util/eventbus: add tests for a subscriber publishing events

As of 2025-11-20, publishing more events than the eventbus's
internal queues can hold may deadlock if a subscriber tries
to publish events itself.

This commit adds a test that demonstrates this deadlock,
and skips it until the bug is fixed.

Updates #18012

Signed-off-by: Nick Khyl <nickk@tailscale.com>

util/eventbus/bus_test.go

@@ -636,6 +636,7 @@ func testPublishWithMutex(t *testing.T, n int) {
 
 		var mu sync.Mutex
 		eventbus.SubscribeFunc[EventA](c, func(e EventA) {
+			// Acquire the same mutex as the publisher.
 			// As of 2025-11-20, this can deadlock if n is large enough
 			// and event queues fill up.
 			mu.Lock()
@@ -648,6 +649,7 @@ func testPublishWithMutex(t *testing.T, n int) {
 
 		p := eventbus.Publish[EventA](c)
 		go func() {
+			// Publish events, acquiring the mutex around each publish.
 			for i := range n {
 				mu.Lock()
 				p.Publish(EventA{Counter: i})
@@ -663,6 +665,64 @@ func testPublishWithMutex(t *testing.T, n int) {
 	})
 }
 
+func TestPublishFromSubscriber(t *testing.T) {
+	t.Run("FewEvents", func(t *testing.T) {
+		// Publishing up to [totalMaxQueuedItems]-1 is fine.
+		testPublishFromSubscriber(t, totalMaxQueuedItems-1)
+	})
+	t.Run("ManyEvents", func(t *testing.T) {
+		// As of 2025-11-20, publishing more than [totalMaxQueuedItems] may deadlock.
+		t.Skip("TODO: fix deadlock in https://github.com/tailscale/tailscale/issues/18012")
+
+		// Using 2x to increase chance of deadlock.
+		testPublishFromSubscriber(t, totalMaxQueuedItems*2)
+	})
+}
+
+// testPublishFromSubscriber publishes the specified number of EventA events.
+// Each EventA causes the subscriber to publish an EventB.
+// The test fails if it loses any events or if a deadlock occurs.
+func testPublishFromSubscriber(t *testing.T, n int) {
+	synctest.Test(t, func(t *testing.T) {
+		b := eventbus.New()
+		defer b.Close()
+
+		c := b.Client("TestClient")
+
+		// Ultimately we expect to receive n EventB events
+		// published as a result of receiving n EventA events.
+		evts := make([]any, n)
+		for i := range evts {
+			evts[i] = EventB{Counter: i}
+		}
+		exp := expectEvents(t, evts...)
+
+		pubA := eventbus.Publish[EventA](c)
+		pubB := eventbus.Publish[EventB](c)
+
+		eventbus.SubscribeFunc[EventA](c, func(e EventA) {
+			// Upon receiving EventA, publish EventB.
+			// As of 2025-11-20, this can deadlock if n is large enough
+			// and event queues fill up.
+			pubB.Publish(EventB{Counter: e.Counter})
+		})
+		eventbus.SubscribeFunc[EventB](c, func(e EventB) {
+			// Mark EventB as received.
+			exp.Got(e)
+		})
+
+		for i := range n {
+			pubA.Publish(EventA{Counter: i})
+		}
+
+		synctest.Wait()
+
+		if !exp.Empty() {
+			t.Errorf("unexpected extra events: %+v", exp.want)
+		}
+	})
+}
+
 type queueChecker struct {
 	t    *testing.T
 	want []any