Prevent multiple invocations of relay cancel function

pubsub.go

@@ -676,12 +676,22 @@ func (p *PubSub) handleAddRelay(req *addRelayReq) {
 		p.rt.Join(topic)
 	}
 
-	req.resp <- func() {
+	// flag used to prevent calling cancel function multiple times
+	isCancelled := false
+
+	relayCancelFunc := func() {
+		if isCancelled {
+			return
+		}
+
 		select {
 		case p.rmRelay <- topic:
+			isCancelled = true
 		case <-p.ctx.Done():
 		}
 	}
+
+	req.resp <- relayCancelFunc
 }
 
 // handleRemoveRelay removes one relay reference from bookkeeping.

topic_test.go

@@ -629,7 +629,20 @@ func TestTopicRelayReuse(t *testing.T) {
 		t.Fatal("incorrect number of relays")
 	}
 
+	// only the first invocation should take effect
 	relay1Cancel()
+	relay1Cancel()
+	relay1Cancel()
+
+	pubsubs[0].eval <- func() {
+		res <- pubsubs[0].myRelays[topic] == 2
+	}
+
+	isCorrectNumber = <-res
+	if !isCorrectNumber {
+		t.Fatal("incorrect number of relays")
+	}
+
 	relay2Cancel()
 	relay3Cancel()