docs: def _authenticate_plain(credentials, sock_info):
      """Authenticate using SASL PLAIN (RFC 4616)
      """
      source = credentials.source
      username = credentials.username
      password = credentials.password
      payload = ('\x00%s\x00%s' % (username, password)).encode('utf-8')
      cmd = SON([('saslStart', 1),
                 ('mechanism', 'PLAIN'),
                 ('payload', Binary(payload)),
                 ('autoAuthorize', 1)])
      sock_info.command(source, cmd)