avoid race between dir existence check and dir creation

tools/generate_stubs/generate_stubs.py

 #!/usr/bin/env python
 # Copyright (c) 2011 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """Creates windows and posix stub files for a given set of signatures.
 
 For libraries that need to be loaded outside of the standard executable startup
 path mechanism, stub files need to be generated for the wanted functions.  In
 windows, this is done via "def" files and the delay load mechanism.  On a posix
@@ skipping 906 matching lines @@
 
   if options.type == FILE_TYPE_POSIX_STUB:
     if options.stubfile_name is None:
       parser.error('Output file name need for %s' % FILE_TYPE_POSIX_STUB)
     if options.path_from_source is None:
       parser.error('Path from source needed for %s' % FILE_TYPE_POSIX_STUB)
 
   return options, args
 
 
+def EnsureDirExists(dir):
+  """Creates a directory. Does not use the more obvious 'if not exists: create'
+  to avoid race with other invocations of the same code, which will error out
+  on makedirs if another invocation has succeeded in creating the directory
+  since the existence check."""
+  try:
+    os.makedirs(dir)
+  except:
+    if not os.path.isdir(dir):
+      raise
+
+
 def CreateOutputDirectories(options):
   """Creates the intermediate and final output directories.
 
   Given the parsed options, create the intermediate and final output
   directories if they do not exist.  Returns the paths to both directories
   as a pair.
 
   Args:
     options: An OptionParser.OptionValues object with the parsed options.
 
   Returns:
     The pair (out_dir, intermediate_dir), both of which are strings.
   """
   out_dir = os.path.normpath(options.out_dir)
   intermediate_dir = os.path.normpath(options.intermediate_dir)
   if intermediate_dir is None:
     intermediate_dir = out_dir
 
-  if not os.path.exists(out_dir):
-    os.makedirs(out_dir)
-  if not os.path.exists(intermediate_dir):
-    os.makedirs(intermediate_dir)
+  EnsureDirExists(out_dir)
+  EnsureDirExists(intermediate_dir)
 
   return out_dir, intermediate_dir
 
 
 def CreateWindowsLibForSigFiles(sig_files, out_dir, intermediate_dir):
   """For each signature file, create a windows lib.
 
   Args:
     sig_files: Array of Strings with the paths to each signature file.
     out_dir: String holding path to directory where the generated libs go.
@@ skipping 85 matching lines @@
   if options.type == FILE_TYPE_WIN:
     CreateWindowsLibForSigFiles(args, out_dir, intermediate_dir)
   elif options.type == FILE_TYPE_POSIX_STUB:
     CreatePosixStubsForSigFiles(args, options.stubfile_name, out_dir,
                                 intermediate_dir, options.path_from_source,
                                 options.extra_stub_header)
 
 
 if __name__ == '__main__':
   main()