def SlaveBuildSet(master_buildbucket_id):
  """Compute the buildset id for all slaves of a master builder.

  Args:
    master_buildbucket_id: The buildbucket id of the master build.

  Returns:
    A string to use as a buildset for the slave builders, or None.
  """
  if not master_buildbucket_id:
    return None

  return 'cros/master_buildbucket_id/%s' % master_buildbucket_id