Added VariableRateCyclicTaskABC and updated ThreadBasedCyclicSendTask

[sschueth]

• Modified bus.py to include an optional argument, period_intra, in send_periodic and _send_periodic_internal
• Implemented VariableCyclicTaskRateABC within broadcastmanager.py to be inherited by ThreadBasedCyclicTask
• Added attributes to CyclicSendTaskABC to help with handling a group of messages
• relaxed arbitration_id requirement within group of messages and updated the _check_modified_messages to check for each updataed message's arbitration_id to preserve that arbitration_id does not change when a message is modified
• Modified ThreadBasedCyclicTask._run() for variable rate transmission of messages in a group

[sschueth]

@tysonite @hardbyte @zariiii9003

I referenced PR #785 related to the relaxation of the uniqueness of the arbitration_id in SocketCAN and was looking for some insight on the reason why the arbitration_id uniqueness checking exists for a Sequence of messages within ModifiableCyclicTaskABC.

• Is this uniqueness restriction related to a hardware / driver limitation?
  • I've only been able to test this on my available hardware (vector and kvaser) and have not had any issues with relaxing the uniqueness of arbitration_id
• Is this uniqueness restriction to ensure a modified or updated message is not changing an existing message arbitration_id and therefore removing it from the bus?
  • If this is the case, I think my changes within _check_modified_messages() should suffice?

I removed the all_same_id check within _check_and_convert_messages() and converted self.arbitration_id to a list of arbitration_id. To preserve the same behavior of "The arbitration ID of new cyclic messages cannot be changed from when the task was created" I added a series of checks within _check_modified_messages() to check that the new sequence of messages have respecitvely matching arbitration_id of the original sequence of messages.

For context, I'm making this change to allow for messages with different arbitration ids to be passed in the same task ensuring the order of transmission and improved timing. For example, with this update the J1939-76 Safety Header Message and Safety Data Message can be explicitly linked and transmitted as a sequence of messages within the same task.

Any feedback / guidance is greatly appreciated. Thanks!

[hardbyte]

I like the direction.

Would appreciate this being tested and the user facing docs being updated too.

[hardbyte]

prefer self.arbitration_ids

[hardbyte]

I haven't tried but do you know if socketcan supports reordering the messages as well? In which case prefer to count the arbitration ids match.

[hardbyte]

Period in seconds between individual message within the sequence.

I assume no impact for sending individual messages? Could you also update the docstring for period 🙏🏼

[DjMike238]

Hi, will this PR ever be merged? I'm trying to use this library to send multipacket J1939 packets (in which the first packet will have a different arbitration ID than the others) periodically, and having the relaxed arbitration_id requirement for this would be very useful for my use case.

I'm currently using a PCAN interface, I just tried commenting out the code that checks all_same_id in _check_and_convert_messages() from broadcastmanager.py and it's working without any issues.

I could also do a smaller PR only with the relaxed arbitration_id requirement if you think it's the best course of action.