Always call the ConnectionPool.shutdown()-callback.

- If shutting down, the callback is being passed an error.

lib/driver.js

@@ -742,14 +742,15 @@ PooledConnection.prototype._getNextCon = function(callback) {
 PooledConnection.prototype.shutdown = function(callback) {
   var self = this;
 
+  callback = callback || function() {};
+
   // Start shutdown mode, causes no new execute()'s to be accepted
   if (self.shuttingDown) {
-    return;
+    var err = new Error('Already shutting down.');
+    return callback(err);
   }
   self.shuttingDown = true;
 
-  callback = callback || function() {};
-
   // Close all open connections as soon as the pool has drained
   self.once('drain', function() {
     self._closeConnections(function() {

test/test_driver.js

@@ -1083,3 +1083,37 @@ exports.testPooledConnectionShutdown = function(test, assert) {
     test.finish();
   });
 };
+
+exports.testPooledConnectionShutdownTwice = function(test, assert) {
+  var hosts = ['127.0.0.1:19170'];
+  var conn = new PooledConnection({'hosts': hosts, 'keyspace': 'Keyspace1'});
+
+  var expected = 100;
+  var cbcount = 0;
+  var spy = function(err, res) {
+    assert.ifError(err);
+    cbcount++;
+  };
+
+  for (var i = 0; i < expected; i++) {
+    (function(index) {
+      conn.execute('UPDATE CfUtf8 SET ? = ? WHERE KEY = ?', ['col', 'val', 'key'+index], spy);
+    })(i);
+  }
+
+  assert.ok(!conn.shuttingDown);
+  conn.shutdown(function(err) {
+    assert.ifError(err);
+    assert.equal(cbcount, expected);
+    assert.ok(secondCbCalledImmediatelyWithError);
+    test.finish();
+  });
+
+  // Make sure second callback gets called immediately with an error
+  var secondCbCalledImmediatelyWithError = false;
+  assert.ok(conn.shuttingDown);
+  conn.shutdown(function(err) {
+    assert.ok(err);
+    secondCbCalledImmediatelyWithError = true;
+  });
+};