dns: throw if hostname is not string or falsey

Fix assertion failure from poor argument parsing logic introduced in
6ea5d16. Add tests to make sure arguments are properly parsed.

Fixes: 6ea5d16 "dns: always set variable family in lookup()"
Reviewed-by: Trevor Norris <trev.norris@gmail.com>

lib/dns.js

@@ -105,7 +105,9 @@ exports.lookup = function lookup(hostname, options, callback) {
   var family = -1;
 
   // Parse arguments
-  if (typeof options === 'function') {
+  if (hostname && typeof hostname !== 'string') {
+    throw TypeError('invalid arguments: hostname must be a string or falsey');
+  } else if (typeof options === 'function') {
     callback = options;
     family = 0;
   } else if (typeof callback !== 'function') {

test/simple/test-dns.js

@@ -69,6 +69,47 @@ assert.throws(function() {
   return !(err instanceof TypeError);
 }, 'Unexpected error');
 
+// dns.lookup should accept falsey and string values
+assert.throws(function() {
+  dns.lookup({}, noop);
+}, 'invalid arguments: hostname must be a string or falsey');
+
+assert.throws(function() {
+  dns.lookup([], noop);
+}, 'invalid arguments: hostname must be a string or falsey');
+
+assert.throws(function() {
+  dns.lookup(true, noop);
+}, 'invalid arguments: hostname must be a string or falsey');
+
+assert.throws(function() {
+  dns.lookup(1, noop);
+}, 'invalid arguments: hostname must be a string or falsey');
+
+assert.throws(function() {
+  dns.lookup(noop, noop);
+}, 'invalid arguments: hostname must be a string or falsey');
+
+assert.doesNotThrow(function() {
+  dns.lookup('', noop);
+});
+
+assert.doesNotThrow(function() {
+  dns.lookup(null, noop);
+});
+
+assert.doesNotThrow(function() {
+  dns.lookup(undefined, noop);
+});
+
+assert.doesNotThrow(function() {
+  dns.lookup(0, noop);
+});
+
+assert.doesNotThrow(function() {
+  dns.lookup(NaN, noop);
+});
+
 /*
  * Make sure that dns.lookup throws if hints does not represent a valid flag.
  * (dns.V4MAPPED | dns.ADDRCONFIG) + 1 is invalid because: