realpath: No sync cb() calling allowed.

lib/fs.js

@@ -1070,7 +1070,7 @@ fs.realpath = function realpath(p, cache, cb) {
   p = pathModule.resolve(p);
 
   if (cache && Object.prototype.hasOwnProperty.call(cache, p)) {
-    return cb(null, cache[p]);
+    return process.nextTick(cb.bind(null, null, cache[p]));
   }
 
   var original = p,
@@ -1088,7 +1088,7 @@ fs.realpath = function realpath(p, cache, cb) {
 
   // walk down the path, swapping out linked pathparts for their real
   // values
-  LOOP();
+  return process.nextTick(LOOP);
   function LOOP() {
     // stop if scanned past end of path
     if (pos >= p.length) {

test/simple/test-fs-realpath.js

@@ -515,10 +515,13 @@ function test_lying_cache_liar(cb) {
   var rps = fs.realpathSync(bluff, cache);
   assert.equal(cache[bluff], rps);
   var nums = path.resolve('/1/2/3/4/5/6/7');
+  var called = false; // no sync cb calling!
   fs.realpath(nums, cache, function(er, rp) {
+    called = true;
     assert.equal(cache[nums], rp);
     if (--n === 0) cb();
   });
+  assert(called === false);
 
   var test = path.resolve('/a/b/c/d'),
       expect = path.resolve('/a/b/d');