win32 rename EPERM: fail fast if target exists

When `rename` is used on Windows, and encounters an EPERM or EACCES
error, graceful-fs will retry for up to 30 seconds with gradual backoff
to get around file system locking due to A/V etc.

This was fixed in 59145ce

However, this is a problem if the target exists, which can cause an
EPERM or EACCES that is not due to folder locking.

This commit causes it to fail fast when an EPERM or EACCES is
raised by rename if the target exists.

Fix #98

polyfills.js

@@ -3,6 +3,9 @@ var constants = require('constants')
 
 var origCwd = process.cwd
 var cwd = null
+
+var platform = process.env.GRACEFUL_FS_PLATFORM || process.platform
+
 process.cwd = function() {
   if (!cwd)
     cwd = origCwd.call(process)
@@ -87,7 +90,7 @@ function patch (fs) {
   // failures. Also, take care to yield the scheduler. Windows scheduling gives
   // CPU to a busy looping process, which can cause the program causing the lock
   // contention to be starved of CPU by node, so the contention doesn't resolve.
-  if (process.platform === "win32") {
+  if (platform === "win32") {
     fs.rename = (function (fs$rename) { return function (from, to, cb) {
       var start = Date.now()
       var backoff = 0;
@@ -96,7 +99,12 @@ function patch (fs) {
             && (er.code === "EACCES" || er.code === "EPERM")
             && Date.now() - start < 60000) {
           setTimeout(function() {
-            fs$rename(from, to, CB);
+            fs.stat(to, function (stater, st) {
+              if (stater && stater.code === "ENOENT")
+                fs$rename(from, to, CB);
+              else
+                cb(er)
+            })
           }, backoff)
           if (backoff < 100)
             backoff += 10;

test/windows-rename-polyfill.js

@@ -0,0 +1,35 @@
+process.env.GRACEFUL_FS_PLATFORM = 'win32'
+
+var fs = require('fs')
+fs.rename = function (a, b, cb) {
+  setTimeout(function () {
+    var er = new Error('EPERM blerg')
+    er.code = 'EPERM'
+    cb(er)
+  })
+}
+
+var gfs = require('../')
+var t = require('tap')
+var a = __dirname + '/a'
+var b = __dirname + '/b'
+
+t.test('setup', function (t) {
+  try { fs.mkdirSync(a) } catch (e) {}
+  try { fs.mkdirSync(b) } catch (e) {}
+  t.end()
+})
+
+t.test('rename', { timeout: 100 }, function (t) {
+  t.plan(1)
+
+  gfs.rename(a, b, function (er) {
+    t.ok(er)
+  })
+})
+
+t.test('cleanup', function (t) {
+  try { fs.rmdirSync(a) } catch (e) {}
+  try { fs.rmdirSync(b) } catch (e) {}
+  t.end()
+})