Drain pending requests on language server termination (#4852)

This prevents a freeze while shutting down when using `efm-langserver`.
`efm-langserver` exits immediately after seeing a shutdown request,
without responding to the request. We block awaiting the reply to the
shutdown request which will never come, so we time out.

This change responds to any pending requests with `Err` saying that the
stream has been closed.

helix-lsp/src/transport.rs

@@ -251,6 +251,16 @@ impl Transport {
                     };
                 }
                 Err(Error::StreamClosed) => {
+                    // Close any outstanding requests.
+                    for (id, tx) in transport.pending_requests.lock().await.drain() {
+                        match tx.send(Err(Error::StreamClosed)).await {
+                            Ok(_) => (),
+                            Err(_) => {
+                                error!("Could not close request on a closed channel (id={:?})", id)
+                            }
+                        }
+                    }
+
                     // Hack: inject a terminated notification so we trigger code that needs to happen after exit
                     use lsp_types::notification::Notification as _;
                     let notification =