Fix fs.stat(), fs.lstat(), and fs.fstat() for Node 7.7+

Node 7.7 changed the behavior of the `binding.{stat,lstat,fstat}` functions (see nodejs/node#11522). This updates the binding functions used in mock-fs to match the new Node binding behavior, while still maintaining compatibility with old Node versions. The new behavior is detected when the second argument to `binding.{stat,lstat,fstat}` is a `Float64Array`, which would be an invalid argument for previous versions of the binding.

lib/binding.js

@@ -20,7 +20,7 @@ var getPathParts = require('./filesystem').getPathParts;
  * @return {*} Return (if callback is not provided).
  */
 function maybeCallback(callback, thisArg, func) {
-  if (callback) {
+  if (callback && (typeof callback === 'function' || typeof callback.oncomplete === 'function')) {
     var err = null;
     var val;
     try {
@@ -266,11 +266,36 @@ Binding.prototype.realpath = function(filepath, encoding, callback) {
   });
 };
 
+/**
+ * Fill a Float64Array with stat information
+ * This is based on the internal FillStatsArray function in Node.
+ * https://github.com/nodejs/node/blob/4e05952a8a75af6df625415db612d3a9a1322682/src/node_file.cc#L533
+ * @param {object} stats An object with file stats
+ * @param {Float64Array} statValues A Float64Array where stat values should be inserted
+ * @returns {void}
+ */
+function fillStatsArray(stats, statValues) {
+  statValues[0] = stats.dev;
+  statValues[1] = stats.mode;
+  statValues[2] = stats.nlink;
+  statValues[3] = stats.uid;
+  statValues[4] = stats.gid;
+  statValues[5] = stats.rdev;
+  statValues[6] = stats.blksize;
+  statValues[7] = stats.ino;
+  statValues[8] = stats.size;
+  statValues[9] = stats.blocks;
+  statValues[10] = +stats.atime;
+  statValues[11] = +stats.mtime;
+  statValues[12] = +stats.ctime;
+  statValues[13] = +stats.birthtime;
+}
 
 /**
  * Stat an item.
  * @param {string} filepath Path.
- * @param {function(Error, Stats)} callback Callback (optional).
+ * @param {function(Error, Stats)|Float64Array} callback Callback (optional). In Node 7.7.0+ this will be a Float64Array
+ * that should be filled with stat values.
  * @return {Stats|undefined} Stats or undefined (if sync).
  */
 Binding.prototype.stat = function(filepath, callback) {
@@ -283,22 +308,41 @@ Binding.prototype.stat = function(filepath, callback) {
     if (!item) {
       throw new FSError('ENOENT', filepath);
     }
-    return new Stats(item.getStats());
+    var stats = item.getStats();
+
+    // In Node 7.7.0+, binding.stat accepts a Float64Array as the second argument,
+    // which should be filled with stat values.
+    // In prior versions of Node, binding.stat simply returns a Stats instance.
+    if (callback instanceof Float64Array) {
+      fillStatsArray(stats, callback);
+    } else {
+      return new Stats(stats);
+    }
   });
 };
 
 
 /**
  * Stat an item.
  * @param {number} fd File descriptor.
- * @param {function(Error, Stats)} callback Callback (optional).
+ * @param {function(Error, Stats)|Float64Array} callback Callback (optional). In Node 7.7.0+ this will be a Float64Array
+ * that should be filled with stat values.
  * @return {Stats|undefined} Stats or undefined (if sync).
  */
 Binding.prototype.fstat = function(fd, callback) {
   return maybeCallback(callback, this, function() {
     var descriptor = this._getDescriptorById(fd);
     var item = descriptor.getItem();
-    return new Stats(item.getStats());
+    var stats = item.getStats();
+
+    // In Node 7.7.0+, binding.stat accepts a Float64Array as the second argument,
+    // which should be filled with stat values.
+    // In prior versions of Node, binding.stat simply returns a Stats instance.
+    if (callback instanceof Float64Array) {
+      fillStatsArray(stats, callback);
+    } else {
+      return new Stats(stats);
+    }
   });
 };
 
@@ -933,7 +977,8 @@ Binding.prototype.readlink = function(pathname, encoding, callback) {
 /**
  * Stat an item.
  * @param {string} filepath Path.
- * @param {function(Error, Stats)} callback Callback (optional).
+ * @param {function(Error, Stats)|Float64Array} callback Callback (optional). In Node 7.7.0+ this will be a Float64Array
+ * that should be filled with stat values.
  * @return {Stats|undefined} Stats or undefined (if sync).
  */
 Binding.prototype.lstat = function(filepath, callback) {
@@ -942,7 +987,16 @@ Binding.prototype.lstat = function(filepath, callback) {
     if (!item) {
       throw new FSError('ENOENT', filepath);
     }
-    return new Stats(item.getStats());
+    var stats = item.getStats();
+
+    // In Node 7.7.0+, binding.stat accepts a Float64Array as the second argument,
+    // which should be filled with stat values.
+    // In prior versions of Node, binding.stat simply returns a Stats instance.
+    if (callback instanceof Float64Array) {
+      fillStatsArray(stats, callback);
+    } else {
+      return new Stats(item.getStats());
+    }
   });
 };