test,dgram: add tests for setBroadcast()

The only tests for `setBroadcast()` (from the `dgram` module) were in
`test/internet` which means they almost never get run. This adds a
minimal test that can check JS-land functionality in `test/parallel`.

I also expanded a comment and did some minor formatting on the existing
`test/internet` test. If there were an easy and reliable way to check
for the BROADCAST flag on an interface, it's possible that a version of
the test could be moved to `test/sequential` or `test/parallel` once it
was modified to only use internal networks.

PR-URL: https://github.com/nodejs/node/pull/6750
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/internet/test-dgram-broadcast-multi-process.js

@@ -20,7 +20,9 @@ if (common.inFreeBSDJail) {
   return;
 }
 
-// take the first non-internal interface as the address for binding
+// Take the first non-internal interface as the address for binding.
+// Ideally, this should check for whether or not an interface is set up for
+// BROADCAST and favor internal/private interfaces.
 get_bindAddress: for (var name in networkInterfaces) {
   var interfaces = networkInterfaces[name];
   for (var i = 0; i < interfaces.length; i++) {

test/parallel/test-dgram-setBroadcast.js

@@ -0,0 +1,39 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const dgram = require('dgram');
+
+const setup = () => {
+  return dgram.createSocket({type: 'udp4', reuseAddr: true});
+};
+
+const teardown = (socket) => {
+  if (socket.close)
+    socket.close();
+};
+
+const runTest = (testCode, expectError) => {
+  const socket = setup();
+  const assertion = expectError ? assert.throws : assert.doesNotThrow;
+  const wrapped = () => { testCode(socket); };
+  assertion(wrapped, expectError);
+  teardown(socket);
+};
+
+// Should throw EBADF if socket is never bound.
+runTest((socket) => { socket.setBroadcast(true); }, /EBADF/);
+
+// Should not throw if broadcast set to false after binding.
+runTest((socket) => {
+  socket.bind(common.PORT, common.localhostIPv4, () => {
+    socket.setBroadcast(false);
+  });
+});
+
+// Should not throw if broadcast set to true after binding.
+runTest((socket) => {
+  socket.bind(common.PORT, common.localhostIPv4, () => {
+    socket.setBroadcast(true);
+  });
+});