Fix crash when a series has no block

block.go

@@ -438,7 +438,7 @@ Outer:
 
 		for _, chk := range chks {
 			if intervalOverlap(mint, maxt, chk.MinTime, chk.MaxTime) {
-				// Delete only until the current vlaues and not beyond.
+				// Delete only until the current values and not beyond.
 				tmin, tmax := clampInterval(mint, maxt, chks[0].MinTime, chks[len(chks)-1].MaxTime)
 				stones[p.At()] = Intervals{{tmin, tmax}}
 				continue Outer

head.go

@@ -587,8 +587,12 @@ func (h *Head) Delete(mint, maxt int64, ms ...labels.Matcher) error {
 	for p.Next() {
 		series := h.series.getByID(p.At())
 
+		t0, t1 := series.minTime(), series.maxTime()
+		if t0 == math.MinInt64 || t1 == math.MinInt64 {
+			continue
+		}
 		// Delete only until the current values and not beyond.
-		t0, t1 := clampInterval(mint, maxt, series.minTime(), series.maxTime())
+		t0, t1 = clampInterval(mint, maxt, t0, t1)
 		stones = append(stones, Stone{p.At(), Intervals{{t0, t1}}})
 	}
 
@@ -1106,11 +1110,18 @@ type memSeries struct {
 }
 
 func (s *memSeries) minTime() int64 {
+	if len(s.chunks) == 0 {
+		return math.MinInt64
+	}
 	return s.chunks[0].minTime
 }
 
 func (s *memSeries) maxTime() int64 {
-	return s.head().maxTime
+	c := s.head()
+	if c == nil {
+		return math.MinInt64
+	}
+	return c.maxTime
 }
 
 func (s *memSeries) cut(mint int64) *memChunk {

head_test.go

@@ -230,6 +230,31 @@ func TestMemSeries_truncateChunks(t *testing.T) {
 	testutil.Assert(t, ok == false, "non-last chunk incorrectly wrapped with sample buffer")
 }
 
+func TestHeadDeleteSeriesWithoutSamples(t *testing.T) {
+	entries := []interface{}{
+		[]RefSeries{
+			{Ref: 10, Labels: labels.FromStrings("a", "1")},
+		},
+		[]RefSample{},
+		[]RefSeries{
+			{Ref: 50, Labels: labels.FromStrings("a", "2")},
+		},
+		[]RefSample{
+			{Ref: 50, T: 80, V: 1},
+			{Ref: 50, T: 90, V: 1},
+		},
+	}
+	wal := &memoryWAL{entries: entries}
+
+	head, err := NewHead(nil, nil, wal, 1000)
+	testutil.Ok(t, err)
+	defer head.Close()
+
+	testutil.Ok(t, head.ReadWAL())
+
+	testutil.Ok(t, head.Delete(0, 100, labels.NewEqualMatcher("a", "1")))
+}
+
 func TestHeadDeleteSimple(t *testing.T) {
 	numSamples := int64(10)