fix(message-router): prevent subscription race condition on gRPC reconnect

When a party re-subscribes (e.g., Android reconnects), the old gRPC stream's
defer Unsubscribe() was accidentally removing the NEW subscription from the
subscribers map, causing the party to miss session_started events.

Fix:
- Subscribe() now returns the channel to the caller
- Unsubscribe() now takes the channel and only removes if it matches
- This prevents older streams from removing newer subscriptions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/mpc-system/services/message-router/adapters/input/grpc/message_grpc_handler.go

@@ -350,8 +350,10 @@ func (s *MessageRouterServer) SubscribeSessionEvents(
 		zap.String("party_id", req.PartyId))
 
 	// Subscribe to events
-	eventCh := s.eventBroadcaster.Subscribe(req.PartyId)
-	defer s.eventBroadcaster.Unsubscribe(req.PartyId)
+	// The channel is used for identity check in Unsubscribe to prevent
+	// accidentally removing a newer subscription when this stream exits
+	eventCh, _ := s.eventBroadcaster.Subscribe(req.PartyId)
+	defer s.eventBroadcaster.Unsubscribe(req.PartyId, eventCh)
 
 	// Stream events
 	for {

backend/mpc-system/services/message-router/domain/session_event_broadcaster.go

@@ -2,6 +2,7 @@ package domain
 
 import (
 	"sync"
+	"time"
 
 	pb "github.com/rwadurian/mpc-system/api/grpc/router/v1"
 	"github.com/rwadurian/mpc-system/pkg/logger"
@@ -22,32 +23,51 @@ func NewSessionEventBroadcaster() *SessionEventBroadcaster {
 }
 
 // Subscribe subscribes a party to session events
-// If the party already has an active subscription, the old channel is closed first
-// to prevent memory leaks and ensure clean reconnection
-func (b *SessionEventBroadcaster) Subscribe(partyID string) <-chan *pb.SessionEvent {
+// Returns the channel for receiving events and a unique subscription ID
+// The subscription ID is used to safely unsubscribe without affecting newer subscriptions
+func (b *SessionEventBroadcaster) Subscribe(partyID string) (<-chan *pb.SessionEvent, int64) {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 
 	// Close existing channel if party is re-subscribing (e.g., after reconnect)
+	// This will cause the old gRPC stream to exit cleanly
 	if oldCh, exists := b.subscribers[partyID]; exists {
 		close(oldCh)
+		logger.Debug("Closed old subscription channel for re-subscribing party",
+			zap.String("party_id", partyID))
 	}
 
 	// Create buffered channel for this subscriber
 	ch := make(chan *pb.SessionEvent, 100)
 	b.subscribers[partyID] = ch
 
-	return ch
+	// Generate a unique subscription ID (using current time in nanoseconds)
+	subscriptionID := time.Now().UnixNano()
+
+	return ch, subscriptionID
 }
 
-// Unsubscribe removes a party's subscription
-func (b *SessionEventBroadcaster) Unsubscribe(partyID string) {
+// Unsubscribe removes a party's subscription only if the channel matches
+// This prevents a race condition where a newer subscription is accidentally removed
+// when an old gRPC stream exits after the party has already re-subscribed
+func (b *SessionEventBroadcaster) Unsubscribe(partyID string, ch <-chan *pb.SessionEvent) {
 	b.mu.Lock()
 	defer b.mu.Unlock()
 
-	if ch, exists := b.subscribers[partyID]; exists {
-		close(ch)
-		delete(b.subscribers, partyID)
+	if currentCh, exists := b.subscribers[partyID]; exists {
+		// Only delete if the channel matches (i.e., this is still our subscription)
+		// If the channel doesn't match, a newer subscription has been created
+		// and we should not delete it
+		if currentCh == ch {
+			// Don't close the channel here - it was already closed by Subscribe
+			// when the new subscription was created, or we're the last one
+			delete(b.subscribers, partyID)
+			logger.Debug("Unsubscribed party from session events",
+				zap.String("party_id", partyID))
+		} else {
+			logger.Debug("Skipping unsubscribe - channel mismatch (newer subscription exists)",
+				zap.String("party_id", partyID))
+		}
 	}
 }