Preprocess tungstenite 'ConnectionClosed' errors.

Tungstenite abuses `Err(ConnectionClosed)` to indicate the non-error
condition of a websocket closing. This commit changes error processing
such that the error is caught and converted into a successful
termination of websocket handling.

examples/upgrade/src/ws.rs

@@ -87,14 +87,23 @@ impl<'r, 'o: 'r, S> Responder<'r, 'o> for MessageStream<'o, S>
     }
 }
 
+/// Returns `Ok(true)` if processing should continue, `Ok(false)` if processing
+/// has terminated without error, and `Err(e)` if an error has occurred.
+fn handle_result(result: Result<()>) -> io::Result<bool> {
+    match result {
+        Ok(_) => Ok(true),
+        Err(Error::ConnectionClosed) => Ok(false),
+        Err(Error::Io(e)) => Err(e),
+        Err(e) => Err(io::Error::new(io::ErrorKind::Other, e))
+    }
+}
+
 #[rocket::async_trait]
 impl IoHandler for Channel<'_> {
     async fn io(&mut self, io: IoStream) -> io::Result<()> {
         let stream = WebSocketStream::from_raw_socket(io, Role::Server, None).await;
-        (self.handler)(stream).await.map_err(|e| match e {
+        let result = (self.handler)(stream).await;
-            Error::Io(e) => e,
+        handle_result(result).map(|_| ())
-            other => io::Error::new(io::ErrorKind::Other, other)
-        })
     }
 }
 
@@ -112,10 +121,9 @@ impl<'r, S> IoHandler for MessageStream<'r, S>
                 Err(e) => Err(e)
             };
 
-            result.map_err(|e| match e {
+            if !handle_result(result)? {
-                Error::Io(e) => e,
+                return Ok(());
-                other => io::Error::new(io::ErrorKind::Other, other)
+            }
-            })?;
         }
 
         Ok(())