Add fs option to allow passing virtual filesystem

PR-URL: #430
Credit: @mmkal
Close: #430
Reviewed-by: @isaacs

Author: mmkal

README.md

@@ -276,6 +276,9 @@ the filesystem.
 * `absolute` Set to true to always receive absolute paths for matched
   files.  Unlike `realpath`, this also affects the values returned in
   the `match` event.
+* `fs` File-system object with Node's `fs` API. By default, the built-in
+  `fs` module will be used. Set to a volume provided by a library like
+  `memfs` to avoid using the "real" file-system.
 
 ## Comparisons to other fnmatch/glob implementations
 

common.js

@@ -10,6 +10,7 @@ function ownProp (obj, field) {
   return Object.prototype.hasOwnProperty.call(obj, field)
 }
 
+var fs = require("fs")
 var path = require("path")
 var minimatch = require("minimatch")
 var isAbsolute = require("path-is-absolute")
@@ -75,6 +76,7 @@ function setopts (self, pattern, options) {
   self.stat = !!options.stat
   self.noprocess = !!options.noprocess
   self.absolute = !!options.absolute
+  self.fs = options.fs || fs
 
   self.maxLength = options.maxLength || Infinity
   self.cache = options.cache || Object.create(null)

glob.js

@@ -40,7 +40,6 @@
 
 module.exports = glob
 
-var fs = require('fs')
 var rp = require('fs.realpath')
 var minimatch = require('minimatch')
 var Minimatch = minimatch.Minimatch
@@ -501,7 +500,7 @@ Glob.prototype._readdirInGlobStar = function (abs, cb) {
   var lstatcb = inflight(lstatkey, lstatcb_)
 
   if (lstatcb)
-    fs.lstat(abs, lstatcb)
+    self.fs.lstat(abs, lstatcb)
 
   function lstatcb_ (er, lstat) {
     if (er && er.code === 'ENOENT')
@@ -542,7 +541,7 @@ Glob.prototype._readdir = function (abs, inGlobStar, cb) {
   }
 
   var self = this
-  fs.readdir(abs, readdirCb(this, abs, cb))
+  self.fs.readdir(abs, readdirCb(this, abs, cb))
 }
 
 function readdirCb (self, abs, cb) {
@@ -746,13 +745,13 @@ Glob.prototype._stat = function (f, cb) {
   var self = this
   var statcb = inflight('stat\0' + abs, lstatcb_)
   if (statcb)
-    fs.lstat(abs, statcb)
+    self.fs.lstat(abs, statcb)
 
   function lstatcb_ (er, lstat) {
     if (lstat && lstat.isSymbolicLink()) {
       // If it's a symlink, then treat it as the target, unless
       // the target does not exist, then treat it as a file.
-      return fs.stat(abs, function (er, stat) {
+      return self.fs.stat(abs, function (er, stat) {
         if (er)
           self._stat2(f, abs, null, lstat, cb)
         else