Fixing a race condition in landmines.py

build/landmines.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """
 This script runs every build as a hook. If it detects that the build should
 be clobbered, it will touch the file <build_dir>/.landmine_triggered. The
 various build scripts will then check for the presence of this file and clobber
 accordingly. The script will also emit the reasons for the clobber to stdout.
 
 A landmine is tripped when a builder checks out a different revision, and the
 diff between the new landmines and the old ones is non-null. At this point, the
 build is clobbered.
 """
 
 import difflib
+import errno
 import logging
 import optparse
 import os
 import sys
 import subprocess
 import time
 
 import landmine_utils
 
 
@@ skipping 22 matching lines @@
     raise NotImplementedError('Unexpected GYP_GENERATORS (%s)' % build_tool)
   return os.path.abspath(ret)
 
 
 def set_up_landmines(target, new_landmines):
   """Does the work of setting, planting, and triggering landmines."""
   out_dir = get_target_build_dir(landmine_utils.builder(), target,
                                  landmine_utils.platform() == 'ios')
 
   landmines_path = os.path.join(out_dir, '.landmines')
-  if not os.path.exists(out_dir):
+  try:
     os.makedirs(out_dir)
+  except OSError as e:
+    if e.errno == errno.EEXIST:
+      pass
 
   if not os.path.exists(landmines_path):
     with open(landmines_path, 'w') as f:
       f.writelines(new_landmines)
   else:
     triggered = os.path.join(out_dir, '.landmines_triggered')
     with open(landmines_path, 'r') as f:
       old_landmines = f.readlines()
     if old_landmines != new_landmines:
       old_date = time.ctime(os.stat(landmines_path).st_ctime)
@@ skipping 51 matching lines @@
                               stdout=subprocess.PIPE)
       output, _ = proc.communicate()
       landmines.extend([('%s\n' % l.strip()) for l in output.splitlines()])
     set_up_landmines(target, landmines)
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())