add Feed test

async_test.go

@@ -2,6 +2,7 @@ package gubgub
 
 import (
 	"context"
+	"sync"
 	"testing"
 	"time"
 
@@ -248,6 +249,48 @@ func TestAsyncTopic_AllPublishedBeforeClosedAreDeliveredAfterClosed(t *testing.T
 	}
 }
 
+func TestAsyncTopic_Feed(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	subscriberReady := make(chan struct{}, 1)
+	defer close(subscriberReady)
+
+	topic := NewAsyncTopic[int](ctx,
+		WithOnSubscribe(func(count int) {
+			subscriberReady <- struct{}{}
+		}),
+	)
+
+	wg := sync.WaitGroup{}
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		seen := make(map[int]struct{})
+		for i := range topic.Feed() {
+			seen[i] = struct{}{}
+			if len(seen) >= 9 {
+				return
+			}
+		}
+	}()
+
+	<-subscriberReady
+
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		for i := range 10 {
+			topic.Publish(i)
+		}
+	}()
+
+	wg.Wait()
+}
+
 func testTimer(t testing.TB, d time.Duration) *time.Timer {
 	t.Helper()