DatagramChannel join without a network interface could use the channel option multicast interface instead of guessing from the local address

[vietj]

Expected behavior

DatagramChannel#joinGroup(multicastAddress) calls DatagramChannel.joinGroup(multicastAddress, networkInterface, source) with the same network interface set on the channel options.

The code would look like

        try {
            return joinGroup(
                    multicastAddress,
                    config().getNetworkInterface(),
                    null, promise);
        } catch (SocketException e) {
            promise.setFailure(e);
        }
        return promise;

Actual behavior

DatagramChannel#joinGroup(multicastAddress) calls DatagramChannel.joinGroup(multicastAddress, networkInterface, source) with NetworkInterface.getByInetAddress(localAddress().getAddress()) that sometimes returns the null reference.

Steps to reproduce

Obvious

Minimal yet complete reproducer code (or URL to code)

Obvious

Netty version

Most

JVM version (e.g. java -version)

OS version (e.g. uname -a)

[NiteshKant]

this sounds reasonable to me, @vietj you want to send a PR, I am happy to review.

[normanmaurer]

@vietj PTAL #11585