feat(master): in-process broker for live UI change signals

internal/master/events/broker_test.go

@@ -0,0 +1,48 @@
+package events
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestBroker_DeliversToAllSubscribers(t *testing.T) {
+	b := NewBroker()
+	a, cancelA := b.Subscribe()
+	defer cancelA()
+	c, cancelC := b.Subscribe()
+	defer cancelC()
+
+	b.Publish(Services)
+	assert.Equal(t, Services, <-a)
+	assert.Equal(t, Services, <-c)
+}
+
+func TestBroker_CancelStopsDelivery(t *testing.T) {
+	b := NewBroker()
+	ch, cancel := b.Subscribe()
+	cancel()
+	cancel() // safe to call twice
+	b.Publish(Agents)
+	assert.Empty(t, ch)
+}
+
+func TestBroker_SlowSubscriberDoesNotBlockPublish(t *testing.T) {
+	b := NewBroker()
+	_, cancel := b.Subscribe()
+	defer cancel()
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		for range 1000 {
+			b.Publish(Services) // must return even though nobody reads
+		}
+	}()
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("Publish blocked on a slow subscriber")
+	}
+}