maintain stable ordering for heap to avoid packet reordering

codel.go

@@ -13,6 +13,7 @@ type codelQueue struct {
 	packets   packetHeap
 	newPacket chan struct{}
 	closed    bool
+	pushCount int
 
 	// CoDel state
 	dropping     bool
@@ -41,7 +42,8 @@ func (q *codelQueue) Enqueue(p *packetWithDeliveryTime) {
 	if q.closed {
 		return
 	}
-	heap.Push(&q.packets, p)
+	q.pushCount++
+	heap.Push(&q.packets, packetWithDeliveryTimeAndOrder{packetWithDeliveryTime: p, count: q.pushCount})
 
 	// Signal that a new packet arrived (non-blocking)
 	select {
@@ -100,7 +102,8 @@ func (q *codelQueue) Dequeue() (*packetWithDeliveryTime, bool) {
 		}
 
 		// Packet is ready, remove from queue and return it
-		p := heap.Pop(&q.packets).(*packetWithDeliveryTime)
+		po := heap.Pop(&q.packets).(packetWithDeliveryTimeAndOrder)
+		p := po.packetWithDeliveryTime
 
 		// Reset CoDel state when queue becomes empty
 		if len(q.packets) == 0 {
@@ -177,21 +180,27 @@ func (q *codelQueue) Close() {
 	close(q.newPacket)
 }
 
+type packetWithDeliveryTimeAndOrder struct {
+	count int
+	*packetWithDeliveryTime
+}
+
 // packetHeap implements heap.Interface ordered by packet delivery time.
-type packetHeap []*packetWithDeliveryTime
+type packetHeap []packetWithDeliveryTimeAndOrder
 
 func (h packetHeap) Len() int { return len(h) }
 
 func (h packetHeap) Less(i, j int) bool {
-	return h[i].DeliveryTime.Before(h[j].DeliveryTime)
+	return (h[i].DeliveryTime.Before(h[j].DeliveryTime) ||
+		h[i].DeliveryTime.Equal(h[j].DeliveryTime) && h[i].count < h[j].count)
 }
 
 func (h packetHeap) Swap(i, j int) {
 	h[i], h[j] = h[j], h[i]
 }
 
 func (h *packetHeap) Push(x any) {
-	*h = append(*h, x.(*packetWithDeliveryTime))
+	*h = append(*h, x.(packetWithDeliveryTimeAndOrder))
 }
 
 func (h *packetHeap) Pop() any {