fix nil pointer in merge iter Err() (#47)

Author: jnyi

pkg/dedup/merge_iter.go

@@ -92,18 +92,23 @@ func (m *mergedSeriesIterator) At() (t int64, v float64) {
 	return m.lastIter.At()
 }
 
-func (it *mergedSeriesIterator) AtHistogram(h *histogram.Histogram) (int64, *histogram.Histogram) {
-	return it.lastIter.AtHistogram(h)
+func (m *mergedSeriesIterator) AtHistogram(h *histogram.Histogram) (int64, *histogram.Histogram) {
+	return m.lastIter.AtHistogram(h)
 }
 
-func (it *mergedSeriesIterator) AtFloatHistogram(fh *histogram.FloatHistogram) (int64, *histogram.FloatHistogram) {
-	return it.lastIter.AtFloatHistogram(fh)
+func (m *mergedSeriesIterator) AtFloatHistogram(fh *histogram.FloatHistogram) (int64, *histogram.FloatHistogram) {
+	return m.lastIter.AtFloatHistogram(fh)
 }
 
-func (it *mergedSeriesIterator) AtT() int64 {
-	return it.lastT
+func (m *mergedSeriesIterator) AtT() int64 {
+	return m.lastT
 }
 
+// Err All At() funcs should panic if called after Next() or Seek() return ValNone.
+// Only Err() should return nil even after Next() or Seek() return ValNone.
 func (m *mergedSeriesIterator) Err() error {
+	if m.lastIter == nil {
+		return nil
+	}
 	return m.lastIter.Err()
 }

pkg/dedup/merge_iter_test.go

@@ -4,13 +4,24 @@
 package dedup
 
 import (
+	"math"
 	"testing"
 
 	"github.com/efficientgo/core/testutil"
 	"github.com/prometheus/prometheus/model/labels"
 	"github.com/prometheus/prometheus/storage"
+	"github.com/prometheus/prometheus/tsdb/chunkenc"
 )
 
+func TestIteratorEdgeCases(t *testing.T) {
+	ms := NewMergedSeries(labels.Labels{}, []storage.Series{})
+	it := ms.Iterator(nil)
+	testutil.Ok(t, it.Err())
+	testutil.Equals(t, int64(math.MinInt64), it.AtT())
+	testutil.Equals(t, chunkenc.ValNone, it.Next())
+	testutil.Ok(t, it.Err())
+}
+
 func TestMergedSeriesIterator(t *testing.T) {
 	for _, tcase := range []struct {
 		name      string