Adding direction to virutal bus messages

[sou1hacker]

This adds can.Message direction (Rx/Tx) to virtual interface

[codecov]

Codecov Report

Merging #779 into develop will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #779      +/-   ##
===========================================
+ Coverage    69.52%   69.54%   +0.01%     
===========================================
  Files           70       70              
  Lines         6544     6547       +3     
===========================================
+ Hits          4550     4553       +3     
  Misses        1994     1994              

[christiansandberg]

A regular copy would suffice here.

[sou1hacker]

@christiansandberg Isn't deepcopy better than copy? I mean doing msg.data = xx after receiving a message in one queue will modify the message for all other virtual interfaces.

Also the original implementation sends the same message instance on every virtual bus. Should we change that to send a different instance since the same argument applies there.

[christiansandberg]

Yes I know we pass the same msg instance to all interfaces (mostly for performance reasons) and that could be a risk if one of them mutates it for some reason. In general I think a message should be seen as immutable and especially the data part but perhaps it is better to deepcopy all messages instead just to be sure.

[sou1hacker]

@christiansandberg changed implementation to create a new deep copied instance of message for each virtual bus queue

[pierreluctg]

Future improvement can be to implement a copy-on-write (CoW) for messages.

[hardbyte]

Perhaps worth adding is_rx as an optional attribute to the module level Message class to coordinate with other backends.

[pierreluctg]

@sou1hacker can you please add unit tests to cover the new features?

1. Test that is_rx flag is correctly set for virtual buses
2. Test that each queue receive it's own copy of the message

Thank you