AsyncTopic guarantees delivery

async.go

@@ -7,8 +7,12 @@ import (
 )
 
 // AsyncTopic allows any message T to be broadcast to subscribers. Publishing as well as
-// subscribing happens asynchronously. Due to the nature of async processes this cannot guarantee
-// message delivery nor delivery order.
+// subscribing happens asynchronously.
+// This guarantees that every published message will be delivered but does NOT guarantee delivery
+// order.
+// In the unlikely scenario where subscribers are being queued very aggressively it is possible
+// that some might never actually receive any message. Subscriber registration order is also not
+// guaranteed.
 type AsyncTopic[T any] struct {
 	options AsyncTopicOptions
 
@@ -20,6 +24,7 @@ type AsyncTopic[T any] struct {
 }
 
 // NewAsyncTopic creates an AsyncTopic that will be closed when the given context is cancelled.
+// After closed calls to Publish or Subscribe will return an error.
 func NewAsyncTopic[T any](ctx context.Context, opts ...AsyncTopicOption) *AsyncTopic[T] {
 	options := AsyncTopicOptions{
 		onClose:     func() {},
@@ -33,29 +38,43 @@ func NewAsyncTopic[T any](ctx context.Context, opts ...AsyncTopicOption) *AsyncT
 	t := AsyncTopic[T]{
 		options:     options,
 		publishCh:   make(chan T, 1),
-		subscribeCh: make(chan Subscriber[T]),
+		subscribeCh: make(chan Subscriber[T], 1),
 	}
 
-	go t.run(ctx)
+	go t.closer(ctx)
+	go t.run()
 
 	return &t
 }
 
-func (t *AsyncTopic[T]) run(ctx context.Context) {
-	defer t.close()
+func (t *AsyncTopic[T]) closer(ctx context.Context) {
+	<-ctx.Done()
 
+	t.mu.Lock()
+	t.closed = true // no more subscribing or publishing
+	t.mu.Unlock()
+
+	close(t.publishCh)
+	close(t.subscribeCh)
+
+	t.options.onClose()
+}
+
+func (t *AsyncTopic[T]) run() {
 	var subscribers []Subscriber[T]
 
 	for {
 		select {
-		case <-ctx.Done():
-			return
-
 		case newCallback := <-t.subscribeCh:
 			subscribers = append(subscribers, newCallback)
 			t.options.onSubscribe(len(subscribers))
 
-		case msg := <-t.publishCh:
+		case msg, more := <-t.publishCh:
+			if !more {
+				// No more published messages, promise was fulfilled and we can return
+				return
+			}
+
 			keepers := make([]Subscriber[T], 0, len(subscribers))
 
 			for _, callback := range subscribers {
@@ -104,37 +123,6 @@ func (t *AsyncTopic[T]) Subscribe(fn Subscriber[T]) error {
 	return nil
 }
 
-func (t *AsyncTopic[T]) close() {
-	var wg sync.WaitGroup
-
-	wg.Add(2)
-
-	go func() {
-		defer wg.Done()
-		for range t.publishCh {
-			// drain publishCh to release all read locks
-		}
-	}()
-
-	go func() {
-		defer wg.Done()
-		for range t.subscribeCh {
-			// drain subscribeCh to release all read locks
-		}
-	}()
-
-	t.mu.Lock()
-	t.closed = true // no more subscribing or publishing
-	t.mu.Unlock()
-
-	close(t.publishCh)
-	close(t.subscribeCh)
-
-	wg.Wait()
-
-	t.options.onClose()
-}
-
 type AsyncTopicOptions struct {
 	onClose     func()
 	onSubscribe func(count int)

async_test.go

@@ -10,9 +10,9 @@ import (
 )
 
 func TestAsyncTopic_SinglePublisherSingleSubscriber(t *testing.T) {
-	const msgCount = 100
+	const msgCount = 10
 
