def cmd_rollback(context):
  """
  Roll back by finding the most recent "stable" tagged version, and putting it again, so that
  it's the new "current" version.
  Args:
    context: a populated EFVersionContext object
  """
  last_stable = get_versions(context, return_stable=True)
  if len(last_stable) != 1:
    fail("Didn't find a version marked stable for key: {} in env/service: {}/{}".format(
         context.key, context.env, context.service_name))
  context.value = last_stable[0].value
  context.commit_hash = last_stable[0].commit_hash
  context.build_number = last_stable[0].build_number
  context.location = last_stable[0].location
  context.stable = True
  cmd_set(context)
Roll back by finding the most recent "stable" tagged version, and putting it again, so that
  it's the new "current" version.
  Args:
    context: a populated EFVersionContext object