FilterChain: Arguments should be a context object, and return a response object

[markmandel]

Passing in a context and returning a result object means we can add extra fields to our Filter methods without breaking the type signatures down the line.

Something like this:

pub struct DownstreamContext {
    endpoints: [EndPoint],
    from: SocketAddr,
    contents: Vec<u8>,
}

pub struct UpstreamContext {
    endpoint: EndPoint,
    from: SocketAddr,
    to: SocketAddr,
    contents: Vec<u8>,
}

pub struct DownstreamResponse {
    endpoints: Vec<EndPoint>,
    contents: Vec<u8>,
}

pub struct UpstreamStreamResponse {
    contents: Vec<u8>,
}

/// Filter is a trait for routing and manipulating packets.
pub trait Filter: Send + Sync {
    /// on_downstream_receive filters packets received from the local port, and potentially sends them
    /// to configured endpoints.
    /// This function should return the array of endpoints that the packet should be sent to,
    /// and the packet that should be sent (which may be manipulated) as well.
    /// If the packet should be rejected, return None.
    fn on_downstream_receive(
        &self,
        ctx: &DownstreamContext
    ) -> Option<DownstreamResponse>;

    /// on_upstream_receive filters packets received from `from`, to a given endpoint,
    /// that are going back to the original sender.
    /// This function should return the packet to be sent (which may be manipulated).
    /// If the packet should be rejected, return None.
    fn on_upstream_receive(
        &self,
        ctx: &UpstreamContext
    ) -> Option<UpstreamstreamResponse>;
}

How do people feel about this?