Fix Lookup instance filtering

client.go

@@ -110,12 +110,10 @@ func (r *Resolver) Browse(ctx context.Context, service, domain string, entries c
 
 // Lookup a specific service by its name and type in a given domain.
 func (r *Resolver) Lookup(ctx context.Context, instance, service, domain string, entries chan<- *ServiceEntry) error {
-	params := defaultParams(service)
-	params.Instance = instance
-	if domain != "" {
-		params.Domain = domain
+	if domain == "" {
+		domain = "local"
 	}
-	params.Entries = entries
+	params := newLookupParams(instance, service, domain, false, entries)
 	ctx, cancel := context.WithCancel(ctx)
 	go r.c.mainloop(ctx, params)
 	err := r.c.query(params)

service_test.go

@@ -91,6 +91,32 @@ func TestNoRegister(t *testing.T) {
 	cancel()
 }
 
+func TestLookupParamsServiceInstanceName(t *testing.T) {
+	instance := "device-one"
+	service := "_lookup-filter._tcp"
+	domain := "local"
+	entries := make(chan *ServiceEntry)
+
+	// Correct path used by Resolver.Lookup: instance is set at construction.
+	params := newLookupParams(instance, service, domain, false, entries)
+	want := "device-one._lookup-filter._tcp.local."
+	if got := params.ServiceInstanceName(); got != want {
+		t.Fatalf("ServiceInstanceName() = %q, want %q", got, want)
+	}
+	if params.Instance != instance {
+		t.Fatalf("Instance = %q, want %q", params.Instance, instance)
+	}
+	if params.ServiceName() != "_lookup-filter._tcp.local." {
+		t.Fatalf("ServiceName() = %q, want %q", params.ServiceName(), "_lookup-filter._tcp.local.")
+	}
+
+	broken := defaultParams(service)
+	broken.Instance = instance
+	if got := broken.ServiceInstanceName(); got != "" {
+		t.Fatalf("mutating Instance after construction must not update ServiceInstanceName; got %q", got)
+	}
+}
+
 func TestSubtype(t *testing.T) {
 	t.Run("browse with subtype", func(t *testing.T) {
 		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)