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use std::collections::HashMap;
use std::sync::Arc;
use serde_json::Value;
use tokio::sync::{mpsc, oneshot};
use tokio::task::JoinHandle;
use tokio_util::sync::CancellationToken;
use super::transport::{McpClientTransportReader, McpClientTransportWriter};
use super::{
CorrelatedResponse, McpClientMessage, MultiplexerClosed, NotificationMessage, PendingRequest,
SERVER_REQUEST_BUFFER_SIZE, SERVER_REQUEST_BUFFER_WARNING, ServerRequestMessage,
};
// ============================================================================
// Message Multiplexer
// ============================================================================
/// Background task that reads from the transport and routes messages.
///
/// - Responses → oneshot channels (by ID)
/// - Server requests → bounded handler channel
/// - Notifications → unbounded channel
///
/// Implements: TJ-SPEC-018 F-002
pub(super) struct MessageMultiplexer {
/// Pending response senders: `id.to_string()` → oneshot sender.
response_senders: Arc<std::sync::Mutex<HashMap<String, oneshot::Sender<CorrelatedResponse>>>>,
/// Why the multiplexer closed (set on loop exit).
close_reason: Arc<std::sync::Mutex<Option<MultiplexerClosed>>>,
/// Join handle for the background task.
///
/// Dropping this handle detaches the task. Call [`Self::abort`] during
/// shutdown for deterministic teardown.
handle: JoinHandle<()>,
}
impl MessageMultiplexer {
/// Spawn the multiplexer background task.
///
/// Takes exclusive ownership of the transport reader.
///
/// Implements: TJ-SPEC-018 F-002
#[allow(clippy::too_many_arguments)]
pub(super) fn spawn(
mut reader: Box<dyn McpClientTransportReader>,
writer: Arc<tokio::sync::Mutex<Box<dyn McpClientTransportWriter>>>,
pending: Arc<std::sync::Mutex<HashMap<String, PendingRequest>>>,
server_request_tx: mpsc::Sender<ServerRequestMessage>,
notification_tx: mpsc::Sender<NotificationMessage>,
response_senders: Arc<
std::sync::Mutex<HashMap<String, oneshot::Sender<CorrelatedResponse>>>,
>,
close_reason: Arc<std::sync::Mutex<Option<MultiplexerClosed>>>,
cancel: CancellationToken,
) -> Self {
let senders = Arc::clone(&response_senders);
let reason = Arc::clone(&close_reason);
let handle = tokio::spawn(async move {
let exit_reason = loop {
tokio::select! {
biased;
() = cancel.cancelled() => {
break MultiplexerClosed::Cancelled;
}
msg = reader.recv(&pending) => {
match msg {
Ok(Some(McpClientMessage::Response { id, method, result, is_error, request_params })) => {
let id_key = id.to_string();
let sender = senders
.lock()
.unwrap_or_else(std::sync::PoisonError::into_inner)
.remove(&id_key);
if let Some(tx) = sender {
let _ = tx.send(CorrelatedResponse { method, result, is_error, request_params });
} else {
// EC-MCPC-001: unmatched response ID
tracing::warn!(id = %id, "received response for unknown request id");
}
}
Ok(Some(McpClientMessage::Notification { method, params })) => {
if notification_tx
.try_send(NotificationMessage { method, params })
.is_err()
{
break MultiplexerClosed::TransportError(
"notification backlog exceeded".to_string()
);
}
}
Ok(Some(McpClientMessage::ServerRequest { id, method, params })) => {
// Backpressure check (§3.7)
if server_request_tx.capacity() < SERVER_REQUEST_BUFFER_WARNING {
tracing::warn!(
capacity = server_request_tx.capacity(),
max = SERVER_REQUEST_BUFFER_SIZE,
"server request buffer nearly full"
);
}
let req = ServerRequestMessage {
id: id.clone(),
method: method.clone(),
params,
};
match server_request_tx.try_send(req) {
Ok(()) => {}
Err(mpsc::error::TrySendError::Full(_)) => {
tracing::warn!(
method = %method,
id = %id,
"server request buffer full, dropping request"
);
// Return error to server so it doesn't hang
let _ = writer.lock().await
.send_error_response(&id, -32000, "Client overwhelmed: server request buffer full")
.await;
}
Err(mpsc::error::TrySendError::Closed(_)) => {
// Handler task crashed — stop the multiplexer
tracing::error!("server request handler channel closed, stopping multiplexer");
break MultiplexerClosed::TransportError(
"server request handler channel closed".to_string()
);
}
}
}
Ok(None) => {
break MultiplexerClosed::TransportEof;
}
Err(e) => {
break MultiplexerClosed::TransportError(e.to_string());
}
}
}
}
};
// Store close reason BEFORE dropping senders
*reason
.lock()
.unwrap_or_else(std::sync::PoisonError::into_inner) = Some(exit_reason);
senders
.lock()
.unwrap_or_else(std::sync::PoisonError::into_inner)
.clear();
});
Self {
response_senders,
close_reason,
handle,
}
}
/// Register a oneshot channel for a response, keyed by request ID.
///
/// Must be called BEFORE sending the request to prevent races.
pub(super) fn register_response(&self, id: &Value) -> oneshot::Receiver<CorrelatedResponse> {
let (tx, rx) = oneshot::channel();
self.response_senders
.lock()
.unwrap_or_else(std::sync::PoisonError::into_inner)
.insert(id.to_string(), tx);
rx
}
/// Returns the reason the multiplexer closed, if it has.
pub(super) fn close_reason(&self) -> MultiplexerClosed {
self.close_reason
.lock()
.unwrap_or_else(std::sync::PoisonError::into_inner)
.clone()
.unwrap_or(MultiplexerClosed::TransportEof)
}
pub(super) fn remove_response(&self, id_key: &str) {
self.response_senders
.lock()
.unwrap_or_else(std::sync::PoisonError::into_inner)
.remove(id_key);
}
/// Aborts the multiplexer background task.
pub(super) fn abort(&self) {
self.handle.abort();
}
}