def parse_lines(self, file, boundary, content_length, cap_at_buffer=True):
        """Generate parts of
        ``('begin_form', (headers, name))``
        ``('begin_file', (headers, name, filename))``
        ``('cont', bytestring)``
        ``('end', None)``

        Always obeys the grammar
        parts = ( begin_form cont* end |
                  begin_file cont* end )*
        """
        next_part = b"--" + boundary
        last_part = next_part + b"--"

        iterator = chain(
            make_line_iter(
                file,
                limit=content_length,
                buffer_size=self.buffer_size,
                cap_at_buffer=cap_at_buffer,
            ),
            _empty_string_iter,
        )

        terminator = self._find_terminator(iterator)

        if terminator == last_part:
            return
        elif terminator != next_part:
            self.fail("Expected boundary at start of multipart data")

        while terminator != last_part:
            headers = parse_multipart_headers(iterator)

            disposition = headers.get("content-disposition")
            if disposition is None:
                self.fail("Missing Content-Disposition header")
            disposition, extra = parse_options_header(disposition)
            transfer_encoding = self.get_part_encoding(headers)
            name = extra.get("name")
            filename = extra.get("filename")

            # if no content type is given we stream into memory.  A list is
            # used as a temporary container.
            if filename is None:
                yield _begin_form, (headers, name)

            # otherwise we parse the rest of the headers and ask the stream
            # factory for something we can write in.
            else:
                yield _begin_file, (headers, name, filename)

            buf = b""
            for line in iterator:
                if not line:
                    self.fail("unexpected end of stream")

                if line[:2] == b"--":
                    terminator = line.rstrip()
                    if terminator in (next_part, last_part):
                        break

                if transfer_encoding is not None:
                    if transfer_encoding == "base64":
                        transfer_encoding = "base64_codec"
                    try:
                        line = codecs.decode(line, transfer_encoding)
                    except Exception:
                        self.fail("could not decode transfer encoded chunk")

                # we have something in the buffer from the last iteration.
                # this is usually a newline delimiter.
                if buf:
                    yield _cont, buf
                    buf = b""

                # If the line ends with windows CRLF we write everything except
                # the last two bytes.  In all other cases however we write
                # everything except the last byte.  If it was a newline, that's
                # fine, otherwise it does not matter because we will write it
                # the next iteration.  this ensures we do not write the
                # final newline into the stream.  That way we do not have to
                # truncate the stream.  However we do have to make sure that
                # if something else than a newline is in there we write it
                # out.
                if line[-2:] == b"\r\n":
                    buf = b"\r\n"
                    cutoff = -2
                else:
                    buf = line[-1:]
                    cutoff = -1
                yield _cont, line[:cutoff]

            else:  # pragma: no cover
                raise ValueError("unexpected end of part")

            # if we have a leftover in the buffer that is not a newline
            # character we have to flush it, otherwise we will chop of
            # certain values.
            if buf not in (b"", b"\r", b"\n", b"\r\n"):
                yield _cont, buf

            yield _end, None
Generate parts of
        ``('begin_form', (headers, name))``
        ``('begin_file', (headers, name, filename))``
        ``('cont', bytestring)``
        ``('end', None)``

        Always obeys the grammar
        parts = ( begin_form cont* end |
                  begin_file cont* end )*