Fix RunTests to not stall on empty test list

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__ = '1.6.2'
 
@@ skipping 460 matching lines @@
   @staticmethod
   def RunTests(tests_mix, parallel=True):
     tests = []
     msgs = []
     for t in tests_mix:
       if isinstance(t, OutputApi.PresubmitResult):
         msgs.append(t)
       else:
         assert issubclass(t.message, _PresubmitResult)
         tests.append(t)
-    if parallel:
+    if tests and parallel:
       pool = multiprocessing.Pool()
       # async recipe works around multiprocessing bug handling Ctrl-C
       msgs.extend(pool.map_async(CallCommand, tests).get(99999))
       pool.close()
       pool.join()
     else:
       msgs.extend(map(CallCommand, tests))
     return [m for m in msgs if m]
 
 
@@ skipping 891 matching lines @@
   except PresubmitFailure, e:
     print >> sys.stderr, e
     print >> sys.stderr, 'Maybe your depot_tools is out of date?'
     print >> sys.stderr, 'If all fails, contact maruel@'
     return 2
 
 
 if __name__ == '__main__':
   fix_encoding.fix_encoding()
   sys.exit(Main(None))