Fix: mcp client background task entering a hot loop on child process crash (aws#146)

Author: dingfeli

crates/chat-cli/src/mcp_client/client.rs

@@ -363,6 +363,16 @@ where
                     },
                     Err(e) => {
                         tracing::error!("Background listening thread for client {}: {:?}", server_name, e);
+                        // If we don't have anything on the other end, we should just end the task
+                        // now
+                        if let TransportError::RecvError(tokio::sync::broadcast::error::RecvError::Closed) = e {
+                            tracing::error!(
+                                "All senders dropped for transport layer for server {}: {:?}. This likely means the mcp server process is no longer running.",
+                                server_name,
+                                e
+                            );
+                            break;
+                        }
                     },
                 }
             }

crates/chat-cli/src/mcp_client/transport/stdio.rs

@@ -51,7 +51,7 @@ impl JsonRpcStdioTransport {
                 // Messages are delimited by newlines and assumed to contain no embedded newlines
                 // See https://spec.modelcontextprotocol.io/specification/2024-11-05/basic/transports/#stdio
                 match buf_reader.read_until(b'\n', &mut buffer).await {
-                    Ok(0) => continue,
+                    Ok(0) => break,
                     Ok(_) => match serde_json::from_slice::<JsonRpcMessage>(buffer.as_slice()) {
                         Ok(msg) => {
                             let _ = tx.send(Ok(msg));