implement recv_raw for Connection wrappers into mavlink-core

[roby2014]

(depends on #284)

So, my goal is to have some sort of way to fetch raw messages from the connection wrappers (AsyncMavConnection, MavConnection) directly, without having to do the extra-step of providing a type and converting between and vice-versa.

At the moment, recv will always use the generic M.

Another thing I was planning was to somehow allow connections without knowing the message type M, but this looks a bit harder to implement (and probably impossible because dialect must be known at compile-time?).

I've already implemented for AsyncMavConnection and its protocols (udp, tcp, direct-serial, file):

async fn recv_raw(&self) -> Result<MAVLinkRawMessage, crate::error::MessageReadError>;

I've also implemented a small example (mavlink-dump-raw-async), which already shows how this can be used with async.

I want to ask if this is a good idea and if possible, to open discussion on this topic here.

[roby2014]

Well 1 possible idea I have is to implement raw connectors, e.g: AsyncMavRawConnection and MavRawConnection instead of adding recv_raw (or keep recv_raw just for API design). This would not care about dialect and just tries to parse a MAVLink message. Obviously, the message check (crc calculation, deserialization, etc), would have to be done manually by the user).

@patrickelectric (and other maintainers) what do you think?

[pv42]

So I had the idea of doing something like that:

pub trait MavRawConnection {
    fn recv_raw(&self) -> Result<MAVLinkVXMessageRaw>
    ...
}

and then have

pub trait MavConnection<M: Message> : MavRawConnection  {
    fn recv(&self) -> Result<(MavHeader, M), crate::error::MessageReadError> {
        M::parse(self.recv_raw())
    }
   ...
}

The issue is that MAVLinkVXMessageRaw does not exist and it probably would have to be a trait object.

[roby2014]

So I had the idea of doing something like that:

pub trait MavRawConnection {
    fn recv_raw(&self) -> Result<MAVLinkVXMessageRaw>
    ...
}

and then have

pub trait MavConnection<M: Message> : MavRawConnection  {
    fn recv(&self) -> Result<(MavHeader, M), crate::error::MessageReadError> {
        M::parse(self.recv_raw())
    }
   ...
}

The issue is that MAVLinkVXMessageRaw does not exist and it probably would have to be a trait object.

I still think you'll need a dialect that way since functions like read_v1_raw_message_async, etc... Depend on the generic M

P.S: My title might be misleading. My goal is to have Connection wrappers that do not depend on a dialect so you can just receive RAW data

[joaoantoniocardoso]

So I had the idea of doing something like that:

pub trait MavRawConnection {
    fn recv_raw(&self) -> Result<MAVLinkVXMessageRaw>
    ...
}

and then have

pub trait MavConnection<M: Message> : MavRawConnection  {
    fn recv(&self) -> Result<(MavHeader, M), crate::error::MessageReadError> {
        M::parse(self.recv_raw())
    }
   ...
}

The issue is that MAVLinkVXMessageRaw does not exist and it probably would have to be a trait object.

I still think you'll need a dialect that way since functions like read_v1_raw_message_async, etc... Depend on the generic M

P.S: My title might be misleading. My goal is to have Connection wrappers that do not depend on a dialect so you can just receive RAW data

I think that'd only be possible if we don't check the CRC. For that, we could surely extract the part of the recv raw functions that deal with the message before the CRC, and expose that to the public lib API.

[roby2014]

I think it would be great to have some sort of API to just receive/send mavlink messages, even if they are not CRC-validated or from a specific dialect. When the goal is to just "route" messages, I dont want to care about a dialect or version.

[joaoantoniocardoso]

I think it would be great to have some sort of API to just receive/send mavlink messages, even if they are not CRC-validated or from a specific dialect. When the goal is to just "route" messages, I dont want to care about a dialect or version.

I think the main reason the CRC (and thus the knowledge of the message dialect) is necessary for us to create a message is because the protocol relies on a single magic byte to define the start of the message, and this byte can also exist within the rest of the packed, so unless we have another reliable algorithm to identify a message, if we follow the current parser idea, it would possibly discard valid messages while wrongly identifying messages.

One idea would be not to remove all bytes from each identified message from the buffer, but to only remove the bytes until the next magic byte, which would result in sending both invalid and valid messages, increasing bandwidth (no idea of how much). This assumes we can pass the responsibility of validating the messages entirely to the clients.

[hamishwillee]

FWIW the design of MAVLink is a known pain-point with routers. In particular the CRC_EXTRA check requires that the router knows about the message - and therefore needs to have all the messages that might be routed to it.. That's annoying because the router doesn't care what the message is but still has to keep up with all new messages.

Further, if a router doesn't have the message definition, the other problem is that it can't determine the target address (since that is embedded in the payload). So it then becomes hard for a router to know where it should send a message. There has been talk of various ways to fix that last one, including pulling the target into the header using an incompatiblility bit.

I don't know how likely having the magic message marker is in the message "by accident". Certainly a risk. I guess a router could send any messages it does "understand" and send the entire block of bytes between the blocks it doesn't understand. But then you'll have to parse every message in order to work out the end points. Yuck.

@tridge Might have some ideas.