Only clear cache locks if the cache exists.

gclient.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.
 
 """Meta checkout manager supporting both Subversion and GIT."""
 # Files
 #   .gclient      : Current client configuration, written by 'config' command.
 #                   Format is a Python script defining 'solutions', a list whose
 #                   entries each are maps binding the strings "name" and "url"
@@ skipping 1204 matching lines @@
     if config_dict.get('target_os_only', False):
       self._enforced_os = tuple(set(target_os))
     else:
       self._enforced_os = tuple(set(self._enforced_os).union(target_os))
 
     cache_dir = config_dict.get('cache_dir')
     if cache_dir:
       cache_dir = os.path.join(self.root_dir, cache_dir)
       cache_dir = os.path.abspath(cache_dir)
       # If running on a bot, force break any stale git cache locks.
-      if os.environ.get('CHROME_HEADLESS'):
+      if os.path.exists(cache_dir) and os.environ.get('CHROME_HEADLESS'):
         subprocess2.check_call(['git', 'cache', 'unlock', '--cache-dir',
                                 cache_dir, '--force', '--all'])
     gclient_scm.GitWrapper.cache_dir = cache_dir
     git_cache.Mirror.SetCachePath(cache_dir)
 
     if not target_os and config_dict.get('target_os_only', False):
       raise gclient_utils.Error('Can\'t use target_os_only if target_os is '
                                 'not specified')
 
     deps_to_add = []
@@ skipping 961 matching lines @@
     print >> sys.stderr, 'Error: %s' % str(e)
     return 1
   finally:
     gclient_utils.PrintWarnings()
 
 
 if '__main__' == __name__:
   sys.exit(Main(sys.argv[1:]))
 
 # vim: ts=2:sw=2:tw=80:et: