Experiment with passing handles between nodes in Protobufs

[wildarch]

This would allow nodes to share channels with each other simply by passing them over an existing channel, encoding them in an ordinary protocol buffer.

The approach I am proposing for now is that we introduce methods in the Oak ABI to generate a kind of 'invite token' for a combination of a channel handle to send and a channel handle over which to send the token. This token can then be transmitted as part of an ordinary protobuf message, and the receiver can call upon the Oak ABI to exchange the invite token for an actual handle.

The experiment should answer the following questions:

• Can we make this work in a way that is ergonomic and safe for users?
• Can we ensure handles are passed in a way that is type-safe?

Related to #426 and #420.

[wildarch]

@tiziano88 could you assign this bug to me btw?

[wildarch]

I have some very early POC in https://github.com/wildarch/handle_extract (requires https://github.com/wildarch/prost as well). Right now this maps a proto like:

message TestMessage {
  oak.handle.Sender sender = 1 [(oak.handle.handle_type) = "TestHandleType"];
}

message TestHandleType {
  // ...
}

to a Rust struct that looks like:

struct TestMessage {
    sender: Sender<TestHandleType>,
}

And the other way around.

Next step will be to implement a procedural macro that generates an implementation of a trait like:

pub trait HandleExtract {
    fn extract(&mut self, handles: &mut Vec<Handle>);
    fn inject(&mut self, handles: &[Handle]);
}

[wildarch]

The POC in https://github.com/wildarch/handle_extract now implements extraction and injection in a type-safe manner 🎉.

I'll now start working towards:

• Minimizing the number of patches necessary in Prost to support the custom code generation that we need
• Integrating the solution with Oak.