iotests: implement QemuIoInteractive class

        sys.stderr.write('qemu-io received signal %i: %s\n' % (-exitcode, ' '.join(args)))

    return subp.communicate()[0]

class QemuIoInteractive:

    def __init__(self, *args):

        self.args = qemu_io_args + list(args)

        self._p = subprocess.Popen(self.args, stdin=subprocess.PIPE,

                                   stdout=subprocess.PIPE,

                                   stderr=subprocess.STDOUT)

        assert self._p.stdout.read(9) == 'qemu-io> '

    def close(self):

        self._p.communicate('q\n')

    def _read_output(self):

        pattern = 'qemu-io> '

        n = len(pattern)

        pos = 0

        s = []

        while pos != n:

            c = self._p.stdout.read(1)

            # check unexpected EOF

            assert c != ''

            s.append(c)

            if c == pattern[pos]:

                pos += 1

            else:

                pos = 0

        return ''.join(s[:-n])

    def cmd(self, cmd):

        # quit command is in close(), '\n' is added automatically

        assert '\n' not in cmd

        cmd = cmd.strip()

        assert cmd != 'q' and cmd != 'quit'

        self._p.stdin.write(cmd + '\n')

        return self._read_output()

def qemu_nbd(*args):

    '''Run qemu-nbd in daemon mode and return the parent's exit code'''

    return subprocess.call(qemu_nbd_args + ['--fork'] + list(args))