Shutdown the runtime on drop

Currently, the runtime does not shutdown if the runtime handle is
dropped. This can happen during a panic or when the value is simply
dropped.

This patch forces the runtime to shutdown if it is not explicitly
shutdown.

Fixes tokio-rs#209

Author: carllerche

src/runtime.rs

@@ -339,17 +339,7 @@ impl Runtime {
     /// [mod]: index.html
     pub fn shutdown_now(mut self) -> Shutdown {
         let inner = self.inner.take().unwrap();
-
-        let inner = Box::new({
-            let pool = inner.pool;
-            let reactor = inner.reactor;
-
-            pool.shutdown_now().and_then(|_| {
-                reactor.shutdown_now()
-            })
-        });
-
-        Shutdown { inner }
+        Shutdown::shutdown_now(inner)
     }
 
     fn inner(&self) -> &Inner {
@@ -361,6 +351,15 @@ impl Runtime {
     }
 }
 
+impl Drop for Runtime {
+    fn drop(&mut self) {
+        if let Some(inner) = self.inner.take() {
+            let shutdown = Shutdown::shutdown_now(inner);
+            let _ = shutdown.wait();
+        }
+    }
+}
+
 // ===== impl TaskExecutor =====
 
 impl TaskExecutor {
@@ -425,6 +424,24 @@ impl ::executor::Executor for TaskExecutor {
 
 // ===== impl Shutdown =====
 
+impl Shutdown {
+    fn shutdown_now(inner: Inner) -> Self {
+        let inner = Box::new({
+            let pool = inner.pool;
+            let reactor = inner.reactor;
+
+            pool.shutdown_now().and_then(|_| {
+                reactor.shutdown_now()
+                    .then(|_| {
+                        Ok(())
+                    })
+            })
+        });
+
+        Shutdown { inner }
+    }
+}
+
 impl Future for Shutdown {
     type Item = ();
     type Error = ();