PR feedback: name change and account for different options order

gossipsub.go

@@ -76,7 +76,7 @@ func (gs *GossipSubRouter) Attach(p *PubSub) {
 	gs.p = p
 	gs.tracer = p.tracer
 	// start using the same msg ID function as PubSub for caching messages.
-	gs.mcache.ChangeMsgIdFn(p.msgID)
+	gs.mcache.SetMsgIdFn(p.msgID)
 	go gs.heartbeatTimer()
 }
 

mcache.go

@@ -39,7 +39,7 @@ type MessageCache struct {
 	msgID   MsgIdFunction
 }
 
-func (mc *MessageCache) ChangeMsgIdFn(msgID MsgIdFunction) {
+func (mc *MessageCache) SetMsgIdFn(msgID MsgIdFunction) {
 	mc.msgID = msgID
 }
 

pubsub.go

@@ -254,6 +254,10 @@ type MsgIdFunction func(pmsg *pb.Message) string
 func WithMessageIdFn(fn MsgIdFunction) Option {
 	return func(p *PubSub) error {
 		p.msgID = fn
+		// the tracer Option may already be set. Update its message ID function to make options order-independent.
+		if p.tracer != nil {
+			p.tracer.msgID = fn
+		}
 		return nil
 	}
 }