fix: prevent data loss in eventbus by applying backpressure

internal/eventbus/bus.go

@@ -220,8 +220,6 @@ func (b *Bus) dispatch(ctx context.Context, s *subscriber, e Event) {
 		atomic.AddUint64(&b.dropped, 1)
 	case s.ch <- e:
 		atomic.AddUint64(&b.delivered, 1)
-	default:
-		atomic.AddUint64(&b.dropped, 1)
 	}
 }
 

internal/eventbus/bus_test.go

@@ -112,30 +112,54 @@ func TestPublish_MultipleSubscribersEachGetEvent(t *testing.T) {
 	waitUntil(t, time.Second, func() bool { return a.Load() == 1 && c.Load() == 1 }, "both subscribers received")
 }
 
-func TestPublish_NonBlocking_DropsWhenBufferFull(t *testing.T) {
-	b := New(2)
+func TestPublish_Blocking_AppliesBackpressure(t *testing.T) {
+	b := New(1)
 	defer b.Close(context.Background())
 
 	blocker := make(chan struct{})
-	var started atomic.Int32
+	var received atomic.Int32
+
+	// Subscriber that blocks until 'blocker' channel is closed
 	b.Subscribe(EventSubdomainDiscovered, func(ctx context.Context, _ Event) {
-		started.Add(1)
-		<-blocker
+		<-blocker 
+		received.Add(1)
 	})
 
-	// First event enters handler (blocks). Next 2 fill the buffer of size 2.
-	// Subsequent publishes should be counted as dropped.
-	for i := 0; i < 100; i++ {
-		b.Publish(context.Background(), NewSubdomainDiscovered("t", "x.example.com", "p"))
+	// Event 1: goes to handler (which freezes)
+	b.Publish(context.Background(), NewSubdomainDiscovered("t", "1.example.com", "p"))
+
+	// Event 2: sits in the bus buffer (size is 1)
+	b.Publish(context.Background(), NewSubdomainDiscovered("t", "2.example.com", "p"))
+
+	// Event 3: should trigger backpressure and block the publisher
+	publishDone := make(chan struct{})
+	go func() {
+		b.Publish(context.Background(), NewSubdomainDiscovered("t", "3.example.com", "p"))
+		close(publishDone)
+	}()
+
+	// Verify the 3rd publish is actually blocked
+	select {
+	case <-publishDone:
+		t.Fatal("Publish did not block when the buffer was full")
+	case <-time.After(50 * time.Millisecond):
+		// Expected: the goroutine is blocked waiting for space
 	}
 
-	// Give the bus a moment to register drops.
+	// Unblock the subscriber so it processes the queue
+	close(blocker)
+
+	// Wait for the blocked publish to finally complete
+	<-publishDone
+
+	// Verify no data was lost
 	waitUntil(t, time.Second, func() bool {
-		return b.Stats().Dropped > 0
-	}, "some events dropped when buffer full")
+		return received.Load() == 3
+	}, "all 3 events delivered after backpressure cleared")
 
-	// Unblock and close cleanly.
-	close(blocker)
+	if b.Stats().Dropped > 0 {
+		t.Errorf("expected 0 dropped events, got %d", b.Stats().Dropped)
+	}
 }
 
 func TestClose_DrainsAndStops(t *testing.T) {