util/eventbus: add test helpers to simplify testing events (#16294)

Instead of every module having to come up with a set of test methods for
the event bus, this handful of test helpers hides a lot of the needed
setup for the testing of the event bus.

The tests in portmapper is also ported over to the new helpers.

Updates #15160

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

net/portmapper/portmapper.go

@@ -515,7 +515,7 @@ func (c *Client) createMapping() {
 			GoodUntil: mapping.GoodUntil(),
 		})
 	}
-	if c.onChange != nil {
+	if c.onChange != nil && c.pubClient == nil {
 		go c.onChange()
 	}
 }

net/portmapper/portmapper_test.go

@@ -12,7 +12,7 @@ import (
 	"time"
 
 	"tailscale.com/control/controlknobs"
-	"tailscale.com/util/eventbus"
+	"tailscale.com/util/eventbus/eventbustest"
 )
 
 func TestCreateOrGetMapping(t *testing.T) {
@@ -142,22 +142,15 @@ func TestUpdateEvent(t *testing.T) {
 		t.Fatalf("Create test gateway: %v", err)
 	}
 
-	bus := eventbus.New()
-	defer bus.Close()
+	bus := eventbustest.NewBus(t)
+	tw := eventbustest.NewWatcher(t, bus)
 
-	sub := eventbus.Subscribe[Mapping](bus.Client("TestUpdateEvent"))
 	c := newTestClient(t, igd, bus)
 	if _, err := c.Probe(t.Context()); err != nil {
 		t.Fatalf("Probe failed: %v", err)
 	}
 	c.GetCachedMappingOrStartCreatingOne()
-
-	select {
-	case evt := <-sub.Events():
-		t.Logf("Received portmap update: %+v", evt)
-	case <-sub.Done():
-		t.Error("Subscriber closed prematurely")
-	case <-time.After(5 * time.Second):
-		t.Error("Timed out waiting for an update event")
+	if err := eventbustest.Expect(tw, eventbustest.Type[Mapping]()); err != nil {
+		t.Error(err.Error())
 	}
 }