How to use the pgctl.daemontools.SvStat function in pgctl

def it_only_starts_the_indicated_services(self, in_example_dir, request):
        check_call(('pgctl', 'start', 'sleep'))

        assert_svstat('playground/sleep', state='up')
        assert_svstat('playground/tail', state=SvStat.UNSUPERVISED)

totally unpredictable error message

    >>> svstat_parse('down (exitcode 0) 0 seconds, normally up, want wat, ready 0 seconds')
    Traceback (most recent call last):
        ...
    ValueError: unexpected value for `process`: 'wat'

    >>> svstat_parse('down (exitcode 0) 0 seconds, normally up, want up\x00, ready 0 seconds')
    down (exitcode 0) 0 seconds, starting
    '''
    status = svstat_string.strip()
    trace('RAW   : %s', status)
    if status.startswith(('up ', 'down ')):
        state, buffer = parse(status, '', ' ')
    elif status.startswith('unable to chdir:'):
        return SvStat(SvStat.INVALID, None, None, None, None)
    elif (
            status.startswith('s6-svstat: fatal: unable to read status for ') and status.endswith((
                ': No such file or directory',
                ': Broken pipe',
            ))
    ):
        # the service has never been started before; it's down.
        return SvStat(SvStat.UNSUPERVISED, None, None, None, None)
    else:  # unknown errors
        return SvStat(status, None, None, None, None)

    pid, buffer = parse(buffer, '(pid ', ') ', int)
    exitcode, buffer = parse(buffer, '(exitcode ', ') ', int)
    _, buffer = parse(buffer, '(signal ', ') ')

    seconds, buffer = parse(buffer, '', ' seconds', int)

status = svstat_string.strip()
    trace('RAW   : %s', status)
    if status.startswith(('up ', 'down ')):
        state, buffer = parse(status, '', ' ')
    elif status.startswith('unable to chdir:'):
        return SvStat(SvStat.INVALID, None, None, None, None)
    elif (
            status.startswith('s6-svstat: fatal: unable to read status for ') and status.endswith((
                ': No such file or directory',
                ': Broken pipe',
            ))
    ):
        # the service has never been started before; it's down.
        return SvStat(SvStat.UNSUPERVISED, None, None, None, None)
    else:  # unknown errors
        return SvStat(status, None, None, None, None)

    pid, buffer = parse(buffer, '(pid ', ') ', int)
    exitcode, buffer = parse(buffer, '(exitcode ', ') ', int)
    _, buffer = parse(buffer, '(signal ', ') ')

    seconds, buffer = parse(buffer, '', ' seconds', int)
    buffer = buffer.lstrip(', ')

    # we actually dont care about this value
    _, buffer = parse(buffer, 'normally ', ', ')

    process, buffer = parse(buffer, 'want ', ', ')
    if process is not None:
        process = process.strip('\x00')  # s6 microbug
        if process == 'up':
            process = 'starting'