-	subscriberReady := make(chan struct{})
+	subscriberReady := make(chan struct{}, 1)
 	defer close(subscriberReady)
 
 	topic := NewAsyncTopic[int](context.Background(), WithOnSubscribe(func(count int) {
@@ -35,12 +35,14 @@ func TestAsyncTopic_SinglePublisherSingleSubscriber(t *testing.T) {
 	}
 
 	count := 0
+	timeout := time.After(time.Second)
+
 	for count < msgCount {
 		select {
 		case <-feedback:
 			count++
 
-		case <-time.After(time.Second):
+		case <-timeout:
 			t.Fatalf("expected %d feedback items by now but only got %d", msgCount, count)
 		}
 	}
@@ -53,7 +55,7 @@ func TestAsyncTopic_MultiPublishersMultiSubscribers(t *testing.T) {
 		msgCount = pubCount * 100 // total messages to publish (delivered to EACH subscriber)
 	)
 
-	subscribersReady := make(chan struct{})
+	subscribersReady := make(chan struct{}, 1)
 	defer close(subscribersReady)
 
 	topic := NewAsyncTopic[int](context.Background(), WithOnSubscribe(func(count int) {
@@ -91,12 +93,14 @@ func TestAsyncTopic_MultiPublishersMultiSubscribers(t *testing.T) {
 	}
 
 	count := 0
+	timeout := time.After(time.Second)
+
 	for count != expFeedbackCount {
 		select {
 		case <-feedback:
 			count++
 
-		case <-time.After(time.Second):
+		case <-timeout:
 			t.Fatalf("expected %d feedback items by now but only got %d", expFeedbackCount, count)
 		}
 	}
@@ -106,7 +110,7 @@ func TestAsyncTopic_WithOnClose(t *testing.T) {
 	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
 	defer cancel()
 
-	feedback := make(chan struct{})
+	feedback := make(chan struct{}, 1)
 	defer close(feedback)
 
 	_ = NewAsyncTopic[int](ctx, WithOnClose(func() { feedback <- struct{}{} }))
@@ -138,13 +142,15 @@ func TestAsyncTopic_WithOnSubscribe(t *testing.T) {
 	}
 
 	count := 0
+	timeout := time.After(time.Second)
+
 	for count < totalSub {
 		select {
 		case c := <-feedback:
 			count++
 			assert.Equal(t, count, c, "expected %d but got %d instead", count, c)
 
-		case <-time.After(time.Second):
+		case <-timeout:
 			t.Fatalf("expected %d feedback items by now but only got %d", totalSub, count)
 		}
 	}
@@ -173,7 +179,7 @@ func TestAsyncTopic_ClosedTopicError(t *testing.T) {
 			ctx, cancel := context.WithCancel(context.Background())
 			defer cancel()
 
-			feedback := make(chan struct{})
+			feedback := make(chan struct{}, 1)
 			defer close(feedback)
 
 			topic := NewAsyncTopic[int](ctx, WithOnClose(func() { feedback <- struct{}{} }))
@@ -192,3 +198,47 @@ func TestAsyncTopic_ClosedTopicError(t *testing.T) {
 		})
 	}
 }
+
+func TestAsyncTopic_AllPublishedBeforeClosedAreDeliveredAfterClosed(t *testing.T) {
+	const msgCount = 10
+
+	subscriberReady := make(chan struct{}, 1)
+	defer close(subscriberReady)
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	topic := NewAsyncTopic[int](ctx, WithOnSubscribe(func(count int) {
+		subscriberReady <- struct{}{}
+	}))
+
+	feedback := make(chan int) // unbuffered will cause choke point for publishers
+	defer close(feedback)
+
+	err := topic.Subscribe(func(i int) bool {
+		feedback <- i
+		return true
+	})
+	require.NoError(t, err)
+
+	<-subscriberReady
+
+	for i := range msgCount {
+		require.NoError(t, topic.Publish(i))
+	}
+
+	cancel()
+
+	values := make(map[int]struct{}, msgCount)
+	timeout := time.After(time.Second)
+
+	for len(values) < msgCount {
+		select {
+		case f := <-feedback:
+			values[f] = struct{}{}
+
+		case <-timeout:
+			t.Fatalf("expected %d unique feedback values by now but only got %d", msgCount, len(values))
+		}
+	}
+}