Add infra_libs as a bootstrap dependency.

infra_libs/experiments.py

-# Copyright 2015 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.
-
-"""Tools for gradually enabling a feature on a deterministic set of machines.
-
-Add a flag to your program to control the percentage of machines that a new
-feature should be enabled on::
-
-    def add_argparse_options(self, parser):
-      parser.add_argument('--myfeature-percent', type=int, default=0)
-
-    def main(self, opts):
-      if experiments.is_active_for_host('myfeature', opts.myfeature_percent):
-        # do myfeature
-"""
-
-import hashlib
-import logging
-import socket
-import struct
-
-
-def _is_active(labels, percent):
-  h = hashlib.md5()
-  for label, value in sorted(labels.iteritems()):
-    h.update(label)
-    h.update(value)
-
-  # The first 8 bytes of the hash digest as an unsigned integer.
-  hash_num = struct.unpack_from('Q', h.digest())[0]
-
-  return (hash_num % 100) < percent
-
-
-def is_active_for_host(experiment_name, percent):
-  ret = _is_active({
-      'name': experiment_name,
-      'host': socket.getfqdn(),
-  }, percent)
-
-  if ret:
-    logging.info('Experiment "%s" is active', experiment_name)
-
-  return ret