fix: Select ctx.Done() when preprocessing to avoid blocking on cancel (#635)

Close #636 

The PR updates the send logic to use a select with ctx.Done() and
t.p.ctx.Done(), ensuring the operation terminates gracefully. A test
case reproducing the issue is included in the PR for verification.

topic.go

@@ -349,8 +349,14 @@ func (t *Topic) validate(ctx context.Context, data []byte, opts ...PubOpt) (*Mes
 	}
 
 	msg := &Message{m, "", t.p.host.ID(), pub.validatorData, pub.local}
-	t.p.eval <- func() {
+	select {
+	case t.p.eval <- func() {
 		t.p.rt.Preprocess(t.p.host.ID(), []*Message{msg})
+	}:
+	case <-t.p.ctx.Done():
+		return nil, t.p.ctx.Err()
+	case <-ctx.Done():
+		return nil, ctx.Err()
 	}
 	err := t.p.val.ValidateLocal(msg)
 	if err != nil {

topic_test.go

@@ -951,6 +951,23 @@ func TestTopicPublishWithKeyInvalidParameters(t *testing.T) {
 	})
 }
 
+func TestTopicPublishWithContextCanceled(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	const topic = "foobar"
+	const numHosts = 5
+
+	hosts := getDefaultHosts(t, numHosts)
+	topics := getTopics(getPubsubs(ctx, hosts), topic)
+	cancel()
+
+	err := topics[0].Publish(ctx, []byte("buff"))
+	if err != context.Canceled {
+		t.Fatal("error should have been of type context.Canceled", err)
+	}
+}
+
 func TestTopicRelayPublishWithKey(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
 	defer cancel()