sync: Add watch, a single value broadcast channel (tokio-rs#922)

A single-producer, multi-consumer channel that only retains the _last_ sent
value. Values are broadcasted out.

This channel is useful for watching for changes to a value from multiple
points in the code base (for example, changes to a configuration value).

src/sync.rs

@@ -9,5 +9,7 @@
 //!   from one task to another.
 //! - [mpsc](mpsc/index.html), a multi-producer, single-consumer channel for
 //!   sending values between tasks.
+//! - [watch](watch/index.html), a single-producer, multi-consumer channel that
+//!   only stores the **most recently** sent value.
 
-pub use tokio_sync::{mpsc, oneshot};
+pub use tokio_sync::{mpsc, oneshot, watch};

tokio-sync/Cargo.toml

@@ -19,6 +19,7 @@ Synchronization utilities.
 categories = ["asynchronous"]
 
 [dependencies]
+fnv = "1.0.6"
 futures = "0.1.19"
 
 [dev-dependencies]

tokio-sync/src/lib.rs

@@ -6,6 +6,8 @@
 //!
 //! This crate provides primitives for synchronizing asynchronous tasks.
 
+extern crate fnv;
+#[macro_use]
 extern crate futures;
 
 macro_rules! debug {
@@ -27,3 +29,4 @@ pub mod mpsc;
 pub mod oneshot;
 pub mod semaphore;
 pub mod task;
+pub mod watch;