fix(j1939-21): respect sequence number for fragmented frames on reass…

[drewr95]

Summary

Fixes #69

This pull request strengthens the Transport Protocol (TP) implementation in j1939_21.py by enforcing strict sequence checking for incoming TP.DT packets, ensuring that out-of-sequence packets are correctly detected and handled according to the J1939-21 specification. It also adds comprehensive tests to verify these behaviors.

Transport Protocol improvements:

• Enforces strict sequence checking in _process_tp_dt: Now tracks the next expected packet and raises a ValueError if a duplicate or invalid sequence number is received, aborting the session and cleaning up resources as appropriate.
• Initializes the next_expected_packet field in the receive buffer during both BAM and CMDT session setup in _process_tp_cm, ensuring consistent state tracking. [1] [2]

Testing enhancements:

• Adds new tests in test/test_ecu.py to verify that out-of-sequence TP.DT packets are correctly rejected for both BAM and peer-to-peer (CMDT) sessions, and that session state is properly cleaned up after errors.
• Ensures that no notifications or messages are sent when an out-of-sequence error occurs, and that the receive buffer is cleared.

Test infrastructure:

• Imports pytest and relevant classes to support the new test cases.

These changes improve protocol compliance, robustness, and test coverage for the J1939 TP implementation.

[Copilot]

Pull request overview

This PR addresses Issue #69 by enforcing strict TP.DT sequence-number ordering during J1939-21 multi-packet reassembly, aborting/terminating sessions when out-of-sequence packets are received to prevent corrupted payload delivery.

Changes:

• Track next_expected_packet in the receive buffer for both BAM and CMDT sessions.
• Validate TP.DT packet sequence numbers in _process_tp_dt, abort/cleanup on violations, and raise a ValueError.
• Add ECU-level tests covering out-of-sequence scenarios for BAM and CMDT sessions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
j1939/j1939_21.py	Adds receive-side sequence tracking and abort/error behavior for out-of-sequence TP.DT packets.
test/test_ecu.py	Adds tests ensuring out-of-sequence TP.DT packets terminate sessions and prevent partial delivery.

Suppressed comments (1)

test/test_ecu.py:128

• This test requests the feeder fixture but never uses it, which adds unnecessary setup/overhead and can hide accidental dependencies. Remove the unused fixture parameter.

def test_peer_to_peer_sequence_gap_after_valid_packet_aborts(feeder):

---

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[khauersp]

Do we need to implement this for j1939_22?

[drewr95]

Do we need to implement this for j1939_22?

I'm honestly not sure if j1939-22 requires this. It can be a follow up issue and I can research into it