Multipart Flag Error

[jondef95]

Unpacking a MultipartRequest (in this case OFPMP_PORT_DESC, type = 13) with flag=0x0 results in an UnpackException, also noted in #429 :

Traceback (most recent call last):
File "hello.py", line 27, in
print(unpack_message(data[:16]).header.message_type)
File "/usr/local/lib/python3.6/dist-packages/python_openflow-2018.2b1-py3.6.egg/pyof/v0x04/common/utils.py", line 183, in unpack_message
message.unpack(msg_buff)
File "/usr/local/lib/python3.6/dist-packages/python_openflow-2018.2b1-py3.6.egg/pyof/v0x04/controller2switch/multipart_request.py", line 118, in unpack
super().unpack(buff[offset:])
File "/usr/local/lib/python3.6/dist-packages/python_openflow-2018.2b1-py3.6.egg/pyof/foundation/base.py", line 824, in unpack
size = self._unpack_attribute(name, value, buff, begin)
File "/usr/local/lib/python3.6/dist-packages/python_openflow-2018.2b1-py3.6.egg/pyof/foundation/base.py", line 636, in _unpack_attribute
raise UnpackException(msg)
pyof.foundation.exceptions.UnpackException: MultipartRequest.flags; 0 is not a valid MultipartRequestFlags; fmt = !H, buff = b'\x00\r\x00\x00\x00\x00\x00\x00', offset = 2.

The MultipartRequestFlags enum does not have a value for when the flag is 0x0, the end of a MultipartRequest. I imagine this is because the OpenFlow specifications only mention OFPMPF_REQ_MORE = 1 << 0. I found a solution is to simply add a 0x0 value in the enum class:

class MultipartRequestFlags(Enum):
    """Flags for MultipartRequest."""

    #: No more requests to follow
    OFPMPF_REQ_NONE = 0

    #: More requests to follow
    OFPMPF_REQ_MORE = 1 << 0

While this is not explicitly specified in the OpenFlow 1.3 specs, it feels like an appropriate way to handle a flag that is not raised. I'm unsure of how to handle flags that are not raised further up the chain, like in pack() or unpack().