. No streaming requirement, just job announcement and discovery with single frag UDP packets, further comm always resorts to TCP connections.

network-multicast seemingly will work for me, but it feels weird I'd need another separate package to do that.

As for API issue, can you consider add one like setSocketOptionBuf, or a same named one in ByteString module? Like that we have sendBuf and send in ByteString superseding that in top-level ?

In Python it's all handled by socket package like:

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

if is_mcast:
    # join the multicast group
    group = socket.inet_pton(socket.AF_INET, swarm_ip)
    ifa = socket.inet_pton(socket.AF_INET, self.mcast_through or "0.0.0.0")
    mreq = group + ifa
    sock.setsockopt(socket.IPPROTO_IP, socket.IP_ADD_MEMBERSHIP, mreq)

sock.bind((swarm_ip, self.swarm_port))

64-bit suffices with mreq for IPv4, so event an Int may probably work, but may be not a good idea.

[kazu-yamamoto]

We will have such APIs when #445 is merged :)

[kazu-yamamoto]

Not correct. master already has getSockOpt and setSockOpt!

[complyue]

sure, then I'll use it.