fix: fix a bug where AppendRequest with no entries triggers flush (#13672)

Author: grobinson-grafana

pkg/storage/wal/manager.go

@@ -156,7 +156,7 @@ func (m *Manager) Append(r AppendRequest) (*AppendResult, error) {
 	s.w.Append(r.TenantID, r.LabelsStr, r.Labels, r.Entries, m.clock.Now())
 	// If the segment exceeded the maximum age or the maximum size, move s to
 	// the closed list to be flushed.
-	if m.clock.Since(s.w.firstAppend) >= m.cfg.MaxAge || s.w.InputSize() >= m.cfg.MaxSegmentSize {
+	if s.w.Age(m.clock.Now()) >= m.cfg.MaxAge || s.w.InputSize() >= m.cfg.MaxSegmentSize {
 		m.move(el, s)
 	}
 	return s.r, nil
@@ -224,7 +224,7 @@ func (m *Manager) move(el *list.Element, s *segment) {
 func (m *Manager) moveFrontIfExpired() bool {
 	if el := m.available.Front(); el != nil {
 		s := el.Value.(*segment)
-		if !s.w.firstAppend.IsZero() && m.clock.Since(s.w.firstAppend) >= m.cfg.MaxAge {
+		if s.w.Age(m.clock.Now()) >= m.cfg.MaxAge {
 			m.move(el, s)
 			return true
 		}

pkg/storage/wal/manager_test.go

@@ -54,6 +54,38 @@ func TestManager_Append(t *testing.T) {
 	require.NoError(t, res.Err())
 }
 
+func TestManager_AppendNoEntries(t *testing.T) {
+	m, err := NewManager(Config{
+		MaxAge:         30 * time.Second,
+		MaxSegments:    1,
+		MaxSegmentSize: 1024, // 1KB
+	}, NewManagerMetrics(nil))
+	require.NoError(t, err)
+
+	// Append no entries.
+	lbs := labels.Labels{{Name: "a", Value: "b"}}
+	res, err := m.Append(AppendRequest{
+		TenantID:  "1",
+		Labels:    lbs,
+		LabelsStr: lbs.String(),
+		Entries:   []*logproto.Entry{},
+	})
+	require.NoError(t, err)
+	require.NotNil(t, res)
+
+	// The data hasn't been flushed, so reading from Done() should block.
+	select {
+	case <-res.Done():
+		t.Fatal("unexpected closed Done()")
+	default:
+	}
+
+	// The segment that was just appended to has neither reached the maximum
+	// age nor maximum size to be flushed.
+	require.Equal(t, 1, m.available.Len())
+	require.Equal(t, 0, m.pending.Len())
+}
+
 func TestManager_AppendFailed(t *testing.T) {
 	m, err := NewManager(Config{
 		MaxAge:         30 * time.Second,

pkg/storage/wal/segment.go

@@ -140,6 +140,9 @@ func NewWalSegmentWriter() (*SegmentWriter, error) {
 
 // Age returns the age of the segment.
 func (b *SegmentWriter) Age(now time.Time) time.Duration {
+	if b.firstAppend.IsZero() {
+		return 0
+	}
 	return now.Sub(b.firstAppend)
 }