8233435: (dc) DatagramChannel should allow IPv6 socket join IPv4 multicast groups (macOS, win)

[Alan Bateman]

If a DatagramChannel is created with the no-arg open method or with the 
protocol family INET6 then the channel cannot join IPv4 multicast groups 
on macOS and Windows (no issue on Linux or Solaris). Both macOS and 
Windows support this now so we can remove this restriction. The 
recommendation for IPv4 multicasting app is to create the 
DatagramChannel with protocol family INET remains of course.

Related is that on Windows, the StandardSocketOptions.IP_TOS and 
IP_MULTICAST_* socket options cannot be accessed when the socket is a 
dual socket that has been bound to an IPv4 local address. The Windows 
networking stack requires using IP_ rather than IPV6_ socket options for 
this scenario.

The webrev with the proposed changes is here. MulticastSendReceiveTests 
has been updated to run the IPv4 multicasting tests when the system 
under test has both IPv4 and IPv6. SocketOptionTests has been updated to 
exercise reading and setting socket options in additional scenarios, 
including a dual socket bound to an IPv4 local address.

http://cr.openjdk.java.net/~alanb/8233435/webrev/

SAP team - I don't have any access to AIX so 
Net.canIPv6SocketJoinIPv4Group() will return false as it has always 
done. If you have cycles to test this change or return value for this 
operating system changed as part of this issue then let me know. A 
follow-on AIX specific issue is okay too.

-Alan

PS: The motivation for this change, and the recent change to the 
DatagramSocket adaptor, is to get closer to the point where it will be 
possible to re-implement legacy DatagramSocket and MulticastSocket.