Add a generic client-server implementation to libextra

[brendanzab]

This commit adds a generic client-server implementation based in part on Erlang's gen_server module. Here is an example of an implementation of how one could implement the cell example described in Concurrent Programming in ML (Reppy, 1999, pp. 42-45):

use extra::comm::server::*;

struct Server { value: int }

struct Get;
struct Put(int);

impl GenericServer<int, (), Get, Put, int, ()> for Server {
    fn init(x: int) -> Result<Server, ()> {
        Ok(Server { value: x })
    }

    fn handle_call(&mut self, _: Get) -> Reply<int, ()> {
        Reply(self.value)
    }

    fn handle_cast(&mut self, Put(x): Put) -> Reply<Continue, ()> {
        self.value = x; Reply(Continue)
    }
}

struct Cell {
    priv client: SharedClient<Get, Put, int>,
}

impl Cell {
    fn new(x: int) -> Cell {
        Cell { client: spawn_server::<int, (), Get, Put, int, (), Server>(x)
                .unwrap().into_shared() }
    }

    fn get(&self) -> int { self.client.call(Get) }
    fn put(&self, x: int) { self.client.cast(Put(x)) }
}

impl Clone for Cell {
    fn clone(&self) -> Cell {
        Cell { client: self.client.clone() }
    }
}

And here is an example of the storage cell in use:

let cell = Cell::new(1);
assert_eq!(cell.get(), 1);
cell.put(2);
assert_eq!(cell.get(), 2);
assert_eq!(cell.get(), 2);

let cell2 = cell.clone();
assert_eq!(cell.get(), 2);
cell2.put(3);
assert_eq!(cell.get(), 3);
assert_eq!(cell2.get(), 3);

[brendanzab]

Just to be clear, I'm looking for discussion and critique, not an r+ just yet.

cc. @metajack

[alexcrichton]

This seems really slick! I'm not sure if we want to continue to expand the horizons of libextra (seeing how we still want to dissolve it), but this is certainly really cool.

One thing I should mention is that in my channel rewrite, SharedPort is going away, but it looks like that only affects the SharedClient structure, so I don't think it'd be too bad.

[brendanzab]

@alexcrichton Yeah, perhaps I could just publish it as a separate lib for now. How would I do SharedClient then?

[alexcrichton]

Closing due to inactivity, and it sounded like this may want to wait for Chan changes to being internally upgradeable (as it would simplify this API)