multistream: Do not wait for negotiation in poll_close (#319)

This PR backports https://github.com/libp2p/rust-libp2p/pull/4019/files,
effectively ensuring that our `poll_close` flushes any data immediately
without waiting for negotiation to complete.

Closes an outdated PR: #62

cc @paritytech/networking

---------

Signed-off-by: Alexandru Vasile <[email protected]>

src/multistream_select/dialer_select.rs

@@ -548,9 +548,7 @@ mod tests {
             io.close().await.unwrap();
         });
 
-        // TODO: Once https://github.com/paritytech/litep2p/pull/62 is merged, this
-        // should be changed to `is_ok`.
-        assert!(tokio::time::timeout(Duration::from_secs(10), client).await.is_err());
+        assert!(tokio::time::timeout(Duration::from_secs(10), client).await.is_ok());
     }
 
     #[tokio::test]

src/multistream_select/negotiated.rs

@@ -323,15 +323,22 @@ where
     }
 
     fn poll_close(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
-        // Ensure all data has been flushed and expected negotiation messages
-        // have been received.
-        ready!(self.as_mut().poll(cx).map_err(Into::<io::Error>::into)?);
+        // Ensure all data has been flushed, including optimistic multistream-select messages.
         ready!(self.as_mut().poll_flush(cx).map_err(Into::<io::Error>::into)?);
 
         // Continue with the shutdown of the underlying I/O stream.
         match self.project().state.project() {
             StateProj::Completed { io, .. } => io.poll_close(cx),
-            StateProj::Expecting { io, .. } => io.poll_close(cx),
+            StateProj::Expecting { io, .. } => {
+                let close_poll = io.poll_close(cx);
+                if let Poll::Ready(Ok(())) = close_poll {
+                    tracing::debug!(
+                        target: LOG_TARGET,
+                        "Stream closed. Confirmation from remote for optimstic protocol negotiation still pending."
+                    );
+                }
+                close_poll
+            }
             StateProj::Invalid => panic!("Negotiated: Invalid state"),
         }
     }