[8.x](backport #41230) Re-enable TestGroup_Go and fix flaky behavior

filebeat/input/filestream/internal/task/group_test.go

@@ -36,15 +36,21 @@ type noopLogger struct{}
 
 func (n noopLogger) Errorf(string, ...interface{}) {}
 
-type testLogger strings.Builder
+type testLogger struct {
+	mu sync.Mutex
+	b  strings.Builder
+}
 
 func (tl *testLogger) Errorf(format string, args ...interface{}) {
-	sb := (*strings.Builder)(tl)
-	sb.WriteString(fmt.Sprintf(format, args...))
-	sb.WriteString("\n")
+	tl.mu.Lock()
+	defer tl.mu.Unlock()
+	tl.b.WriteString(fmt.Sprintf(format, args...))
+	tl.b.WriteString("\n")
 }
 func (tl *testLogger) String() string {
-	return (*strings.Builder)(tl).String()
+	tl.mu.Lock()
+	defer tl.mu.Unlock()
+	return tl.b.String()
 }
 
 func TestNewGroup(t *testing.T) {
@@ -67,7 +73,6 @@ func TestNewGroup(t *testing.T) {
 }
 
 func TestGroup_Go(t *testing.T) {
-	t.Skip("Flaky tests: https://github.com/elastic/beats/issues/41218")
 	t.Run("don't run more than limit goroutines", func(t *testing.T) {
 		done := make(chan struct{})
 		defer close(done)
@@ -227,38 +232,37 @@ func TestGroup_Go(t *testing.T) {
 
 	t.Run("all workloads return an error", func(t *testing.T) {
 		logger := &testLogger{}
-		runCunt := atomic.Uint64{}
-		wg := sync.WaitGroup{}
+		var count atomic.Uint64
 
 		wantErr := errors.New("a error")
 		workload := func(i int) func(context.Context) error {
 			return func(_ context.Context) error {
-				defer runCunt.Add(1)
-				defer wg.Done()
+				defer count.Add(1)
 				return fmt.Errorf("[%d]: %w", i, wantErr)
 			}
 		}
 
 		want := uint64(2)
 		g := NewGroup(want, time.Second, logger, "errorPrefix")
 
-		wg.Add(1)
 		err := g.Go(workload(1))
 		require.NoError(t, err)
-		wg.Wait()
 
-		wg.Add(1)
 		err = g.Go(workload(2))
 		require.NoError(t, err)
-		wg.Wait()
 
-		err = g.Stop()
+		assert.Eventually(t, func() bool {
+			return count.Load() == want && logger.String() != ""
+		}, 100*time.Millisecond, time.Millisecond)
 
+		err = g.Stop()
 		require.NoError(t, err)
+
 		logs := logger.String()
 		assert.Contains(t, logs, wantErr.Error())
 		assert.Contains(t, logs, "[2]")
 		assert.Contains(t, logs, "[1]")
+
 	})
 
 	t.Run("some workloads return an error", func(t *testing.T) {
@@ -268,17 +272,26 @@ func TestGroup_Go(t *testing.T) {
 
 		g := NewGroup(want, time.Second, logger, "")
 
-		err := g.Go(func(_ context.Context) error { return nil })
+		var count atomic.Uint64
+		err := g.Go(func(_ context.Context) error {
+			count.Add(1)
+			return nil
+		})
 		require.NoError(t, err)
-		err = g.Go(func(_ context.Context) error { return wantErr })
+		err = g.Go(func(_ context.Context) error {
+			count.Add(1)
+			return wantErr
+		})
 		require.NoError(t, err)
 
-		time.Sleep(time.Millisecond)
+		assert.Eventually(t, func() bool {
+			return count.Load() == want && logger.String() != ""
+		}, 100*time.Millisecond, time.Millisecond, "not all workloads finished")
 
-		err = g.Stop()
+		assert.Contains(t, logger.String(), wantErr.Error())
 
+		err = g.Stop()
 		assert.NoError(t, err)
-		assert.Contains(t, logger.String(), wantErr.Error())
 	})
 
 	t.Run("workload returns no error", func(t *testing.T) {