fix(serialization): normalize function stringification

A non-default option for serialization allows for the automatically
stringifying javascript functions. Function.prototype.toString has
changed between versions of node, so we need to normalize so as to
not potentially break compatibility with earlier expectations. This
should have marginal performance effects only in the case where
users are actually using this functionality.

NODE-1499

lib/bson/parser/calculate_size.js

@@ -13,6 +13,8 @@ var Long = require('../long').Long,
   DBRef = require('../db_ref').DBRef,
   Binary = require('../binary').Binary;
 
+var normalizedFunctionString = require('./utils').normalizedFunctionString;
+
 // To ensure that 0.4 of node works correctly
 var isDate = function isDate(d) {
   return typeof d === 'object' && Object.prototype.toString.call(d) === '[object Date]';
@@ -221,7 +223,7 @@ function calculateElement(name, value, serializeFunctions, isArray, ignoreUndefi
             1 +
             4 +
             4 +
-            Buffer.byteLength(value.toString(), 'utf8') +
+            Buffer.byteLength(normalizedFunctionString(value), 'utf8') +
             1 +
             calculateObjectSize(value.scope, serializeFunctions, ignoreUndefined)
           );
@@ -230,7 +232,7 @@ function calculateElement(name, value, serializeFunctions, isArray, ignoreUndefi
             (name != null ? Buffer.byteLength(name, 'utf8') + 1 : 0) +
             1 +
             4 +
-            Buffer.byteLength(value.toString(), 'utf8') +
+            Buffer.byteLength(normalizedFunctionString(value), 'utf8') +
             1
           );
         }

lib/bson/parser/serializer.js

@@ -6,6 +6,8 @@ var writeIEEE754 = require('../float_parser').writeIEEE754,
   Map = require('../map'),
   Binary = require('../binary').Binary;
 
+const normalizedFunctionString = require('./utils').normalizedFunctionString;
+
 // try {
 //   var _Buffer = Uint8Array;
 // } catch (e) {
@@ -443,7 +445,8 @@ var serializeFunction = function(buffer, key, value, index, checkKeys, depth, is
   index = index + numberOfWrittenBytes;
   buffer[index++] = 0;
   // Function string
-  var functionString = value.toString();
+  var functionString = normalizedFunctionString(value);
+
   // Write the string
   var size = buffer.write(functionString, index + 4, 'utf8') + 1;
   // Write the size of the string to buffer

lib/bson/parser/utils.js

@@ -0,0 +1,13 @@
+'use strict';
+
+/**
+ * Normalizes our expected stringified form of a function across versions of node
+ * @param {Function} fn The function to stringify
+ */
+function normalizedFunctionString(fn) {
+  return fn.toString().replace('function(', 'function (');
+}
+
+module.exports = {
+  normalizedFunctionString
+};