fix: preserve cross-pod message order

server/session.go

@@ -168,6 +168,9 @@ type Session struct {
 
 	// Type of proxy to master request being handled.
 	proxyReq ProxyReqType
+	// Ensures that one multiplexing session is drained by at most one cluster
+	// write task at a time, preserving per-topic response order.
+	clusterWriteScheduled atomic.Bool
 }
 
 // Subscription is a mapper of sessions to topics.
@@ -332,7 +335,7 @@ func (s *Session) isCluster() bool {
 }
 
 func (s *Session) scheduleClusterWriteLoop() {
-	if globals.cluster != nil && globals.cluster.proxyEventQueue != nil {
+	if globals.cluster != nil && globals.cluster.proxyEventQueue != nil && s.clusterWriteScheduled.CompareAndSwap(false, true) {
 		globals.cluster.proxyEventQueue.Schedule(
 			func() { s.clusterWriteLoop(s.proxiedTopic) })
 	}