fix: absorb multiplex close bursts

server/cluster_order_test.go

@@ -112,3 +112,54 @@ func TestClusterWriteLoopRejectsWritesAfterTermination(t *testing.T) {
 		t.Fatalf("terminated multiplexing session retained %d outbound messages", len(sess.send))
 	}
 }
+
+func TestMultiplexEnqueueWaitsForDrain(t *testing.T) {
+	sess := &Session{proto: MULTIPLEX, send: make(chan any, 1)}
+	sess.send <- "existing"
+
+	go func() {
+		time.Sleep(10 * time.Millisecond)
+		<-sess.send
+	}()
+
+	payload := &ServerComMessage{TenantID: types.TenantID(1)}
+	if !sess.enqueueMultiplexOutbound(payload, 200*time.Millisecond) {
+		t.Fatal("multiplex enqueue failed while the drain was making progress")
+	}
+	if got := <-sess.send; got != payload {
+		t.Fatalf("unexpected queued payload: %#v", got)
+	}
+}
+
+func TestMultiplexEnqueueTimesOutWithoutDrain(t *testing.T) {
+	sess := &Session{proto: MULTIPLEX, send: make(chan any, 1)}
+	sess.send <- "existing"
+
+	started := time.Now()
+	if sess.enqueueMultiplexOutbound("next", 20*time.Millisecond) {
+		t.Fatal("multiplex enqueue succeeded without queue capacity")
+	}
+	if elapsed := time.Since(started); elapsed < 15*time.Millisecond || elapsed > 200*time.Millisecond {
+		t.Fatalf("multiplex enqueue timeout outside expected bounds: %s", elapsed)
+	}
+}
+
+func TestMultiplexEnqueueStopsWhenSessionTerminates(t *testing.T) {
+	sess := &Session{proto: MULTIPLEX, send: make(chan any, 1)}
+	sess.send <- "existing"
+
+	go func() {
+		time.Sleep(10 * time.Millisecond)
+		sess.clusterQueueMu.Lock()
+		atomic.StoreInt32(&sess.terminating, 1)
+		<-sess.send
+		sess.clusterQueueMu.Unlock()
+	}()
+
+	if !sess.enqueueMultiplexOutbound("next", 200*time.Millisecond) {
+		t.Fatal("terminating multiplex session should discard the pending write")
+	}
+	if len(sess.send) != 0 {
+		t.Fatalf("terminating multiplex session retained %d outbound messages", len(sess.send))
+	}
+}