Fix: fs.open callback did not fire

Problem: Omitting the mode parameter causes the provided callback
parameter to never fire. This was originally fixed in 6078c37b and
later broken in 5f2e9093.

Solution: Overwriting the value of a parameter also overwrites the
reference in the arguments object. This patch works arround this
fact by not touching the mode parameter until a reference to the
callback has been established.

lib/fs.js

@@ -185,10 +185,11 @@ function modeNum(m, def) {
 }
 
 fs.open = function(path, flags, mode, callback) {
-  mode = modeNum(mode, '0666');
   var callback_ = arguments[arguments.length - 1];
   var callback = (typeof(callback_) == 'function' ? callback_ : null);
 
+  mode = modeNum(mode, '0666');
+
   binding.open(path, stringToFlags(flags), mode, callback || noop);
 };
 

test/simple/test-fs-open.js

@@ -0,0 +1,17 @@
+var common = require('../common');
+var assert = require('assert');
+var fs = require('fs');
+
+var openFd;
+fs.open(__filename, 'r', function(err, fd) {
+  if (err) {
+    throw err;
+  }
+
+  openFd = fd;
+});
+
+process.addListener('exit', function() {
+  assert.ok(openFd);
+});
+