tokio-sync: Add async mutual exclusion primitive (#964)

This PR introduces `Lock`: A concurrency primitive built on top of `Semaphore` that provides a `Mutex`-like primitive that interacts nicely with futures. Specifically, `LockGuard` (in contrast to `MutexGuard`) does _not_ borrow the `Lock`, and can thus be passed into a future where it will later be unlocked.

This replaces #958, which attempted to introduce a less generic version. The primitive proposed there will instead live in [`async-lease`](https://github.com/jonhoo/async-lease).

2 files changed, 3 insertions, 2 deletions

diff --git a/tokio/Cargo.toml b/tokio/Cargo.toml
index c2792971..e2901c3c 100644
--- a/tokio/Cargo.toml
+++ b/tokio/Cargo.toml
@@ -73,7 +73,7 @@ tokio-fs = { version = "0.1.6", optional = true }
 tokio-io = { version = "0.1.6", optional = true }
 tokio-executor = { version = "0.1.7", optional = true }
 tokio-reactor = { version = "0.1.1", optional = true }
-tokio-sync = { version = "0.1.3", optional = true }
+tokio-sync = { version = "0.1.3", optional = true, path = "../tokio-sync" }
 tokio-threadpool = { version = "0.1.13", optional = true }
 tokio-tcp = { version = "0.1.0", optional = true }
 tokio-udp = { version = "0.1.0", optional = true }
diff --git a/tokio/src/sync.rs b/tokio/src/sync.rs
index ff7438a2..c8fb7524 100644
--- a/tokio/src/sync.rs
+++ b/tokio/src/sync.rs
@@ -9,7 +9,8 @@
 //!   from one task to another.
 //! - [mpsc](mpsc/index.html), a multi-producer, single-consumer channel for
 //!   sending values between tasks.
+//! - [lock](lock/index.html), an asynchronous `Mutex`-like type.
 //! - [watch](watch/index.html), a single-producer, multi-consumer channel that
 //!   only stores the **most recently** sent value.
 
-pub use tokio_sync::{mpsc, oneshot, watch};
+pub use tokio_sync::{lock, mpsc, oneshot, watch};