[Kinflou]

Lets a custom (namespaced) handshake run on an async connection. Until now Handshake worked only over the blocking Transport, and AsyncConnection could run only comline@1 or skip the handshake (new_unverified).

API

// comline_runtime::handshake, feature `async`
pub trait AsyncHandshake {
    fn name(&self) -> &str;
    fn initiate<T: AsyncTransport>(&mut self, transport: &mut T, local: &Facts<'_>)
        -> impl Future<Output = Result<(), RuntimeError>> + Send;
    fn respond<T: AsyncTransport>(&mut self, transport: &mut T, local: &Facts<'_>)
        -> impl Future<Output = Result<(), RuntimeError>> + Send;
}

impl<T: AsyncTransport, W, F> AsyncConnection<T, W, F> {
    pub async fn connect_with<H: AsyncHandshake>(transport: T, format: W, framing: F, handshake: &mut H, ir_hash: u64) -> Result<Self, RuntimeError>;
    pub async fn accept_with<H: AsyncHandshake>(transport: T, format: W, framing: F, handshake: &mut H, ir_hash: u64) -> Result<Self, RuntimeError>;
}

• The shapes match the blocking Handshake and Connection::connect_with/accept_with. Nothing from an executor is needed (the futures crates only, as with AsyncDispatch), and implementors write async fns.
• ComlineHandshake and NoHandshake implement it. comline@1 builds and checks its frame with the same code the blocking exchange uses, so it sends the same bytes.
• AsyncConnection::connect, accept and handshake now run through connect_with/accept_with with ComlineHandshake. Their behaviour is unchanged.
• Documented in the handshake and async_connection module docs.

Tests

The new tests/async_handshake.rs covers:

• A custom handshake (com.acme.hello@1-style) runs on both ends of an async connection, and a call goes through after it. A mismatch refuses both ends.
• ComlineHandshake and NoHandshake work through the _with constructors.
• Wire bytes: for every conformance vector in conformance/vectors/handshake.json, comline@1 sends exactly that vector's bytes, both over async (initiate and respond) and blocking. The async exchange also refuses each reject vector. The vectors are unchanged.

All of these pass locally: cargo test, cargo test -p comline-runtime --features tokio, the --no-default-features --features alloc and alloc,jsonrpc tests, and cargo metadata --locked.

Unblocks the async half of comline-rust#81's mapped handshakes.