fix(server): add error enum while deal server info (#51)

1. wrap the error type for more standardized
2. add more information in error for debug trace
3. wrap helper func for more user-friendly code

Signed-off-by: jokemanfire <hu.dingyang@zte.com.cn>

crates/rmcp/src/service/server.rs

@@ -1,14 +1,15 @@
 use futures::{SinkExt, StreamExt};
+use thiserror::Error;
 
 use super::*;
 use crate::model::{
-    CancelledNotification, CancelledNotificationParam, ClientInfo, ClientNotification,
-    ClientRequest, ClientResult, CreateMessageRequest, CreateMessageRequestParam,
-    CreateMessageResult, ListRootsRequest, ListRootsResult, LoggingMessageNotification,
-    LoggingMessageNotificationParam, ProgressNotification, ProgressNotificationParam,
-    PromptListChangedNotification, ResourceListChangedNotification, ResourceUpdatedNotification,
-    ResourceUpdatedNotificationParam, ServerInfo, ServerMessage, ServerNotification, ServerRequest,
-    ServerResult, ToolListChangedNotification,
+    CancelledNotification, CancelledNotificationParam, ClientInfo, ClientJsonRpcMessage,
+    ClientMessage, ClientNotification, ClientRequest, ClientResult, CreateMessageRequest,
+    CreateMessageRequestParam, CreateMessageResult, ListRootsRequest, ListRootsResult,
+    LoggingMessageNotification, LoggingMessageNotificationParam, ProgressNotification,
+    ProgressNotificationParam, PromptListChangedNotification, ResourceListChangedNotification,
+    ResourceUpdatedNotification, ResourceUpdatedNotificationParam, ServerInfo, ServerMessage,
+    ServerNotification, ServerRequest, ServerResult, ToolListChangedNotification,
 };
 
 #[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
@@ -26,6 +27,24 @@ impl ServiceRole for RoleServer {
     const IS_CLIENT: bool = false;
 }
 
+/// It represents the error that may occur when serving the server.
+///
+/// if you want to handle the error, you can use `serve_server_with_ct` or `serve_server` with `Result<RunningService<RoleServer, S>, ServerError>`
+#[derive(Error, Debug)]
+pub enum ServerError {
+    #[error("expect initialized request, but received: {0:?}")]
+    ExpectedInitRequest(Option<ClientMessage>),
+
+    #[error("expect initialized notification, but received: {0:?}")]
+    ExpectedInitNotification(Option<ClientMessage>),
+
+    #[error("connection closed: {0}")]
+    ConnectionClosed(String),
+
+    #[error("IO error: {0}")]
+    Io(#[from] std::io::Error),
+}
+
 pub type ClientSink = Peer<RoleServer>;
 
 impl<S: Service<RoleServer>> ServiceExt<RoleServer> for S {
@@ -55,6 +74,46 @@ where
     serve_server_with_ct(service, transport, CancellationToken::new()).await
 }
 
+/// Helper function to get the next message from the stream
+async fn expect_next_message<S>(stream: &mut S, context: &str) -> Result<ClientMessage, ServerError>
+where
+    S: StreamExt<Item = ClientJsonRpcMessage> + Unpin,
+{
+    Ok(stream
+        .next()
+        .await
+        .ok_or_else(|| ServerError::ConnectionClosed(context.to_string()))?
+        .into_message())
+}
+
+/// Helper function to expect a request from the stream
+async fn expect_request<S>(
+    stream: &mut S,
+    context: &str,
+) -> Result<(ClientRequest, RequestId), ServerError>
+where
+    S: StreamExt<Item = ClientJsonRpcMessage> + Unpin,
+{
+    let msg = expect_next_message(stream, context).await?;
+    let msg_clone = msg.clone();
+    msg.into_request()
+        .ok_or(ServerError::ExpectedInitRequest(Some(msg_clone)))
+}
+
+/// Helper function to expect a notification from the stream
+async fn expect_notification<S>(
+    stream: &mut S,
+    context: &str,
+) -> Result<ClientNotification, ServerError>
+where
+    S: StreamExt<Item = ClientJsonRpcMessage> + Unpin,
+{
+    let msg = expect_next_message(stream, context).await?;
+    let msg_clone = msg.clone();
+    msg.into_notification()
+        .ok_or(ServerError::ExpectedInitNotification(Some(msg_clone)))
+}
+
 pub async fn serve_server_with_ct<S, T, E, A>(
     service: S,
     transport: T,
@@ -70,54 +129,45 @@ where
     let mut stream = Box::pin(stream);
     let id_provider = <Arc<AtomicU32RequestIdProvider>>::default();
 
-    // service
-    let (request, id) = stream
-        .next()
-        .await
-        .ok_or(std::io::Error::new(
-            std::io::ErrorKind::UnexpectedEof,
-            "expect initialize request",
-        ))?
-        .into_message()
-        .into_request()
-        .ok_or(std::io::Error::new(
-            std::io::ErrorKind::InvalidData,
-            "expect initialize request",
-        ))?;
-    let ClientRequest::InitializeRequest(peer_info) = request else {
-        return Err(std::io::Error::new(
-            std::io::ErrorKind::InvalidData,
-            "expect initialize request",
-        )
-        .into());
+    // Convert ServerError to std::io::Error, then to E
+    let handle_server_error = |e: ServerError| -> E {
+        match e {
+            ServerError::Io(io_err) => io_err.into(),
+            other => std::io::Error::new(std::io::ErrorKind::Other, format!("{}", other)).into(),
+        }
     };
+
+    // Get initialize request
+    let (request, id) = expect_request(&mut stream, "initialized request")
+        .await
+        .map_err(handle_server_error)?;
+
+    let ClientRequest::InitializeRequest(peer_info) = request else {
+        return Err(handle_server_error(ServerError::ExpectedInitRequest(Some(
+            ClientMessage::Request(request, id),
+        ))));
+    };
+
+    // Send initialize response
     let init_response = service.get_info();
     sink.send(
         ServerMessage::Response(ServerResult::InitializeResult(init_response), id)
             .into_json_rpc_message(),
     )
     .await?;
-    // waiting for notification
-    let notification = stream
-        .next()
+
+    // Wait for initialize notification
+    let notification = expect_notification(&mut stream, "initialize notification")
         .await
-        .ok_or(std::io::Error::new(
-            std::io::ErrorKind::UnexpectedEof,
-            "expect initialize notification",
-        ))?
-        .into_message()
-        .into_notification()
-        .ok_or(std::io::Error::new(
-            std::io::ErrorKind::InvalidData,
-            "expect initialize notification",
-        ))?;
+        .map_err(handle_server_error)?;
+
     let ClientNotification::InitializedNotification(_) = notification else {
-        return Err(std::io::Error::new(
-            std::io::ErrorKind::InvalidData,
-            "expect initialize notification",
-        )
-        .into());
+        return Err(handle_server_error(ServerError::ExpectedInitNotification(
+            Some(ClientMessage::Notification(notification)),
+        )));
     };
+
+    // Continue processing service
     serve_inner(service, (sink, stream), peer_info.params, id_provider, ct).await
 }