From http://jira.codehaus.org/browse/JRUBY-7070.

The problem is triggered in the code below by changing to a multicast socket after the bind has happened. In our logic, we defer creating the JDK-level socket object as long as possible, since it is generally not possible to change socket type after that point. setsockopt with IP_ADD_MEMBERSHIP triggers us to flip to using multicast sockets, but without a subsequent bind the actual multicast socket object is never created. That leads to the NPE.

The sequence used in this code is found (for example) in rinda/ring.rb, which binds and then adds membership (at least in the 2.2 version).

The code below should exit normally and the recvfrom should block without raising NPE.

require 'socket'
require 'ipaddr'

MULTICAST_ADDR = '239.255.255.250'
LOCATOR_PORT = 24067

membership = IPAddr.new(MULTICAST_ADDR).hton + IPAddr.new("0.0.0.0").hton
sock = UDPSocket.open
sock.setsockopt(Socket::SOL_SOCKET, Socket::SO_REUSEADDR, 1)

begin
  sock.bind(Socket::INADDR_ANY, LOCATOR_PORT)
rescue Errno::EADDRINUSE
  p :here
  raise Error, "Unable to listen on port #{LOCATOR_PORT},"
end

sock.setsockopt(Socket::IPPROTO_IP, Socket::IP_ADD_MEMBERSHIP, membership)

Thread.new {
  sleep 5
  puts "Exited normally"
  exit  
}


sock.recvfrom(100)