Fix race condition in Directory.create.

runtime/bin/directory_win.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "bin/directory.h"
 
 #include <errno.h>
 #include <sys/stat.h>
 
 #include "bin/log.h"
@@ skipping 326 matching lines @@
   wchar_t* current = new wchar_t[length + 1];
   GetCurrentDirectoryW(length + 1, current);
   char* result = StringUtils::WideToUtf8(current);
   delete[] current;
   return result;
 }
 
 
 bool Directory::Create(const char* dir_name) {
   const wchar_t* system_name = StringUtils::Utf8ToWide(dir_name);
-  // If the directory already exists and is a directory do not
-  // attempt to create it again and treat it as a success.
-  if (ExistsHelper(system_name) == EXISTS) {
+  int create_status = CreateDirectoryW(system_name, NULL);
+  // If the directory already existed, treat it as a success.
+  if (create_status == 0 &&
+      GetLastError() == ERROR_ALREADY_EXISTS &&
+      ExistsHelper(system_name) == EXISTS) {
     free(const_cast<wchar_t*>(system_name));
     return true;
   }
-  int create_status = CreateDirectoryW(system_name, NULL);
   free(const_cast<wchar_t*>(system_name));
   return (create_status != 0);
 }
 
 
 char* Directory::CreateTemp(const char* const_template) {
   // Returns a new, unused directory name, modifying the contents of
   // dir_template.  Creates this directory, with a default security
   // descriptor inherited from its parent directory.
   // The return value must be freed by the caller.
@@ skipping 73 matching lines @@
     bool success = DeleteRecursively(system_new_path);
     if (!success) return false;
   }
   DWORD flags = MOVEFILE_WRITE_THROUGH;
   int move_status =
       MoveFileExW(system_path, system_new_path, flags);
   free(const_cast<wchar_t*>(system_path));
   free(const_cast<wchar_t*>(system_new_path));
   return (move_status != 0);
 }