Constructing an MP_JOIN option raises AttributeError: TCP._make builds the options before assigning self._flags

[JarryShaw]

Building any MP_JOIN option raises AttributeError, because TCP._make constructs the options before it assigns the self._flags those option makers read.

Mechanism

pcapkit/protocols/transport/tcp.py:547 builds the options first:

options_value, total_length = self._make_tcp_options(options)

and self._flags is only assigned twenty lines later, at tcp.py:567:

_flag = cast('Enum_Flags', 0)
for key, val in flags.items():
    if val == 1:
        _flag |= Enum_Flags.get(key.upper())
self._flags = _flag

But _make_mptcp_join, reached from _make_tcp_options, branches on it at tcp.py:2695-2699:

if Enum_Flags.SYN in self._flags and Enum_Flags.ACK not in self._flags:   # MP_JOIN-SYN
if Enum_Flags.SYN in self._flags and Enum_Flags.ACK in self._flags:       # MP_JOIN-SYN/ACK
if Enum_Flags.SYN not in self._flags and Enum_Flags.ACK in self._flags:   # MP_JOIN-ACK

The read path has the same pair of branches at tcp.py:1521-1525, where the ordering is fine because _read assigns self._flags at tcp.py:485 before parsing options. Only the make path is inverted.

Reproduction

from pcapkit.protocols.transport.tcp import TCP
from pcapkit.const.tcp.option import Option
from pcapkit.const.tcp.mp_tcp_option import MPTCPOption

TCP(srcport=1, dstport=2, seq=0, ack=0, syn=True,
    options=[(Option.Multipath_TCP, {'subtype': MPTCPOption.MP_JOIN, 'backup': False,
                                     'addr_id': 1, 'token': 7, 'nonce': 9})])

AttributeError: 'TCP' object has no attribute '_flags'

Executed on origin/main (9c240a60e), CPython 3.14.7. On a fresh instance the attribute does not exist at all, so this is an outright failure rather than a stale-value read — though an instance that has already parsed a packet would instead silently pick up that packet's flags, which is the worse of the two outcomes and is why the fix should not simply be to initialise _flags to zero.

Notes

• The MP_JOIN variant is genuinely flag-dependent — RFC 8684 §3.2 gives three different option layouts for SYN, SYN/ACK and ACK — so the fix is to make the flags available before the options are built, not to remove the branch.
• This is why tcp-mptcp/MP_JOIN remains in EXPECTED_FAILURES in tests/protocols/test_option_roundtrip_unit.py and must stay there until this is fixed. Do not delete that entry as part of _make_mptcp_*/_read_mptcp_* length arithmetic is wrong at six more sites beyond MP_CAPABLE #576/fix(tcp): correct MPTCP option length arithmetic at all six #576 sites #585.
• Found while reviewing fix(tcp): correct MPTCP option length arithmetic at all six #576 sites #585's CI. Distinct from _make_mptcp_*/_read_mptcp_* length arithmetic is wrong at six more sites beyond MP_CAPABLE #576, which is about length arithmetic; this is an ordering defect and needs its own fix.