Class H2PaddedPriorityHeadersFrame

scapy/contrib/http2.py:1591–1648

H2PaddedPriorityHeadersFrame is the variant of H2HeadersFrame where both priority and padding flags are set

class H2PaddedPriorityHeadersFrame(H2AbstractHeadersFrame):
    """ H2PaddedPriorityHeadersFrame is the variant of H2HeadersFrame where
    both priority and padding flags are set
    """
    __slots__ = ['s_len']

    name = 'HTTP/2 Headers Frame with Padding and Priority'
    fields_desc = [
        fields.FieldLenField('padlen', None, length_of='padding', fmt='B'),
        fields.BitField('exclusive', 0, 1),
        fields.BitField('stream_dependency', 0, 31),
        fields.ByteField('weight', 0),
        fields.PacketListField('hdrs', [], HPackHeaders,
                               length_from=lambda pkt: pkt.get_hdrs_len()
                               ),
        fields.StrLenField('padding', '',
                           length_from=lambda pkt: pkt.getfieldval('padlen')
                           )
    ]

    def get_hdrs_len(self):
        # type: () -> int
        """ get_hdrs_len computes the length of the hdrs field

        To do this computation, the length of the padlen field, the priority
        information fields and the actual padding is subtracted to the string
        that was provided to the pre_dissect fun of the pkt parameter.
        :return: int: the length of the hdrs field
        :raises: AssertionError
        """

        padding_len = self.getfieldval('padlen')
        fld, fval = self.getfield_and_val('padlen')
        padding_len_len = fld.i2len(self, fval)
        bit_cnt = self.get_field('exclusive').size
        bit_cnt += self.get_field('stream_dependency').size
        fld, fval = self.getfield_and_val('weight')
        weight_len = fld.i2len(self, fval)
        ret = int(self.s_len -
                  padding_len_len -
                  padding_len -
                  (bit_cnt / 8) -
                  weight_len
                  )
        assert ret >= 0
        return ret

    def pre_dissect(self, s):
        # type: (str) -> str
        """pre_dissect is filling the s_len property of this instance. This
        property is later used during the parsing of the hdrs PacketListField
        when trying to evaluate the length of the PacketListField! This "trick"
        works because the underlayer packet (H2Frame) is assumed to override the  # noqa: E501
        "extract_padding" method and to only provide to this packet the data
        necessary for this packet. Tricky, tricky, will break some day probably!  # noqa: E501
        """
        self.s_len = len(s)
        return s