Require 'CHROME_HEADLESS' to use 'git-number'.

git_number.py

 #!/usr/bin/env python
 # Copyright 2013 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.
 
 """Usage: %prog [options] [<commitref>]*
 
 If no <commitref>'s are supplied, it defaults to HEAD.
 
 Calculates the generation number for one or more commits in a git repo.
@@ skipping 234 matching lines @@
                     help='Do not actually cache anything we calculate.')
   parser.add_option('--reset', action='store_true',
                     help='Reset the generation number cache and quit.')
   parser.add_option('-v', '--verbose', action='count', default=0,
                     help='Be verbose. Use more times for more verbosity.')
   opts, args = parser.parse_args()
 
   levels = [logging.ERROR, logging.INFO, logging.DEBUG]
   logging.basicConfig(level=levels[min(opts.verbose, len(levels) - 1)])
 
+  # 'git number' should only be used on bots.
+  if os.getenv('CHROME_HEADLESS') != '1':
+    logging.error("'git-number' is an infrastructure tool that is only "
+                  "intended to be used internally by bots. Developers should "
+                  "use the 'Cr-Commit-Position' value in the commit's message.")
+    return 1
+
   try:
     if opts.reset:
       clear_caches(on_disk=True)
       return
 
     try:
       targets = git.parse_commitrefs(*(args or ['HEAD']))
     except git.BadCommitRefException as e:
       parser.error(e)
 
     load_generation_numbers(targets)
     if not opts.no_cache:
       finalize(targets)
 
     print '\n'.join(map(str, map(get_num, targets)))
     return 0
   except KeyboardInterrupt:
     return 1
 
 
 if __name__ == '__main__':  # pragma: no cover
   sys.exit(main())