Do not assume that all presubmit directories still exist.

presubmit_support.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.
 
 """Enables directory-specific presubmit checks to run at upload and/or commit.
 """
 
 __version__ = '1.8.0'
 
@@ skipping 1075 matching lines @@
         break
       parent_dir = os.path.dirname(directory)
       if parent_dir == directory:
         # We hit the system root directory.
         break
       directory = parent_dir
 
   # Look for PRESUBMIT.py in all candidate directories.
   results = []
   for directory in sorted(list(candidates)):
-    for f in os.listdir(directory):
-      p = os.path.join(directory, f)
-      if os.path.isfile(p) and re.match(
-          r'PRESUBMIT.*\.py$', f) and not f.startswith('PRESUBMIT_test'):
-        results.append(p)
+    try:
+      for f in os.listdir(directory):

[tandrii(chromium), 2016/08/17 10:01:56]

the only thing that can raise here is this line, so I don't see a reason to wrap
everything in try-catch. In fact, you could do which is even better:

if not os.path.isdir(directory):
  continue
for f in os.listdir......
  ...

+        p = os.path.join(directory, f)
+        if os.path.isfile(p) and re.match(
+            r'PRESUBMIT.*\.py$', f) and not f.startswith('PRESUBMIT_test'):
+          results.append(p)
+    except OSError:
+      pass
 
   logging.debug('Presubmit files: %s', ','.join(results))
   return results
 
 
 class GetTrySlavesExecuter(object):
   @staticmethod
   def ExecPresubmitScript(script_text, presubmit_path, project, change):
     """Executes GetPreferredTrySlaves() from a single presubmit script.
 
@@ skipping 711 matching lines @@
     return 2
 
 
 if __name__ == '__main__':
   fix_encoding.fix_encoding()
   try:
     sys.exit(main())
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(2)