fune/netwerk/test/unit/test_addr_in_use_error.js

// Opening a second listening socket on the same address as an extant
// socket should elicit NS_ERROR_SOCKET_ADDRESS_IN_USE on non-Windows
// machines.

"use strict";

var CC = Components.Constructor;

const ServerSocket = CC(
  "@mozilla.org/network/server-socket;1",
  "nsIServerSocket",
  "init"
);

function testAddrInUse() {
  // Windows lets us have as many sockets listening on the same address as
  // we like, evidently.
  if (mozinfo.os == "win") {
    return;
  }

  // Create listening socket:
  // any port (-1), loopback only (true), default backlog (-1)
  let listener = ServerSocket(-1, true, -1);
  Assert.ok(listener instanceof Ci.nsIServerSocket);

  // Try to create another listening socket on the same port, whatever that was.
  do_check_throws_nsIException(
    () => ServerSocket(listener.port, true, -1),
    "NS_ERROR_SOCKET_ADDRESS_IN_USE"
  );
}

function run_test() {
  testAddrInUse();
}