Notify connection on connection window expansion (#86)

When capacity is released back to the connection and a connection level
window update needs to be sent out, the connection task needs to be
notified in order for the send to actually happen.

tests/flow_control.rs

@@ -669,3 +669,112 @@ fn reserved_capacity_assigned_in_multi_window_updates() {
 
     let _ = h2.join(srv).wait().unwrap();
 }
+
+#[test]
+fn connection_notified_on_released_capacity() {
+    use futures::sync::oneshot;
+    use std::thread;
+    use std::sync::mpsc;
+
+    let _ = ::env_logger::init();
+    let (io, srv) = mock::new();
+
+    // We're going to run the connection on a thread in order to isolate task
+    // notifications. This test is here, in part, to ensure that the connection
+    // receives the appropriate notifications to send out window updates.
+
+    let (tx, rx) = mpsc::channel();
+
+    // Because threading is fun
+    let (settings_tx, settings_rx) = oneshot::channel();
+
+    let th1 = thread::spawn(move || {
+        srv.assert_client_handshake().unwrap()
+            .recv_settings()
+            .map(move |v| {
+                settings_tx.send(()).unwrap();
+                v
+            })
+            // Get the first request
+            .recv_frame(
+                frames::headers(1)
+                    .request("GET", "https://example.com/a")
+                    .eos())
+            // Get the second request
+            .recv_frame(
+                frames::headers(3)
+                    .request("GET", "https://example.com/b")
+                    .eos())
+            // Send the first response
+            .send_frame(frames::headers(1).response(200))
+            // Send the second response
+            .send_frame(frames::headers(3).response(200))
+
+            // Fill the connection window
+            .send_frame(frames::data(1, vec![0u8; 16_384]).eos())
+            .idle_ms(100)
+            .send_frame(frames::data(3, vec![0u8; 16_384]).eos())
+
+            // The window update is sent
+            .recv_frame(frames::window_update(0, 16_384))
+            .map(drop)
+            .wait().unwrap();
+    });
+
+
+    let th2 = thread::spawn(move || {
+        let h2 = Client::handshake(io).wait().unwrap();
+
+        let (mut h2, _) = h2.drive(settings_rx).wait().unwrap();
+
+        let request = Request::get("https://example.com/a")
+            .body(())
+            .unwrap();
+
+        tx.send(h2.request(request, true).unwrap()).unwrap();
+
+        let request = Request::get("https://example.com/b")
+            .body(())
+            .unwrap();
+
+        tx.send(h2.request(request, true).unwrap()).unwrap();
+
+        // Run the connection to completion
+        h2.wait().unwrap();
+    });
+
+    // Get the two requests
+    let a = rx.recv().unwrap();
+    let b = rx.recv().unwrap();
+
+    // Get the first response
+    let response = a.wait().unwrap();
+    assert_eq!(response.status(), StatusCode::OK);
+    let (_, a) = response.into_parts();
+
+    // Get the next chunk
+    let (chunk, mut a) = a.into_future().wait().unwrap();
+    assert_eq!(16_384, chunk.unwrap().len());
+
+    // Get the second response
+    let response = b.wait().unwrap();
+    assert_eq!(response.status(), StatusCode::OK);
+    let (_, b) = response.into_parts();
+
+    // Get the next chunk
+    let (chunk, b) = b.into_future().wait().unwrap();
+    assert_eq!(16_384, chunk.unwrap().len());
+
+    // Wait a bit
+    thread::sleep(Duration::from_millis(100));
+
+    // Release the capacity
+    a.release_capacity(16_384).unwrap();
+
+    th1.join().unwrap();
+    th2.join().unwrap();
+
+    // Explicitly drop this after the joins so that the capacity doesn't get
+    // implicitly released before.
+    drop(b);
+}