feat: Add test for validating the fix

Author: murarustefaan

scheduler/batchsender/batch_sender_test.go

@@ -0,0 +1,43 @@
+package batchsender
+
+import (
+	"sync"
+	"testing"
+	"time"
+)
+
+// This test verifies there is no data race between Send() and the timer-triggered flush.
+func TestSend_ConcurrentWithTimerFlush(t *testing.T) {
+	// The race occurs when:
+	//   1. a Send() call schedules a timer via time.AfterFunc
+	//   2. the timer fires and calls flush() on a separate goroutine
+	//   3. another Send() reads bs.items concurrently.
+	//
+	// To trigger this, we send items from multiple goroutines with delays around batchTimeout so the timer fires between Sends.
+	var mu sync.Mutex
+	var received []any
+
+	const numGoroutines = 5
+	const sendsPerGoroutine = 20
+
+	bs := NewBatchSender(func(items any) {
+		mu.Lock()
+		defer mu.Unlock()
+		received = append(received, items)
+	})
+
+	var wg sync.WaitGroup
+	wg.Add(numGoroutines)
+	for range numGoroutines {
+		go func() {
+			defer wg.Done()
+			for range sendsPerGoroutine {
+				bs.Send("item")
+				time.Sleep(batchTimeout + 10*time.Millisecond)
+			}
+		}()
+	}
+
+	wg.Wait()
+	bs.Close()
+}