Fix telemetry not responding to pings (#3272)

substrate/core/telemetry/src/worker/node.rs

@@ -87,7 +87,7 @@ impl<TTrans: Transport> Node<TTrans> {
 
 impl<TTrans: Transport, TSinkErr> Node<TTrans>
 where TTrans: Clone + Unpin, TTrans::Dial: Unpin,
-	TTrans::Output: Sink<BytesMut, Error = TSinkErr> + Unpin, TSinkErr: fmt::Debug {
+	TTrans::Output: Sink<BytesMut, Error = TSinkErr> + Stream + Unpin, TSinkErr: fmt::Debug {
 	/// Sends a WebSocket frame to the node. Returns an error if we are not connected to the node.
 	///
 	/// After calling this method, you should call `poll` in order for it to be properly processed.
@@ -175,7 +175,7 @@ fn gen_rand_reconnect_delay() -> Delay {
 }
 
 impl<TTrans: Transport, TSinkErr> NodeSocketConnected<TTrans>
-where TTrans::Output: Sink<BytesMut, Error = TSinkErr> + Unpin {
+where TTrans::Output: Sink<BytesMut, Error = TSinkErr> + Stream + Unpin {
 	/// Processes the queue of messages for the connected socket.
 	///
 	/// The address is passed for logging purposes only.
@@ -200,6 +200,7 @@ where TTrans::Output: Sink<BytesMut, Error = TSinkErr> + Unpin {
 					item_len, my_addr
 				);
 				self.need_flush = true;
+
 			} else if self.need_flush {
 				match Sink::poll_flush(Pin::new(&mut self.sink), cx) {
 					Poll::Pending => return Poll::Pending,
@@ -207,6 +208,11 @@ where TTrans::Output: Sink<BytesMut, Error = TSinkErr> + Unpin {
 					Poll::Ready(Ok(())) => self.need_flush = false,
 				}
 
+			} else if let Poll::Ready(_) = Stream::poll_next(Pin::new(&mut self.sink), cx) {
+				// We poll the telemetry `Stream` because the underlying implementation relies on
+				// this in order to answer PINGs.
+				// We don't do anything with incoming messages, however.
+
 			} else {
 				break
 			}