fs: make fs.access() flags read only

These flags were defined as constants, but could be
overwritten when exported from fs. This commit exports
the flags as read only properties of fs.

PR-URL: #507
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/fs.js

@@ -27,10 +27,6 @@ const O_RDWR = constants.O_RDWR || 0;
 const O_SYNC = constants.O_SYNC || 0;
 const O_TRUNC = constants.O_TRUNC || 0;
 const O_WRONLY = constants.O_WRONLY || 0;
-const F_OK = constants.F_OK || 0;
-const R_OK = constants.R_OK || 0;
-const W_OK = constants.W_OK || 0;
-const X_OK = constants.X_OK || 0;
 
 const isWindows = process.platform === 'win32';
 
@@ -164,18 +160,20 @@ fs.Stats.prototype.isSocket = function() {
   return this._checkModeProperty(constants.S_IFSOCK);
 };
 
-fs.F_OK = F_OK;
-fs.R_OK = R_OK;
-fs.W_OK = W_OK;
-fs.X_OK = X_OK;
+// Don't allow mode to accidentally be overwritten.
+['F_OK', 'R_OK', 'W_OK', 'X_OK'].forEach(function(key) {
+  Object.defineProperty(fs, key, {
+    enumerable: true, value: constants[key] || 0, writable: false
+  });
+});
 
 fs.access = function(path, mode, callback) {
   if (!nullCheck(path, callback))
     return;
 
   if (typeof mode === 'function') {
     callback = mode;
-    mode = F_OK;
+    mode = fs.F_OK;
   } else if (typeof callback !== 'function') {
     throw new TypeError('callback must be a function');
   }
@@ -190,7 +188,7 @@ fs.accessSync = function(path, mode) {
   nullCheck(path);
 
   if (mode === undefined)
-    mode = F_OK;
+    mode = fs.F_OK;
   else
     mode = mode | 0;