Plugin for TUNNEL messages

[mortenfyhn]

I wanted to use MAVLink TUNNEL messages to communicate between ROS and some hardware connected to the FCU. I made a MAVROS plugin that forwards tunnels between ROS and the FCU, in both directions, and a Tunnel message in mavros_msgs that mirrors the MAVLink message.

One thing I'm not sure about is how to best deal with the variable length payload field. In MAVLink, it's defined as a 128 size array, but I suppose the serializer makes sure to avoid sending a bunch of zeros if only some of the size is used. I've made it a uint8[128] in ros to enforce the maximum size, but I think that might mean the ROS messages always send the full 128 bytes. I could make it uint8[], but then I would need to handle the max size somewhere. (Maybe just a comment in the message definition, and a check in the plugin when it copies between the ROS and MAVLink messages?)

[mortenfyhn]

I've fixed the cog generation now, and I've added a check on the payload length, so I don't try to std::copy out of bounds data if payload_length is accidentally set higher than what's allowed.

[vooon]

LGTM, Thanks!

[vooon]

Hmm, i'm thinking, what if make Tunnel.payload dynamically sized, then we could remove Tunnel.payload_length (as we have it in payload.size()).

[mortenfyhn]

That's absolutely an option.

So the error handling would then be:

• Plugin receives a mavros tunnel message
• Check payload.size(), print error and don't send if > 128

and

• Plugin receives a mavlink tunnel message
• Check payload_length, print error and don't send if > 128

The only downside I can see is that the mavros and mavlink messages would be less similar, so the user of the plugin needs to figure out that payload_length is just encoded in the array size and not in a separate field.

[vooon]

I think first option better.

In my opinion payload_length makes handling of that message harder, and only needed for mavlink to express dynamically sized payload.
So i suppose that we can just add a note to the message and that's it.

[vooon]

Since payload is std::array i think better to use ::size().