def attach(self, buffer, read_line=None):
        """
        Read buffer until end (read() returns '') and sends it to self.logger and self.job_backend.

        :param buffer: a buffer instance with block read() or readline() method
        :param read_line: callable or True to read line per line. If callable is given, it will be executed per line
                          and ignores does not redirect the line to stdout/logger when callable returns False.
        """

        bid = id(buffer)
        self.attach_last_messages[bid] = b''

        def reader():
            current_line = b''

            def handle_line(buf):
                if chunk == b'':
                    return

                if read_line and callable(read_line):
                    res = read_line(buf)
                    if res is False:
                        return False

                    elif res is not None:
                        buf = res
                        if hasattr(buf, 'encode'):
                            buf = buf.encode('utf-8')

                self.attach_last_messages[bid] += buf

                if len(self.attach_last_messages[bid]) > 21 * 1024:
                    self.attach_last_messages[bid] = self.attach_last_messages[bid][-20 * 1024:]

                self.write(buf)

            flush_char = b'\n'

            while True:
                try:
                    # needs to be 1 so we fetch data in near real-time
                    chunk = buffer.read(1)
                    if chunk == b'':
                        if current_line:
                            handle_line(current_line)
                        return

                    current_line += chunk

                    while flush_char in current_line:
                        pos = current_line.find(flush_char)
                        line = current_line[:pos+1]
                        current_line = current_line[pos+1:]
                        handle_line(line)

                    # todo, periodically flush by '\r' only (progress bars for example)
                    # and make sure only necessary data is sent (by applying \r and \b control characters)

                except (KeyboardInterrupt, SystemExit):
                    raise
                except Exception:
                    # we need to make sure, we continue to read otherwise the process of this buffer
                    # will block and we have a stuck process.
                    sys.__stderr__.write(traceback.format_exc() + '\n')
                    sys.__stderr__.flush()

        thread = Thread(target=reader)
        thread.daemon = True
        thread.start()

        def wait():
            thread_join_non_blocking(thread)
            self.send_buffer()

        return wait